free memory of c++ lambda when execute finished

January 17, 2022

I’m coding a network function on C++, the HTTP request in background thread, use lambda callback when receive HTTP data. But I don’t know how to release the lambda, hopes some help.

void foo()
{
    // the `func` must be a heap variable for asynchronously.
    auto func = new auto ([&](std::string response){
        printf("recv data: %s", response.c_str());
    });
    
    std::thread t([&]{
        sleep(2);   // simulate a HTTP request.
        std::string ret = "http result";
        (*func)(ret);
    });
    t.detach();
    
    // The foo function was finished. bug `func` lambda still in memory ?
}

int main()
{
    foo();
    getchar(); // simulate UI Event Loop.
    return 0;
}

>Solution :

You can capture lambda inside a lambda:

void foo()
{
    std::thread t(
        [func = [](std::string response) {
            printf("recv data: %s", response.c_str());
        }](){
        sleep(2);   // simulate a HTTP request.
        std::string ret = "http result";
        func(ret);
    });
    t.detach();
    // The foo function was finished. bug `func` lambda still in memory ?
}

or if it’s supposed to be shared, you can use shared ownership semantics via shared_ptr and then capture it into the lambda by value in order to increase its reference count:

void foo()
{
    auto lambda = [&](std::string response){
                printf("recv data: %s", response.c_str());
            };
    
    std::shared_ptr<decltype(lambda)> func{
        std::make_shared<decltype(lambda)>(std::move(lambda))
    };

    std::thread t([func]{
        sleep(2);   // simulate a HTTP request.
        std::string ret = "http result";
        (*func)(ret);
    });
    t.detach();
}

Of for non-capturing lambdas one can just turn it into a function pointer and don’t really care

void foo()
{
    auto func_{
        [](std::string response){
            printf("recv data: %s", response.c_str());
        }
    };
    
    std::thread t([func=+func_]{ //note the + to turn lambda into function pointer
        sleep(2);   // simulate a HTTP request.
        std::string ret = "http result";
        (*func)(ret);
    });
    t.detach();