使用std :: invoke调用模板化函数

Question

I'm trying to call a templated member function using std::invoke as in the code snipped below.

#include <functional>
#include <iostream>

struct Node
{
    template <typename T>
    T eval(T lhs, T rhs) { return lhs + rhs; }
};

int main(void)
{
    std::cout << std::invoke(&Node::eval<int>, 1, 2);

    return 0;
}

gcc 8.1 is giving me the error

no matching function for call to 'invoke(unresolved overloaded function type, int, int)'

which I think it means that the function template is not actually instantiated.

What am I missing to make the call possible?

Answer 1

The problem is that you try to call a member function with no objects. There are two solutions:

1. Mark eval as static .

2. Create a Node object and std::invoke the eval method on that object. It would look like this:

    int main() { Node n{}; std::cout << std::invoke(&Node::eval<int>, n, 1, 2); return 0; } 

Here, the n object is passed as a object for this pointer to eval method.

Answer 2

Two problems:

First, your function template takes one type parameter, not two.
Second, your function is not static , hence you can't use it without an object.

struct Node
{
    template <typename T>
    static T eval(T lhs, T rhs) { return lhs + rhs; }
    ^^^^^^
};

int main(void)
{
    std::cout << std::invoke(&Node::eval<int>, 1, 2);
                                         ^^^
    return 0;
}