Use a lambda as a parameter for a C++ function

Question

I would like to use a lambda as a parameter for a C++ function, but I don't know which type to specify in the function declaration. What I would like to do is this:

void myFunction(WhatToPutHere lambda){     //some things } 

I have tried void myFunction(auto lambda) and void myFunction(void lambda) but none of these codes compiled. In case it matters, the lambda doesn't return anything.

How can I use a lambda as a parameter in a C++ function?

Answer 1

You have 2 ways: make your function template:

template <typename F> void myFunction(F&& lambda) {     //some things } 

or erase type (with std::function for example):

void myFunction(const std::function<void()/*type of your lamdba::operator()*/>& f) {     //some things } 

Answer 2

You have two choices, basically.

Make it a template:

template<typename T> void myFunction(T&& lambda){ } 

or, if you do not want (or can't) do that, you can use type-erased std::function:

void myFunction(std::function<void()> const& lambda){ } 

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Conversely, your attempt with auto would've been correct under the concepts TS as currently implemented in gcc, where it'd be an abbreviated template.

// hypothetical C++2x code void myFunction(auto&& lambda){ } 

or with a concept:

// hypothetical C++2x code void myFunction(Callable&& lambda){ }