C++ explicit template specialization of templated constructor of templated class

问题

I have a class like

template <class T>
struct A{
    template <class U>
    A(U u);
};

I would like to write an explicit specialization of this for a declaration like

A<int>::A(float);

In the following test code, if I comment out the specialization, it compiles with g++. Otherwise, it says I have the wrong number of template parameters:

#include <iostream>

template <class T>
struct A{
    template <class U>
    A(T t, U *u){
        *u += U(t);
    }
};

template <>
template <>
A<int>::A<int,float>(int t, float *u){
    *u += float(2*t);
}

int main(){
    float f = 0;
    int i = 1;
    A<int>(i, &f);
    std::cout << f << std::endl;
    return 0;
}

回答1:

Try

template <>
template <>
A<int>::A(int t, float *u){
     *u += float(2*t);
}

That seems to work for me.

回答2:

The definition's function parameter list should match the declaration's.

template <>
template <>
A<int>::A<float>(int t, float *u){
    *u += U(2*t);
}

来源：https://stackoverflow.com/questions/2563165/c-explicit-template-specialization-of-templated-constructor-of-templated-class