Forward declaration of lambdas in C++

Question

In C++, it is possible to separate the declaration and definition of functions. For example, it is quite normal to declare a function:

int Foo(int x);

in Foo.h and implement it in Foo.cpp. Is it possible to do something similar with lambdas? For example, define a

std::function<int(int)> bar;

in bar.h and implement it in bar.cpp as:

std::function<int(int)> bar = [](int n)
{
    if (n >= 5) 
        return n;
    return n*(n + 1);
};

Disclaimer: I have experience with lambdas in C#, but I have not used them in C++ very much.

Answer 1

You can't separate declaration and definition of lambdas, neither forward declare it. Its type is a unique unnamed closure type which is declared with the lambda expression. But you could do that with std::function objects, which is designed to be able to store any callable target, including lambdas.

As your sample code shown you've been using std::function, just note that for this case bar is a global variable indeed, and you need to use extern in header file to make it a declaration (not a definition).

// bar.h
extern std::function<int(int)> bar;     // declaration

and

// bar.cpp
std::function<int(int)> bar = [](int n) // definition
{
    if (n >= 5) return n;
    return n*(n + 1);
};

Note again that this is not separate declaration and definition of lambda; It's just separate declaration and definition of a global variable bar with type std::function<int(int)>, which is initialized from a lambda expression.