SWIG Library

To help build extension modules, SWIG is packaged with a library of support files that you can include in your own interfaces. These files often define new SWIG directives or provide utility functions that can be used to access parts of the standard C and C++ libraries. This chapter provides a reference to the current set of supported library files.

Compatibility note: Older versions of SWIG included a number of library files for manipulating pointers, arrays, and other structures. Most these files are now deprecated and have been removed from the distribution. Alternative libraries provide similar functionality. Please read this chapter carefully if you used the old libraries.

12.1 The %include directive and library search path

Library files are included using the %include directive. When searching for files, directories are searched in the following order:

Within directories mentioned in points 3-5, SWIG first looks for a subdirectory corresponding to a target language (e.g., python, tcl, etc.). If found, SWIG will search the language specific directory first. This allows for language-specific implementations of library files.

You can ignore the installed SWIG library by setting the SWIG_LIB environment variable. Set the environment variable to hold an alternative library directory.

The directories that are searched are displayed when using -verbose commandline option.

12.2 C arrays and pointers

This section describes library modules for manipulating low-level C arrays and pointers. The primary use of these modules is in supporting C declarations that manipulate bare pointers such as int *, double *, or void *. The modules can be used to allocate memory, manufacture pointers, dereference memory, and wrap pointers as class-like objects. Since these functions provide direct access to memory, their use is potentially unsafe and you should exercise caution.

12.2.1 argcargv.i

The argcargv.i library is a simple library providing multi-argument typemaps for handling C argc argv command line argument C string arrays. The argc parameter contains the argument count and argv contains the argument vector array.

12.2.2 cpointer.i

The cpointer.i module defines macros that can be used to generate wrappers around simple C pointers. The primary use of this module is in generating pointers to primitive datatypes such as int and double.

Generates a collection of four functions for manipulating a pointer type *:

type *new_name()

Creates a new object of type type and returns a pointer to it. In C, the object is created using calloc(). In C++, new is used.

type *copy_name(type value)

Creates a new object of type type and returns a pointer to it. An initial value is set by copying it from value. In C, the object is created using calloc(). In C++, new is used.

type *delete_name(type *obj)

Deletes an object type type.

void name_assign(type *obj, type value)

Assigns *obj = value.

type name_value(type *obj)

Returns the value of *obj.

When using this macro, type may be any type and name must be a legal identifier in the target language. name should not correspond to any other name used in the interface file.

Here is a simple example of using %pointer_functions():

%module example
%include "cpointer.i"

/* Create some functions for working with "int *" */
%pointer_functions(int, intp);

/* A function that uses an "int *" */
void add(int x, int y, int *result);

Now, in Python:

>>> import example
>>> c = example.new_intp()     # Create an "int" for storing result
>>> example.add(3, 4, c)       # Call function
>>> example.intp_value(c)      # Dereference
7
>>> example.delete_intp(c)     # Delete

Wraps a pointer of type * inside a class-based interface. This interface is as follows:

struct name {
  name();                            // Create pointer object
  ~name();                           // Delete pointer object
  void assign(type value);           // Assign value
  type value();                      // Get value
  type *cast();                      // Cast the pointer to original type
  static name *frompointer(type *);  // Create class wrapper from existing
                                     // pointer
};

When using this macro, type is restricted to a simple type name like int, float, or Foo. Pointers and other complicated types are not allowed. name must be a valid identifier not already in use. When a pointer is wrapped as a class, the "class" may be transparently passed to any function that expects the pointer.

If the target language does not support proxy classes, the use of this macro will produce the example same functions as %pointer_functions() macro.

It should be noted that the class interface does introduce a new object or wrap a pointer inside a special structure. Instead, the raw pointer is used directly.

Here is the same example using a class instead:

%module example
%include "cpointer.i"

/* Wrap a class interface around an "int *" */
%pointer_class(int, intp);

/* A function that uses an "int *" */
void add(int x, int y, int *result);

Now, in Python (using proxy classes)

>>> import example
>>> c = example.intp()         # Create an "int" for storing result
>>> example.add(3, 4, c)       # Call function
>>> c.value()                  # Dereference
7

Of the two macros, %pointer_class is probably the most convenient when working with simple pointers. This is because the pointers are access like objects and they can be easily garbage collected (destruction of the pointer object destroys the underlying object).

Creates a casting function that converts type1 to type2. The name of the function is name. For example:

%pointer_cast(int *, unsigned int *, int_to_uint);

In this example, the function int_to_uint() would be used to cast types in the target language.

