Question

Suppose I have the following function, the value it produces is needed by many other functions in the code:

float mean(foo param1, bar param2);

My other functions look like this:

float foobar(foo param1, bar param2, float meanValue);

What I want to do is the following:

float foobar(foo param1, bar param2, float meanValue = mean(param1, param2));

While passing in a function as a default parameter is legal, the compiler (VS2008) complains that param1 and param2 are not declared – because the parameters get pushed onto the stack right-to-left, starting with meanValue. Of course, I can’t reverse the order of parameters, since in that case I wouldn’t be able to specify a default value for the first parameter.

I could pass in some value like -10000 for the mean to tell myself to call the mean() function; or I could overload every function (and there are 10+ to overload), but that’s not very neat. I think it’s really cool that you can call a function as a default parameter and I’m wondering if there’s a neat way to accomplish what I’m trying to do.

Answer 1

In which order parameters are pushed to the stack, or whether they are pushed at all, or whether there even is a stack on the architecture, is not specified by the C++ standard.

You don’t give much information about why you have so many such functions, and how they differ, but maybe the following would help reduce the number of overloads:

template< typename T1, typename T2 >
float foobar(T1 param1, T2 param2)
{
    return foobar(param1, param2, mean(param1, param2));
}

Note that you can make the result type and the function called, and the mean() function, all template parameters, too, if you need to.