Error on implicit cast from std:unique_ptr to bool

Question

I am using Allegro to create a simple game. When I try to validate that my pointer to the display is not null I get a compiler error telling me

error C2664: 'void validate(bool,std::string)' : cannot convert argument 1 from 'std::unique_ptr< ALLEGRO_DISPLAY,main::< lambda_996846ce92067e506da99cad36e610cf>>' to 'bool'

Here is my code

#include <iostream>
#include <memory>
#include <string>

#include <allegro5\allegro.h>

using namespace std;

const int WIDTH = 512;
const int HEIGHT = 512;

void validate(bool ptr, string errorMessage) {
    if (!ptr) {
        cerr << errorMessage << endl;
        exit(-1);
    }
}

int main() {
    auto deleter = [](ALLEGRO_DISPLAY* d) { al_destroy_display(d); };
    unique_ptr<ALLEGRO_DISPLAY, decltype(deleter)> display;

    validate(al_init(), "Failed to initialize Allegro");
    display = unique_ptr<ALLEGRO_DISPLAY, decltype(deleter)>(al_create_display(WIDTH, HEIGHT), deleter);
    validate(display, "Failed to create display");

    return 0;
}

If I pass validate "!display" instead of "display" it works. I realize I could call validate with display.get(), but I wanted to know why it isn't working when I pass a smart pointer.

I found this bug report. I am using Visual Studio 2013. https://connect.microsoft.com/VisualStudio/feedbackdetail/view/775810/c-11-std-unique-ptr-cast-to-bool-fails-with-deleter-lambda

Answer 1

std::unique_ptr is not implicitly convertible to bool. It is contextually convertible to bool (due to its explicit conversion operator), which is why you can use it in an if statement, or put a ! in front of it, but you cannot pass it as an argument to a function which is expecting a bool.