SWIG:Examples:python:reference

SWIG/Examples/python/reference/

C++ Reference Handling

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This example tests SWIG's handling of C++ references. Since C++ references are closely related to pointers (as both refer to a location in memory), SWIG simply collapses all references into pointers when creating wrappers.

Some examples

References are most commonly used as function parameter. For example, you might have an operator like this:

Vector operator+(const Vector &a, const Vector &b) {
   Vector result;
   result.x = a.x + b.x;
   result.y = a.y + b.y;
   result.z = a.z + b.z;
   return result;
}

or a function:

Vector addv(const Vector &a, const Vector &b) {
   Vector result;
   result.x = a.x + b.x;
   result.y = a.y + b.y;
   result.z = a.z + b.z;
   return result;
}

In these cases, SWIG transforms everything into a pointer and creates a wrapper that looks like this:

Vector wrap_addv(Vector *a, Vector *b) {
    return addv(*a,*b);
}

Occasionally, a reference is used as a return value of a function when the return result is to be used as an lvalue in an expression. The prototypical example is an operator like this:

Vector &operator[](int index);

or a method:

Vector &get(int index);

For functions returning references, a wrapper like this is created:

Vector *wrap_Object_get(Object *self, int index) {
    Vector &result = self->get(index);
    return &result;
}

The following header file contains some class definitions with some operators and use of references.

SWIG Interface

SWIG does NOT support overloaded operators so it can not directly build an interface to the classes in the above file. However, a number of workarounds can be made. For example, an overloaded operator can be stuck behind a function call such as the addv() function above. Array access can be handled with a pair of set/get functions like this:

class VectorArray {
public:
 ...
   %addmethods {
    Vector &get(int index) {
      return (*self)[index];
    }
    void set(int index, Vector &a) {
      (*self)[index] = a;
    }
   }
   ...
}

Click here to see a SWIG interface file with these additions.

Sample Python script

Click here to see a script that manipulates some C++ references.

Notes:

C++ references primarily provide notational convenience for C++ source code. However, Python only supports the 'x.a' notation so it doesn't much matter.
When a program returns a reference, a pointer is returned. Unlike return by value, memory is not allocated to hold the return result.
SWIG has particular trouble handling various combinations of references and pointers. This is side effect of an old parsing scheme and type representation that will be replaced in future versions.