How can I store a lambda expression as a field of a class in C++11?

Question

I\'d like to create a class where the client can store a lambda expression like []() -> void {} as a field of the class, but I can\'t figure out how to do so

Answer 1

In case of void () simple function pointer can be used (declaration syntax is void (*pointer_name) ();). Lambda with empty capture block is implicitly convertible to function pointer with same signature. And std::function have runtime and size (std::function is at least three times larger) overhead.

struct S
{
  void (*f_p)() {}; // `{}` means `= nullptr`;
};

int main()
{
    S s { [] { std::cout << "Lambda called\n"; }};

    s.f_p();

    S s2;
    if (s2.f_p) // check for null
        s.f_p();

    s2.f_p = [] { std::cout << "Lambda2 called\n"; };
    s2.f_p();

    s2.f_p = std::terminate; // you can use regular functions too

    s2.f_p();
}

Output

Lambda called
Lambda2 called
terminate called without an active exception

Answer 2

If you want a class member to be a lambda expression, consider using the std::function<> wrapper type (from the <functional> header), which can hold any callable function. For example:

std::function<int()> myFunction = []() { return 0; }
myFunction(); // Returns 0;

This way, you don't need to know the type of the lambda expression. You can just store a std::function<> of the appropriate function type, and the template system will handle all the types for you. More generally, any callable entity of the appropriate signature can be assigned to a std::function<>, even if the the actual type of that functor is anonymous (in the case of lambdas) or really complicated.

The type inside of the std::function template should be the function type corresponding to the function you'd like to store. So, for example, to store a function that takes in two ints and returns void, you'd make a std::function<void (int, int)>. For a function that takes no parameters and returns an int, you'd use std::function<int()>. In your case, since you want a function that takes no parameters and returns void, you'd want something like this:

class MyClass { 
public:
    std::function<void()> function;
    MyClass(std::function<void()> f) : function(f) {
        // Handled in initializer list
    }
};

int main() {
    MyClass([] {
        printf("hi")
    }) mc; // Should be just fine.
}

Hope this helps!

Answer 3

The only way I can think of to store a lambda in a class is to use a template with a helper make_ function:

#include <cstdio>
#include <utility>

template<class Lambda>
class MyClass {
    Lambda _t;
public:
    MyClass(Lambda &&t) : _t(std::forward<Lambda>(t)) {
        _t();
    }
};

template<class Lambda>
MyClass<Lambda> make_myclass(Lambda &&t) {
    return { std::forward<Lambda>(t) };
}

int main() {
    make_myclass([] {
        printf("hi");
    });
}