Default Arguments

| Default Arguments

Boost.Python wraps (member) function pointers. Unfortunately, C++ function pointers carry no default argument info. Take a function `f` with default arguments:

`

        int f(int, double = 3.14, char const* = "hello");

`

But the type of a pointer to the function `f` has no information about its default arguments:

`

        int(*g)(int,double,char const*) = f;    // defaults lost!

`

When we pass this function pointer to the `def` function, there is no way to retrieve the default arguments:

`

        def("f", f);                            // defaults lost!

`

Because of this, when wrapping C++ code, we had to resort to manual wrapping as outlined in the previous section, or writing thin wrappers:

`

        // write "thin wrappers"
        int f1(int x) { f(x); }
        int f2(int x, double y) { f(x,y); }

        /*...*/

            // in module init
            def("f", f);  // all arguments
            def("f", f2); // two arguments
            def("f", f1); // one argument

`

When you want to wrap functions (or member functions) that either:

- have default arguments, or
- are overloaded with a common sequence of initial arguments

## BOOST_PYTHON_FUNCTION_OVERLOADS

Boost.Python now has a way to make it easier. For instance, given a function:

`

        int foo(int a, char b = 1, unsigned c = 2, double d = 3)
        {
            /*...*/
        }

`

The macro invocation:

`

        BOOST_PYTHON_FUNCTION_OVERLOADS(foo_overloads, foo, 1, 4)

`

will automatically create the thin wrappers for us. This macro will create a class `foo_overloads` that can be passed on to `def(...)`. The third and fourth macro argument are the minimum arguments and maximum arguments, respectively. In our `foo` function the minimum number of arguments is 1 and the maximum number of arguments is 4. The `def(...)` function will automatically add all the foo variants for us:

`

        def("foo", foo, foo_overloads());

`

## BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS

Objects here, objects there, objects here there everywhere. More frequently than anything else, we need to expose member functions of our classes to Python. Then again, we have the same inconveniences as before when default arguments or overloads with a common sequence of initial arguments come into play. Another macro is provided to make this a breeze.

Like `BOOST_PYTHON_FUNCTION_OVERLOADS`, `BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS` may be used to automatically create the thin wrappers for wrapping member functions. Let's have an example:

`

        struct george
        {
            void
            wack_em(int a, int b = 0, char c = 'x')
            {
                /*...*/
            }
        };

`

The macro invocation:

`

        BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS(george_overloads, wack_em, 1, 3)

`

will generate a set of thin wrappers for george's `wack_em` member function accepting a minimum of 1 and a maximum of 3 arguments (i.e. the third and fourth macro argument). The thin wrappers are all enclosed in a class named `george_overloads` that can then be used as an argument to `def(...)`:

`

        .def("wack_em", &george::wack_em, george_overloads());

`

See the overloads reference for details.

## init and optional

A similar facility is provided for class constructors, again, with default arguments or a sequence of overloads. Remember `init<...>`? For example, given a class X with a constructor:

`

        struct X
        {
            X(int a, char b = 'D', std::string c = "constructor", double d = 0.0);
            /*...*/
        }

`

You can easily add this constructor to Boost.Python in one shot:

`

        .def(init<int, optional<char, std::string, double> >())

`

Notice the use of `init<...>` and `optional<...>` to signify the default (optional arguments).

Copyright © 2002-2003 David Abrahams
Copyright © 2002-2003 Joel de Guzman
Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.