Note: None of these macros can be used to safely work with strings (char * or char **).

Note: When working with simple pointers, typemaps can often be used to provide more seamless operation.

12.2.3 carrays.i

This module defines macros that assist in wrapping ordinary C pointers as arrays. The module does not provide any safety or an extra layer of wrapping--it merely provides functionality for creating, destroying, and modifying the contents of raw C array data.

Creates four functions.

type *new_name(size_t nelements)

Creates a new array of objects of type type. In C, the array is allocated using calloc(). In C++, new [] is used.

type *delete_name(type *ary)

Deletes an array. In C, free() is used. In C++, delete [] is used.

type name_getitem(type *ary, size_t index)

Returns the value ary[index].

void name_setitem(type *ary, size_t index, type value)

Assigns ary[index] = value.

When using this macro, type may be any type and name must be a legal identifier in the target language. name should not correspond to any other name used in the interface file.

Here is an example of %array_functions(). Suppose you had a function like this:

void print_array(double x[10]) {
  int i;
  for (i = 0; i < 10; i++) {
    printf("[%d] = %g\n", i, x[i]);
  }
}

To wrap it, you might write this:

%module example

%include "carrays.i"
%array_functions(double, doubleArray);

void print_array(double x[10]);

Now, in a scripting language, you might write this:

a = new_doubleArray(10)               # Create an array
for i in range(0, 10):
    doubleArray_setitem(a, i, 2 * i)  # Set a value
print_array(a)                        # Pass to C
delete_doubleArray(a)                 # Destroy array

Wraps a pointer of type * inside a class-based interface. This interface is as follows:

struct name {
  name(size_t nelements);               // Create an array
  ~name();                              // Delete array
  type getitem(size_t index);           // Return item
  void setitem(size_t index, type value);  // Set item
  type *cast();                         // Cast to original type
  static name *frompointer(type *);     // Create class wrapper from
                                        // existing pointer
};

When using this macro, type is restricted to a simple type name like int or float. Pointers and other complicated types are not allowed. name must be a valid identifier not already in use. When a pointer is wrapped as a class, it can be transparently passed to any function that expects the pointer.

When combined with proxy classes, the %array_class() macro can be especially useful. For example:

%module example
%include "carrays.i"
%array_class(double, doubleArray);

void print_array(double x[10]);

Allows you to do this:

import example
c = example.doubleArray(10)  # Create double[10]
for i in range(0, 10):
    c[i] = 2 * i             # Assign values
example.print_array(c)       # Pass to C

Note: These macros do not encapsulate C arrays inside a special data structure or proxy. There is no bounds checking or safety of any kind. If you want this, you should consider using a special array object rather than a bare pointer.

Note: %array_functions() and %array_class() should not be used with types of char or char *. SWIG's default handling of these types is to handle them as character strings and the two macros do not do enough to change this.

12.2.4 cmalloc.i

This module defines macros for wrapping the low-level C memory allocation functions malloc(), calloc(), realloc(), and free().

Creates a wrapper around malloc() with the following prototype:

type *malloc_name(int nbytes = sizeof(type));

If type is void, then the size parameter nbytes is required. The name parameter only needs to be specified when wrapping a type that is not a valid identifier (e.g., "int *", "double **", etc.).

Creates a wrapper around calloc() with the following prototype:

type *calloc_name(int nobj =1, int sz = sizeof(type));

If type is void, then the size parameter sz is required.

Creates a wrapper around realloc() with the following prototype:

type *realloc_name(type *ptr, int nitems);

Note: unlike the C realloc(), the wrapper generated by this macro implicitly includes the size of the corresponding type. For example, realloc_int(p, 100) reallocates p so that it holds 100 integers.

Creates a wrapper around free() with the following prototype:

void free_name(type *ptr);

Creates the constant:

%constant int sizeof_name = sizeof(type);

Generates wrappers for all five of the above operations.

12.2.5 cdata.i

The cdata.i module defines functions for converting raw C data to and from a target language type. The target language type is either:

The primary applications of this module would be packing/unpacking of binary data structures---for instance, if you needed to extract data from a buffer.

Converts nbytes of data at ptr into the target language type. ptr can be any pointer.

This is actually a wrapper of the standard C library memmove function, void *memmove(void *ptr, const void *src, size_t n), which copies n bytes of data from src into the destination pointed to by ptr. Multi-argument typemaps are used so that the last two parameters, src and n are replaced by s, a string or byte array target language type, mentioned earlier. The string/byte array may contain embedded NULL bytes. Unlike the C library function nothing is returned by the wrapper function.

