CODEWITHSUNDEEP
c++lambda
Zebrafish
Zebrafish

Reputation: 13936

Is there a way this lambda can capture a copy of the pointer?

In a lambda I'd like to capture a copy of a pointer, but can't because it's a static variable (can't capture it):

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

class Window
{public:
    Window(std::string name) : name(name) {}
    std::function<void(int)> eventHandlerFunction;
    std::string name;
};

Window* ptr;

int main()
{
    Window* wnd1 = new Window("wnd1");
    Window* wnd2 = new Window("wnd2");
    ptr = wnd1;

    wnd1->eventHandlerFunction = [=](int n) { std::cout << ptr->name; }; 
// I'd like to capture a copy of the ptr value to wnd1 through the ptr variable

   // BECAUSE THEN
    ptr = wnd2;

    // SOME TIME DOWN THE TRACK THE MOUSE CURSOR ROLLS OVER wnd1, calls eventHandlerFunction
    // and prints "wnd2" instead of "wnd1".
}

Upvotes: 3

Views: 167

Answers (1)

Ted Lyngmo
Ted Lyngmo

Reputation: 117318

You can copy the current value of ptr with the lambda syntax [local_copy=variable_to_copy_from]:

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

class Window {
public:
    Window(std::string name) : name(name) {}
    std::function<void(int)> eventHandlerFunction;
    std::string name;
};

Window* ptr;

auto get_ev_handler() {
    return [ptr=ptr](int n) { std::cout << n << ' ' << ptr->name << '\n'; };

    // You can name it something else if you'd like:
    //return [a_copy=ptr](int n) { std::cout << n << ' ' << a_copy->name << '\n'; };
}

int main() {
    Window* wnd1 = new Window("wnd1");
    Window* wnd2 = new Window("wnd2");

    ptr = wnd1;
    wnd1->eventHandlerFunction = get_ev_handler();
    ptr = wnd2;
    wnd2->eventHandlerFunction = get_ev_handler();
    ptr = nullptr;

    // here ptr == nullptr, but these still hold copies of the old values:
    wnd1->eventHandlerFunction(1);
    wnd2->eventHandlerFunction(2);
}

Upvotes: 6

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