Question

I am trying to use a function with a default argument as a function pointer template parameter:

template <void (*F)()>
class A {};

void foo1(int a = 0) {}
void foo2() {}

int main() 
{
    //A<foo1> a1;   <-- doesn't work
    A<foo2> a2;
}

The compiler error is:

main.cpp:7:7: error: could not convert template argument ‘foo1’ to ‘void (*)()’

Is there specific syntax for this to work? Or a specific language limitation? Otherwise, the alternative is to have two separate functions instead of a default parameter:

void foo1(int a) {}
void foo1() { foo1(0); }

Update
I understand that the signatures are different, but I’m wondering if there is a way to make this work conveniently without needing to modify all the functions with default parameters?

Answer 1

According to section 8.3.6 of the C++ standard,

If an expression is specified in a parameter declaration this expression is used as a default argument. Default arguments will be used in calls where trailing arguments are missing.

Since A<foo1> is not a call of the function, default arguments are ignored. In fact, they are ignored in all contexts except the calls of the function, for example

typedef void (*FFF)();
FFF x = foo1;

will not compile, and produce the same message that you get when trying to use foo1 as a template parameter:

error: invalid conversion from ‘void (*)(int)’ to ‘void (*)()’

This makes sense, because evaluating default arguments is a separate step in the invocation:

8.3.6.9: Default arguments will be evaluated each time the function is called.

The presence of default arguments does not alter the signature of your function. For example, you cannot use a single-argument function with a default argument to override a no-argument virtual member function.