One use of these functions is packing and unpacking data from memory. Here is a short example:

Since the size of data is not always known, the following macro is also defined:

Generates the following function for extracting C data for a given type.

char *cdata_name(type* ptr, int nitems)

nitems is the number of items of the given type to extract.

Note: These functions provide direct access to memory and can be used to overwrite data. Clearly they are unsafe.

12.3 C string handling

A common problem when working with C programs is dealing with functions that manipulate raw character data using char *. In part, problems arise because there are different interpretations of char *---it could be a NULL-terminated string or it could point to binary data. Moreover, functions that manipulate raw strings may mutate data, perform implicit memory allocations, or utilize fixed-sized buffers.

The problems (and perils) of using char * are well-known. However, SWIG is not in the business of enforcing morality. The modules in this section provide basic functionality for manipulating raw C strings.

12.3.1 Default string handling

The default wrapping behavior for this function is to set s to a raw char * that refers to the internal string data in the target language. In other words, if you were using a language like Tcl, and you wrote this,

then s would point to the representation of "Hello" inside the Tcl interpreter. When returning a char *, SWIG assumes that it is a NULL-terminated string and makes a copy of it. This gives the target language its own copy of the result.

There are obvious problems with the default behavior. First, since a char * argument points to data inside the target language, it is NOT safe for a function to modify this data (doing so may corrupt the interpreter and lead to a crash). Furthermore, the default behavior does not work well with binary data. Instead, strings are assumed to be NULL-terminated.

12.3.2 Passing a string with length

you can wrap the parameters (char *str, size_t len) as a single argument using a typemap. Just do this:

In the wrapper function, the passed string will be expanded to a pointer and length parameter. The (char *STRING, int LENGTH) multi-argument typemap is also available in addition to (char *STRING, size_t LENGTH).

12.3.3 Using %newobject to release memory

then the SWIG generated wrappers will have a memory leak--the returned data will be copied into a string object and the old contents ignored.

This will release the result if the appropriate target language support is available. SWIG provides the appropriate "newfree" typemap for char * so that the memory is released, however, you may need to provide your own "newfree" typemap for other types. See Object ownership and %newobject for more details.

12.3.4 cstring.i

The cstring.i library file provides a collection of macros for dealing with functions that either mutate string arguments or which try to output string data through their arguments. An example of such a function might be this rather questionable implementation:

(Off topic rant: If your program really has functions like this, you would be well-advised to replace them with safer alternatives involving bounds checking).

The macros defined in this module all expand to various combinations of typemaps. Therefore, the same pattern matching rules and ideas apply.

Turns parameter parm into an output value. The output string is assumed to be NULL-terminated and smaller than maxsize characters. Here is an example:

%cstring_bounded_output(char *path, 1024);
...
void get_path(char *path);

In the target language:

>>> get_path()
/home/beazley/packages/Foo/Bar
>>>

Internally, the wrapper function allocates a small buffer (on the stack) of the requested size and passes it as the pointer value. Data stored in the buffer is then returned as a function return value. If the function already returns a value, then the return value and the output string are returned together (multiple return values). If more than maxsize bytes are written, your program will crash with a buffer overflow!

Turns parameter parm into an output value. The output string is always chunksize and may contain binary data. Here is an example:

%cstring_chunk_output(char *packet, PACKETSIZE);
...
void get_packet(char *packet);

In the target language:

>>> get_packet()
'\xa9Y:\xf6\xd7\xe1\x87\xdbH;y\x97\x7f\xd3\x99\x14V\xec\x06\xea\xa2\x88'
>>>

This macro is essentially identical to %cstring_bounded_output. The only difference is that the result is always chunksize characters. Furthermore, the result can contain binary data. If more than maxsize bytes are written, your program will crash with a buffer overflow!

Turns parameter parm into a mutable string argument. The input string is assumed to be NULL-terminated and smaller than maxsize characters. The output string is also assumed to be NULL-terminated and less than maxsize characters.

%cstring_bounded_mutable(char *ustr, 1024);
...
void make_upper(char *ustr);

In the target language:

>>> make_upper("hello world")
'HELLO WORLD'
>>>

Internally, this macro is almost exactly the same as %cstring_bounded_output. The only difference is that the parameter accepts an input value that is used to initialize the internal buffer. It is important to emphasize that this function does not mutate the string value passed---instead it makes a copy of the input value, mutates it, and returns it as a result. If more than maxsize bytes are written, your program will crash with a buffer overflow!

