Candidate template ignored because template argument could not be inferred

Question

What is wrong with the following piece of code?

#include 

template
struct A {
    struct X { K p; };
    struct Y { K q; }         


        

Answer 1

Firstly, using :: for referencing a nested struct is not correct. The right way is to use A.x, where x is a member of type X. But for this to work, you need to declare two members x, y of type X and Y, respectively.

Secondly, I think that a good practice is to separate the template declarations of struct X and struct Y from the declaration of struct A in order to avoid the nested struct declaration.

In short, I would rewrite your code as followings:

template
struct X {
    K p;
};

template
struct Y {
    K q;
};

template
struct A {
    X x;
    Y y;
};

template
void foo(const X& x, const Y& y) {
    std::cout << "A" << std::endl;
}

int main() {
    A a;
    foo(a.x, a.y);
    return 0;
}