非返回lambda，捕获为函数指针

Question

My code (C++) need to pass lambda as function pointer. My lambdas always: a) return void; b) never takes any parameter; and c) can have zero or more capture arguments. I want to pass this lambda as a function pointer but unable to do so. Any advice ?

Sample code:

void fptrfunc(void (*fptr)()){ 
  fptr();
}

int main(){
  int x = 2;
  fptrfunc([](){cout << "LAMBDA CALL--1"<< endl; });            // Works
  fptrfunc([x](){cout << "LAMBDA CALL--2"<< endl; });           // Does not compile
  return 0;
} 

Answer 1

Lambda functions which are really just functions, ie, don't carry and data, can be converted to function pointers. To not have any data clearly the capture has to be empty. The other parts of the signature do't matter: as long as the types match, you can assign a lambda function [with an empty capture] to a function pointer.

The moment you need to have data with your lambda function you won't be able to convert it to a function pointer because there is no place to store the additional data. If you have a sensible interface where you need to pass the lambda function you may have some user data being passed along, often a void* . You could a combination of an auxiliary function pointer and a lambda, probably wrapped by a suitable std::function<Sig> , pointed to by the user data to still call a lambda function.

Answer 2

What you want, is not possible in C++. Lambdas that capture variables are not convertible to a function pointer.

You need to do drop the requirement c) and only use lambdas without captures. Or you must change the declaration of fptrfunc . You can change the parameter type to std::function or make it a template that can accept any type of functor that implements the operator() .