Turns parameter parm into a mutable string argument. The input string is assumed to be NULL-terminated. An optional parameter expansion specifies the number of extra characters by which the string might grow when it is modified. The output string is assumed to be NULL-terminated and less than the size of the input string plus any expansion characters.

%cstring_mutable(char *ustr);
...
void make_upper(char *ustr);

%cstring_mutable(char *hstr, HEADER_SIZE);
...
void attach_header(char *hstr);

In the target language:

>>> make_upper("hello world")
'HELLO WORLD'
>>> attach_header("Hello world")
'header: Hello world'
>>>

This macro differs from %cstring_bounded_mutable() in that a buffer is dynamically allocated (on the heap using malloc/new). This buffer is always large enough to store a copy of the input value plus any expansion bytes that might have been requested. It is important to emphasize that this function does not directly mutate the string value passed---instead it makes a copy of the input value, mutates it, and returns it as a result. If the function expands the result by more than expansion extra bytes, then the program will crash with a buffer overflow!

This macro is used to handle bounded character output functions where both a char * and a maximum length parameter are provided. As input, a user simply supplies the maximum length. The return value is assumed to be a NULL-terminated string.

%cstring_output_maxsize(char *path, int maxpath);
...
void get_path(char *path, int maxpath);

In the target language:

>>> get_path(1024)
'/home/beazley/Packages/Foo/Bar'
>>>

This macro provides a safer alternative for functions that need to write string data into a buffer. User supplied buffer size is used to dynamically allocate memory on heap. Results are placed into that buffer and returned as a string object.

This macro is used to handle bounded character output functions where both a char * and a pointer int * are passed. Initially, the int * parameter points to a value containing the maximum size. On return, this value is assumed to contain the actual number of bytes. As input, a user simply supplies the maximum length. The output value is a string that may contain binary data.

%cstring_output_withsize(char *data, int *maxdata);
...
void get_data(char *data, int *maxdata);

In the target language:

>>> get_data(1024)
'x627388912'
>>> get_data(1024)
'xyzzy'
>>>

This macro is a somewhat more powerful version of %cstring_output_chunk(). Memory is dynamically allocated and can be arbitrary large. Furthermore, a function can control how much data is actually returned by changing the value of the maxparm argument.

This macro is used to return strings that are allocated within the program and returned in a parameter of type char **. For example:

void foo(char **s) {
  *s = (char *) malloc(64);
  sprintf(*s, "Hello world\n");
}

The returned string is assumed to be NULL-terminated. release specifies how the allocated memory is to be released (if applicable). Here is an example:

%cstring_output_allocate(char **s, free(*$1));
...
void foo(char **s);

In the target language:

>>> foo()
'Hello world\n'
>>>

This macro is used to return strings that are allocated within the program and returned in two parameters of type char ** and int *. For example:

void foo(char **s, int *sz) {
  *s = (char *) malloc(64);
  *sz = 64;
  // Write some binary data
  ...
}

The returned string may contain binary data. release specifies how the allocated memory is to be released (if applicable). Here is an example:

%cstring_output_allocate_size(char **s, int *slen, free(*$1));
...
void foo(char **s, int *slen);

In the target language:

>>> foo()
'\xa9Y:\xf6\xd7\xe1\x87\xdbH;y\x97\x7f\xd3\x99\x14V\xec\x06\xea\xa2\x88'
>>>

This is the safest and most reliable way to return binary string data in SWIG. If you have functions that conform to another prototype, you might consider wrapping them with a helper function. For example, if you had this:

char  *get_data(int *len);

You could wrap it with a function like this:

void my_get_data(char **result, int *len) {
  *result = get_data(len);
}

12.4 C standard library

12.4.1 Complex floating types

SWIG has some support for complex floating types. By default the keyword _Complex is understood by the parser but complex is not treated as a keyword because it may be used as an identifier.

SWIG is only able to fully wrap complex floating types for some target languages. If you are using such a target language for complex floating types, you should include complex.i to enable this support:

If SWIG is in C++ code this will actually include std_complex.i for you; otherwise it will include ccomplex.i.

For target languages without support this will fail to find a file to include. In this case, if you are wrapping headers which use complex then you'll need to add a define to your SWIG interface file to get SWIG to parse the headers:

Then if wrapping as opaque types is not useful to you, you can use %ignore to tell SWIG not to wrap the functions and/or variables which use complex floating types.

