Question

I have a template-structure that I want to “overload” like this:

#include <iostream>

template <typename T, typename U = int>
struct foo {
    void operator()(T, U);
}

template <typename T, typename U = int>
void foo::operator()(T a, U b){
    std::cout << "T, U ()\n";
}

template <typename T>
struct foo<T, int> {
    void operator()(T);
}

template <typename T>
void foo<T, int>::operator()(T a){
    std::cout << "T ()\n";
}

int main(int argc, char **argv){
    foo<int> a;
    foo<int, char> b;

    a(1);
    b(2, 'b');

    return false;
}

But on compiling I get the following error:

($ g++ test.cpp -o test)
test.cpp:11:6: error: 'template<class T, class U> struct foo' used without template parameters
test.cpp:11:30: error: 'void operator()(T, U)' must be a nonstatic member function

This is strange since the definition of foo< T, int >::operator() seems to work perfectly. Also, when I define the function inline like this:

template <typename T, typename U = int>
struct foo {
    void operator()(T a, U b){ std::cout << "T, U ()\n"; }
}

It works without problems.

Answer 1

You have to use those template parameters to specify which foo, foo<T,U>::operator(). And remove the default template parameter value from the definition.

template <typename T, typename U>    // don't use a default parameter
void foo<T,U>::operator()(T a, U b){ // don't forget the <T,U> here
    std::cout << "T, U ()\n";
}

You also forgot the semicolons after the template class definitions.