Can the result of a function call be used as a default parameter value?

Question

Is there a good method for writing C / C++ function headers with default parameters that are function calls?

I have some header with the function:

int foo(int x, int y = 0);

I am working in a large code base where many functions call this function and depend on this default value. This default value now needs to change to something dynamic and I am looking for a way to do:

int foo(int x, int y = bar());

Where bar() is some function that generates the default value based on some system parameters. Alternatively this function prototype would look like:

int foo(int x, int y = baz.bar());

Where baz is a function belonging to an object that has not been instantiated within the header file.

Answer 1

Go figure! It does work. Default arguments in C++ functions

Answer 2

I would use two overloaded functions:

int foo(int x, int y);

int foo(int x){return foo(x,bar);}

If you allow the forwarding function to be inlined, then the performance penalty is likely to small to zero. If you keep the body of it out of line in a non-header file there may be a performance cost (likely to be small), but much more flexibility in implementation and reduced coupling.