12.5 STL/C++ library

The library modules in this section provide access to parts of the standard C++ library including the STL. SWIG support for the STL is an ongoing effort. Support is quite comprehensive for some language modules but some of the lesser used modules do not have quite as much library code written.

The following table shows which C++ classes are supported and the equivalent SWIG interface library file for the C++ library.

The list is by no means complete; some language modules support a subset of the above and some support additional STL classes. Please look for the library files in the appropriate language library directory.

12.5.1 std::string

The std_string.i library provides typemaps for converting C++ std::string objects to and from strings in the target scripting language. For example:

A common problem that people encounter is that of classes/structures containing a std::string. This can be overcome by defining a typemap. For example:

This module only supports types std::string and const std::string &. Pointers and non-const references are left unmodified and returned as SWIG pointers.

This library file is fully aware of C++ namespaces. If you export std::string or rename it with a typedef, make sure you include those declarations in your interface. For example:

12.5.2 std::string_view

The std_string_view.i library provides typemaps for converting C++17 std::string_view objects to and from strings in the target scripting language. For example:

For target languages for which SWIG supports directors, directorout typemaps are provided for std::string_view, but these require extra care to use safely. The issue is that returning std::string_view effectively returns a pointer to string data but doesn't own the pointed to data. For target languages where there isn't a native narrow string representation (e.g. C#, Java) a static std::string is used to cache the data, which works but isn't thread/reentrant safe. For target languages where there is a native narrow string representation SWIG will return a std::string_view pointing to that data, so you need to store the string to return somewhere which will persist for the lifetime the caller needs (e.g. put it in a member variable) - you can't return a temporary target language string. In both cases SWIG will issue a warning by default.

12.5.3 std::vector

The std_vector.i library provides support for the C++ std::vector class in the STL. Using this library involves the use of the %template directive. All you need to do is to instantiate different versions of vector for the types that you want to use. For example:

/* File : example.h */

#include <vector>
#include <algorithm>
#include <functional>
#include <numeric>

double average(std::vector<int> v) {
  return std::accumulate(v.begin(), v.end(), 0.0)/v.size();
}

std::vector<double> half(const std::vector<double>& v) {
  std::vector<double> w(v);
  for (unsigned int i=0; i<w.size(); i++)
    w[i] /= 2.0;
  return w;
}

void halve_in_place(std::vector<double>& v) {
  for (std::vector<double>::iterator it = v.begin(); it != v.end(); ++it)
    *it /= 2.0;
}

Now, to illustrate the behavior in the scripting interpreter, consider this Python example:

>>> from example import *
>>> iv = IntVector(4)         # Create an vector<int>
>>> for i in range(0, 4):
...      iv[i] = i
>>> average(iv)               # Call method
1.5
>>> average([0, 1, 2, 3])        # Call with list
1.5
>>> half([1, 2, 3])             # Half a list
(0.5, 1.0, 1.5)
>>> halve_in_place([1, 2, 3])   # Oops
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: Type error. Expected _p_std__vectorTdouble_t
>>> dv = DoubleVector(4)
>>> for i in range(0, 4):
...       dv[i] = i
>>> halve_in_place(dv)       # Ok
>>> for i in dv:
...       print i
...
0.0
0.5
1.0
1.5
>>> dv[20] = 4.5
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "example.py", line 81, in __setitem__
    def __setitem__(*args): return apply(examplec.DoubleVector___setitem__, args)
IndexError: vector index out of range
>>>

This library module is fully aware of C++ namespaces. If you use vectors with other names, make sure you include the appropriate using or typedef directives. For example:

Note: This module makes use of several advanced SWIG features including templatized typemaps and template partial specialization. If you are trying to wrap other C++ code with templates, you might look at the code contained in std_vector.i. Alternatively, you can show them the code if you want to make their head explode.

Note: This module is defined for all SWIG target languages. However argument conversion details and the public API exposed to the interpreter vary.

12.5.4 STL exceptions

Many of the STL wrapper functions add parameter checking and will throw a language dependent error/exception should the values not be valid. The classic example is array bounds checking. The library wrappers are written to throw a C++ exception in the case of error. The C++ exception in turn gets converted into an appropriate error/exception for the target language. By and large this handling should not need customising, however, customisation can easily be achieved by supplying appropriate "throws" typemaps. For example:

The custom exception handler might, for example, log the exception then convert it into a specific error/exception for the target language.

