Nested lambda function

Question

I have the following code:

auto get_functor = [&](const bool check) {
    return  [&](const foo& sr)->std::string {
        if(check){
            return "some string";
        }
        return "another string"
    };
};
run(get_functor(true));

The run function signature:

void run(std::function<std::string(const foo&)> func);

I am getting the following error which is not so clear for me:

error C2440: 'return' : cannot convert from 'main::<lambda_6dfbb1b8dd2e41e1b6346f813f9f01b5>::()::<lambda_59843813fe4576d583c9b2877d7a35a7>' to 'std::string (__cdecl *)(const foo &)'

P.S. I am on MSVS 2013

Edit:

if I edit the code by replacing auto with the real type:

std::function<std::string(const foo&)> get_functor1 = [&](const bool check) {
    return  [&](const foo& sr)->std::string {
        if (check) {
            return "some string";
        }
        return "another string";
    };
};
run(get_functor1(true));

I am getting another error:

error C2664: 'std::string std::_Func_class<_Ret,const foo &>::operator
()(const foo &) const' : cannot convert argument 1 from 'bool' to
'const foo &'

Which is totally messed up!

Answer 1

I was able to reproduce the same error on VS 2013 with the following MVCE:

#include <iostream>
#include <functional>
#include <string>

struct foo {};

std::string run(std::function<std::string(foo const&)> f) {
    return f(foo());
}

int main() {

    auto get_functor = [&](bool const check) {
        return [=](foo const&) -> std::string { // Line of the compiler error
            if (check) {
                return "CHECK!";
            }
            else {
                return "NOT CHECK!";
            }
        };
    };

    std::cout << run(std::function<std::string(foo const&)>(get_functor(true)));
    return 0;
}

I then get the error:

Error   1   error C2440: 'return' : cannot convert from 'main::<lambda_1bc0a1ec72ce6dc00f36e05599609bf6>::()::<lambda_4e0981efe0d720bad902313b44329b79>' to 'std::string (__cdecl *)(const foo &)'

The problem lies with MSVC's inability to handle returned lambdas: when you do not specify the return type, it is trying to decay it into a regular function pointer. This fails because your lambda does capture elements!

Moreover, your fix is wrong since std::function<std::string(foo const&)> is not the type of get_functor but rather the type you want to return from it.

Forcing the embedding into an std::function of the returned lambda directly in get_functor will resolve your issue:

auto get_functor = [&](bool const check) -> std::function<std::string(foo const&)> {
    return [=](foo const&) -> std::string {
        if (check) {
            return "some string";
        } else {
            return "another string";
        }
    };
};
std::cout << run(get_functor(true));