Boolean lambdas?

Question

How come this code compiles???

#include <iostream>


int main() {
    auto lambda1 = []{};
    auto lambda2 = []{};

    if(lambda1 && lambda2) {
        std::cout << "BOOLEAN LAMBDAS!!!" << std::endl;
    }

    if(lambda1 || lambda2) {
        std::cout << "BOOLEAN LAMBDAS AGAIN FTW!!!" << std::endl;
    }

    bool b1 = lambda1;
    bool b2 = lambda2;

    std::cout << b1 << ", " << b2 << std::endl;
}

Boolean lambdas! (Or boolambdas, if you will... *shies away*)

How come this works? Is this another GCC bug? If not, is this standard?

@SingerOfTheFall Well, I see lambdas and booleans as two totally different things. — Mark Garcia, Sep 19 '13 at 08:44
will `int * t = new int(0); int * t1 = new int(0); bool b1 = t; bool b2 = t1; std::cout << b1 << b2;` also be surprising then?) — SingerOfTheFall, Sep 19 '13 at 08:46
@SingerOfTheFall Not like that between lambdas and booleans. `:)` — Mark Garcia, Sep 19 '13 at 08:47
Is there now going to be a storm of self-answered questions about lambda's conversion to function pointer? "Whaaaat? I can compare a lambda to a zero?? What does `&*[]{}` mean?" and so on? — avakar, Sep 19 '13 at 08:53
@avakar You foresaw something great! But let's not talk too much about it... they might hear and make your prediction come true... — Mark Garcia, Sep 19 '13 at 08:55
@avakar: Just wait until they [combine them with digraphs](http://stackoverflow.com/q/15736282/500104)... — Xeo, Sep 19 '13 at 10:05

score 7 · Accepted Answer · edited May 23 '17 at 12:29

It turns out that it is standard!

If you refer to this answer^[1], non-capturing lambdas are convertible to function pointers. And it turns out again that function pointers, being pointers themselves, are implicitly convertible to bool!

4.12 Boolean conversions [conv.bool]

¹ A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true. A prvalue of type std::nullptr_t can be converted to a prvalue of type bool; the resulting value is false.

To give a supporting proof that the conversion to function pointer is what makes all of this happen, I've tried doing the same thing with capturing lambdas. Then "can't convert to bool" errors are generated.

LIVE CODE

int main() {
    int i;
    auto lambda = [i]{};

    bool b = lambda;

    if(lambda) {}
}

^[1] _{Which, honestly, gave me the idea to write this.}

Isn't the point of asking a question... to expect an answer from other people? — Ophir Gvirtzer, May 11 '14 at 11:18

Boolean lambdas?

1 Answers1

4.12 Boolean conversions [conv.bool]

Linked

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