When using the STL it is advisable to add in an exception handler to catch all STL exceptions. The %exception directive can be used by placing the following code before any other methods or libraries to be wrapped:

Any thrown STL exceptions will then be gracefully handled instead of causing a crash.

12.5.5 shared_ptr smart pointer

12.5.5.1 shared_ptr basics

Some target languages have support for handling the shared_ptr reference counted smart pointer. This smart pointer is available in the standard C++11 library as std::shared_ptr. It was also in TR1 as std::tr1::shared_ptr before it was fully standardized. Support for the widely used boost::shared_ptr is also available.

In order to use std::shared_ptr, the std_shared_ptr.i library file should be included:

The pre-standard std::tr1::shared_ptr can be used by including the following macro before including the std_shared_ptr.i library file:

In order to use boost::shared_ptr, the boost_shared_ptr.i library file should be included:

You can only use one of these variants of shared_ptr in your interface file at a time (also SWIG doesn't currently support using both %shared_ptr(T) and %unique_ptr<T> on the same type T). All three variants must be used in conjunction with the %shared_ptr(T) macro, where T is the underlying pointer type equating to usage shared_ptr<T>. The type T must be non-primitive. A simple example demonstrates usage:

Note that the %shared_ptr(IntValue) declaration occurs after the inclusion of the boost_shared_ptr.i library which provides the macro and, very importantly, before any usage or declaration of the type, IntValue. The %shared_ptr macro provides, a few things for handling this smart pointer, but mostly a number of typemaps. These typemaps override the default typemaps so that the underlying proxy class is stored and passed around as a pointer to a shared_ptr instead of a plain pointer to the underlying type. This approach means that any instantiation of the type can be passed to methods taking the type by value, reference, pointer or as a smart pointer. The interested reader might want to look at the generated code, however, usage is simple and no different handling is required from the target language. For example, a simple use case of the above code from Java would be:

12.5.5.2 shared_ptr and inheritance

The shared_ptr library works quite differently to SWIG's normal, but somewhat limited, smart pointer handling. The shared_ptr library does not generate extra wrappers, just for smart pointer handling, in addition to the proxy class. The normal proxy class including inheritance relationships is generated as usual. The only real change introduced by the %shared_ptr macro is that the proxy class stores a pointer to the shared_ptr instance instead of a raw pointer to the instance. A proxy class derived from a base which is being wrapped with shared_ptr can and must be wrapped as a shared_ptr too. In other words all classes in an inheritance hierarchy must all be used with the %shared_ptr macro. For example the following code can be used with the base class shown earlier:

A shared_ptr of the derived class can now be passed to a method where the base is expected in the target language, just as it can in C++:

If the %shared_ptr macro is omitted for any class in the inheritance hierarchy, SWIG will warn about this and the generated code may or may not result in a C++ compilation error. For example, the following input:

12.5.5.3 shared_ptr and method overloading

A C++ compiler can disambiguate a method overloaded by a shared_ptr and one using the raw underlying type. For example, either one of these methods can be called in C++:

When wrapped by SWIG, disambiguation is not possible using the overloaded names as there is just one equivalent type (GrandParent) in the target language. SWIG will choose to wrap just the first method by default. Ambiguity in overloading discusses ways to control which method(s) gets wrapped using %ignore or %rename. For the interested reader, SWIG detects that they are equivalent types via the typecheck typemaps in the shared_ptr library.

12.5.5.4 shared_ptr and templates

The %shared_ptr macro should be used for all the required instantiations of the template before each of the %template instantiations. For example, consider number.h containing the following illustrative template:

12.5.5.5 shared_ptr and directors

The languages that support shared_ptr also have support for using shared_ptr with directors.

12.5.6 unique_ptr smart pointer

The std_unique_ptr.i library file provides SWIG's unique_ptr support. It provides move semantics for the smart pointer's underlying object, both from C++ to the target language and vice versa.

The library defines typemaps and a macro, %unique_ptr(T), to use for handling std::unique_ptr<T> for a type T. The type T must be non-primitive. This macro should be used before any code declaring or using type T. Ordering requirements for using this smart pointer macro are the same as the equivalent %shared_ptr(T) macro covered in the previous section. The ownership and move semantics described here can of course be modified if not suitable by copying and customising the typemaps in the appropriate std_unique_ptr.i library file.

