passing lambda as argument - by reference or value

Question

I've written a template code that takes a functor as an argument and after some processing, executes it. Although someone else might pass that function a lambda, a function pointer or even an std::function but it is meant primarily for lambda(not that I ban other formats). I want to ask how should I take that lambda - by value? by reference? or something else.

Example code -

#include <iostream>
#include <functional>
using namespace std;

template<typename Functor>
void f(Functor functor)
{
    functor();
}

void g()
{
    cout << "Calling from Function\n";
}

int main() 
{
    int n = 5;

    f([](){cout << "Calling from Temp Lambda\n";});

    f([&](){cout << "Calling from Capturing Temp Lambda\n"; ++n;});

    auto l = [](){cout << "Calling from stored Lambda\n";};
    f(l);

    std::function<void()> funcSTD = []() { cout << "Calling from std::Function\n"; };
    f(funcSTD);

    f(g);
}

In above code, I've a choice of making it either of these -

template<typename Functor>
    void f(Functor functor)

template<typename Functor>
    void f(Functor &functor)

template<typename Functor>
    void f(Functor &&functor)

What would be the better way and why? Are there any limitations to any of these?

Answer 1

As a possible drawback, note that passing by copy could not work if the lambda isn't copyable. If you can get away with it, passing by copy is just fine.
As an example:

#include<memory>
#include<utility>

template<typename F>
void g(F &&f) {
    std::forward<F>(f)();
}

template<typename F>
void h(F f) {
    f();
}

int main() {
    auto lambda = [foo=std::make_unique<int>()](){};

    g(lambda);
    //h(lambda);
}

In the snippet above, lambda isn't copyable because of foo. Its copy constructor is deleted as a consequence of the fact that the copy constructor of a std::unique_ptr is deleted.
On the other side, F &&f accepts both lvalue and rvalue references being it a forwarding reference, as well as const references.
In other terms, if you want to reuse the same lambda as an argument more than once, you cannot if your functions get your object by copy and you must move it for it's not copyable (well, actually you can, it's a matter of wrapping it in a lambda that captures the outer one by reference).

Answer 2

As lambda expressions can have their own fields (like classes), copying / using reference can cause different results. Here's simple example:

template<class func_t>
size_t call_copy(func_t func) {
    return func(1);
}

template<class func_t>
size_t call_ref(func_t& func) {
    return func(1);
}

int main() {
    auto lambda = [a = size_t{0u}] (int val) mutable {
        return (a += val);
    };
    lambda(5);

    // call_ref(lambda); – uncomment to change result from 5 to 6
    call_copy(lambda);

    std::cout << lambda(0) << std::endl;

    return 0;
}

Btw if you want to judge it by performance there's actually no difference, lambdas are very small and easy to copy.

Also – if you want to pass lambda as parameter (not a variable that contains it) you should use forwarding reference so it could work.