Passing and Returning Structures by Value

$Header: /cvs/projects/SWIG/Examples/tcl/value/index.html,v 1.1 2000/08/31 18:07:07 beazley Exp $

Occasionally, a C program will manipulate structures by value such as shown in the following code:

/* File : example.c */

typedef struct Vector {
   double x, y, z;
} Vector;

double dot_product(Vector a, Vector b) {
  return (a.x*b.x + a.y*b.y + a.z*b.z);

Vector vector_add(Vector a, Vector b) {
  Vector r;
  r.x = a.x + b.x;
  r.y = a.y + b.y;
  r.z = a.z + b.z;
  return r;
Since SWIG only knows how to manage pointers to structures (not their internal representation), the following translations are made when wrappers are created:
double wrap_dot_product(Vector *a, Vector *b) {
    return dot_product(*a,*b);

Vector *wrap_vector_add(Vector *a, Vector *b) {
    Vector *r = (Vector *) malloc(sizeof(Vector));
    *r = vector_add(*a,*b);
    return r;
The functions are then called using pointers from the scripting language interface. It should also be noted that any function that returns a structure by value results in an implicit memory allocation. This will be a memory leak unless you take steps to free the result (see below).

The SWIG interface

Click here to see a SWIG interface file that wraps these two functions. In this file, there are a few essential features:
  • A wrapper for the free() function is created so that we can clean up the return result created by vector_add() function.

  • The %inline directive is used to create a few helper functions for creating new Vector objects and to print out the value (for debugging purposes).

A Tcl Script

Click here to see a script that uses these functions from Tcl.


  • When the '-c++' option is used, the resulting wrapper code for the return value changes to the following:
    Vector *wrap_vector_add(Vector *a, Vector *b) {
        Vector *r = new Vector(vector_add(*a,*b));
        return r;
    Similarly, it would be a mistake to use the free() function from C++. A safer approach would be to write a helper function like this:
    %inline %{
       void delete_Vector(Vector *v) {
           delete v;

  • Passing parameters by value like this really isn't the best C programming style. If possible, you might change your application to use pointers.

  • Similar translations are made when C++ references are used.