Note that SWIG doesn't currently support using both %shared_ptr(T) and %unique_ptr<T> on the same type T.

12.5.6.1 unique_ptr passed by value

Example usage of a std::unique_ptr being returned from a function by value is shown below.

The returned objects can be used naturally from the target language, e.g. from C#:

The implementation simply calls std::unique_ptr::release() to obtain the underlying raw pointer. The pointer is then used to create a target language proxy class in the same way that SWIG handles a C++ function returning a class by value. The target language proxy class then owns the memory pointed to by the raw pointer and memory handling is identical to normal SWIG proxy class handling of the underlying C++ memory. Note that an object returned by value is first copied/moved from the stack onto the heap in order to obtain a raw pointer on the heap, whereas the underlying raw pointer in std::unique_ptr already points to an object the heap.

Note that the implementation is quite different to the std::shared_ptr smart pointer, where the proxy class manages the underlying C++ memory as a pointer to a shared_ptr instead of a plain raw pointer.

A possibly less common usage of this smart pointer is as a parameter to a function. When used like this it indicates that memory usage of the object pointed to by the underlying pointer is transferred to the function being called. The code that SWIG generates assumes this happens. First, it is assumed that a proxy class already owns the underlying C++ object and is used to pass the object to the C++ function being called. Second, the ownership is transferred from the proxy class to the C++ function being called and lifetime is then controlled by the function. Finally, it is assumed the lifetime of the object may not last beyond returning from the C++ function and hence the proxy class can no longer be used.

Consider expanding the example above with a function that takes a std::unique_ptr as follows:

Attempts to use k after the ownership has been passed into the take function should not be attempted. The implementation sets the proxy class to an invalid state by setting the class's underlying C++ pointer to null after the return from the take function. Subsequent use of an invalid proxy class instance is very much dependent on the implementation in the target language and ranges from a segfault to giving a nice error. Consider implementing additional checks via the 'check' typemap.

Attempts to pass ownership from a proxy class to a std::unique parameter more than once will result in a "Cannot release ownership as memory is not owned" exception. For example, if example.take(k) in the example above is called twice.

12.5.6.2 unique_ptr passed by reference

The effect of passing a std::unique_ptr by rvalue reference into a function is identical to passing it by value. The ownership of the memory of the object being pointed to by the underyling pointer is transferred from the proxy class to the C++ function being called. Example:

Passing non-const lvalue references into a function is a bit quirky and not perfect due to ambiguities as to what the function may do. The approach taken is the ownership is transferred out of the target language from the proxy class into C++ space and the proxy class can then no longer be used after the wrapped function returns. In summary it works much like passing a std::unique_ptr by value into a function. The assumption is the function will not modify the std::unique_ptr. If this is not true and the underlying pointer is changed, such as calling the member functions, swap, reset or release, then the modified std::unique_ptr will effectively be ignored. It is destroyed when the function exits C++ space on return to the target language. Example:

Passing const lvalue references into a function works much like passing any wrapped class. The proxy class owning the underling C++ object continues to own the underying C++ object after calling the function, the function cannot modify the std::unique_ptr or take ownership. Example:

Move semantics are not provided when wrapping a C++ function that returns a std::unique_ptr by reference. The target language proxy class wrapper that is returned does not own the underlying C++ object. This applies to all reference types, such as:

Compatibility note: Support for std::unique_ptr was first added in SWIG-4.1.0. This initial support contained the move semantics when passing a std::unique_ptr around by value. Support for passing a std::unique_ptr around by reference was added in SWIG-4.3.0.

12.5.7 auto_ptr smart pointer

While std::auto_ptr is deprecated in C++11, some existing code may still be using it. SWIG provides support for this class which is nearly identical to std::unique_ptr.

The std_auto_ptr.i library file provides SWIG's auto_ptr support. It defines typemaps and a macro, %auto_ptr(T), to use for handling std::auto_ptr<T> for a type T. The type T must be non-primitive. This macro should be used before any code declaring or using type T. Ordering requirements for using this smart pointer macro are the same as the equivalent %shared_ptr(T) and %unique_ptr macros covered in the previous two sections.

Example usage of a std::auto_ptr being returned from a function is shown below.

The returned objects can be used naturally from the target language, e.g. from C#:

The implementation simply calls std::auto_ptr::release() to obtain the underlying raw pointer. That is, it works the same way covered in the previous section for std::unique_ptr.

Input parameters also work the same way as std::unique_ptr covered in the previous section.

