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c++function-pointersc++17pointer-to-member
Petok Lorand
Petok Lorand

Reputation: 973

Using std::invoke to call templated function

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?

Upvotes: 2

Views: 1094

Answers (2)

Fureeish
Fureeish

Reputation: 13434

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.

Upvotes: 3

Ivaylo Valchev
Ivaylo Valchev

Reputation: 10425

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;
}

Upvotes: 3

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