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

The argument K in const typename A<K>::X is not deducible. Basically, everything left of a :: is not deducible (if :: separates a nested-name).

It's trivial to see why it makes no sense to ask for deduction by running through this thought experiment:

struct A { typedef int type; }
struct B { typedef int type; }

template <typename T> void foo(typename T::type);

foo(5);   // is T == A or T == B ??

There's no one-to-one mapping from types to nested types: Given any type (such as int), there could be many ambient types of which it is a nested type, or there needn't be any.

Answer 2

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

template<class K>
struct Y {
    K q;
};

template<class K>
struct A {
    X<K> x;
    Y<K> y;
};

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

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

Answer 3

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

K cannot been deduced, since it's in non-deduced context.

n3337 14.8.2.5/4

In certain contexts, however, the value does not participate in type deduction, but instead uses the values of template arguments that were either deduced elsewhere or explicitly specified. If a template parameter is used only in non-deduced contexts and is not explicitly specified, template argument deduction fails.

n3337 14.8.2.5/5

The non-deduced contexts are:

— The nested-name-specifier of a type that was specified using a qualified-id.