std::invoke of a function template/lambda fails

June 29, 2023

How can I get this working? (C++17/20)
Note: I have to use std::invoke here because in the real code, the only guarantee I have is that this callable is invokable.

#include <bits/stdc++.h> // to include everything

int main() {
    const auto func = [&]<typename T>(const std::string& h) {
        T t;
        std::cout << t << " " << h << "\n";
    };
    std::invoke(func<std::string>, std::string("Hello"));
}

Error:

<source>:17:33: error: expected primary-expression before '>' token
   17 |     std::invoke(func<std::string>, std::string("Hello"));

>Solution :

As mentioned in the comment func is not a function template, but rather has a templated operator().

One way to use std::invoke the way you attempted, would be to actually change func into a function template:

#include <string>
#include <iostream>
#include <functional>

template <typename T>
void func(const std::string& h)
{
    T t;
    std::cout << t << " " << h << "\n";
}

int main() 
{
    std::invoke(func<std::string>, std::string("Hello"));
}

Live demo – Godbolt

A side note: better to avoid #include <bits/stdc++.h>.