Cast lambda to std::function with parameter pack

Question

There are several questions on SO that relate to casting lambdas to std::functions, but I have yet to see one that uses a parameter pack for the argument list.

Answer 1

This fails:

#include 

template 
void Functor(std::function f) {}

int main(int argc, char * argv[]) {
    auto x = [] (int a, int b) { return a * b; };
    Functor(x);
    return 0;
}

because you're not specifying that the entirety of TArgs... is {int, int}. What you are doing is specifying that the first two types are {int, int}. Effectively, by providing those three types, we've turned the deduction problem into:

template 
void Functor(std::function f) {}

int main(int argc, char * argv[]) {
    auto x = [] (int a, int b) { return a * b; };
    Functor(x);
    return 0;
}

This doesn't compile because a lambda isn't a std::function (or derived from one), which is the same reason you couldn't have called this without having provided any types to begin with.

The non-variadic version doesn't have this problem, since you've provided all the types.

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But really, what you want is:

template 
void Functor(F ) {}

This doesn't lose you any type safety. It's using std::function that loses type information, since that class template exists to type erase.