12.6 Utility Libraries

12.6.1 exception.i

The exception.i library provides a language-independent function for raising a run-time exception in the target language. This library is largely used by the SWIG library writers. If possible, use the error handling scheme available to your target language as there is greater flexibility in what errors/exceptions can be thrown.

Raises an exception in the target language. code is one of the following symbolic constants:

SWIG_MemoryError
SWIG_IOError
SWIG_RuntimeError
SWIG_IndexError
SWIG_TypeError
SWIG_DivisionByZero
SWIG_OverflowError
SWIG_SyntaxError
SWIG_ValueError
SWIG_SystemError
SWIG_NullReferenceError

message is a string indicating more information about the problem.

The primary use of this module is in writing language-independent exception handlers. For example:

12.6.2 attribute.i

The attribute library contains a set of macros to convert a pair of set/get methods into a "native" attribute/property.

Use %attribute when you have a pair of get/set methods to a primitive type like:

and you want to provide that variable as an attribute in the target language. This example only works for primitive types, not derived types. Now you can use the attributes like so (in Python):

If you don't provide a 'set' method, a 'read-only' attribute is generated, ie, like:

Use %attributeref when you have const/non-const reference access methods for primitive types or class/structs, like:

if the internal C++ reference methods have a different name from the attribute you want, so

is the same as the last example, but instead of the attribute 'c' being called 'c', it is called 'd'.

Use %attribute2 instead of %attribute to indicate that reference-pointer translation is required. Use %attribute2 instead of %attribute in cases like this:

Here, the data type of the property is a wrapped type MyFoo and on the C++ side it is passed by reference. The problem is that the SWIG wrapper will pass around a pointer (MyFoo *) which is not compatible with the reference type of the accessors (MyFoo &). Therefore, if you use %attribute, you'll get an error from your C/C++ compiler. %attribute2 translates between a pointer and a reference to eliminate the error. In case you're confused, let's make it simple: just use %attribute at first, but if the C/C++ compiler gives an error while compiling the wrapper, try %attribute2 instead.

NOTE: remember that if the type contains commas, such as std::pair<int, int>, you need to use the macro like:

where %arg() 'normalizes' the type to be understood as a single argument, otherwise the macro will get confused by the comma.

The %attributeval is the same as %attribute, but should be used when the type is a class/struct (ie a non-primitive type) and when the get and set methods return/pass by value. The following is very similar to the above example, but note that the access is by value rather than reference.

The %attributestring is the same as %attributeval, but should be used for string class types, which are unusual as they are a class on the C++ side, but normally an immutable/primitive type in the target language. Example usage for std::string:

%include <std_string.i>
%attributestring(MyStringyClass, std::string, ReadWriteString, GetString, SetString);
%attributestring(MyStringyClass, std::string, ReadOnlyString, GetString);
%inline %{
  class MyStringyClass {
    std::string str;
  public:
    MyStringyClass(const std::string &val) : str(val) {}
    std::string GetString() { return str; }
    void SetString(std::string other) { str = other; }
  };
%}

The %attributestring also works for class types that have %naturalvar turned on and so is also useful for shared_ptr which has %naturalvar turned on in %shared_ptr.

12.6.2.1 %attribute and C++ templates

%attribute and friends have to be used on fully specified classes. For example

Note the use of a template-id (i.e., A<int> not A<T> or just A). This means that %attribute statements have to be repeated for any template-id that you want to use with %template.

C++ class	C++ Library file	SWIG Interface library file
std::array (C++11)	array	std_array.i
std::auto_ptr	memory	std_auto_ptr.i
std::complex	complex	std_complex.i
std::deque	deque	std_deque.i
std::list	list	std_list.i
std::map	map	std_map.i
std::multimap (C++11)	multimap	std_multimap.i
std::multiset (C++11)	multiset	std_multiset.i
std::pair	utility	std_pair.i
std::set	set	std_set.i
std::shared_ptr (C++11)	shared_ptr	std_shared_ptr.i
std::string	string	std_string.i
std::string_view (C++17)	string_view	std_string_view.i
std::unordered_map (C++11)	unordered_map	std_unordered_map.i
std::unordered_multimap (C++11)	unordered_multimap	std_unordered_multimap.i
std::unordered_multiset (C++11)	unordered_multiset	std_unordered_multiset.i
std::unordered_set (C++11)	unordered_set	std_unordered_set.i
std::vector	vector	std_vector.i
std::wstring	wstring	std_wstring.i