Posted on

Objective-C++: mixing lambdas and blocks

ObjC++ is great and it’s possibilities to shoot one’s legs are countless. One of them is returning C++ objects by value from ObjC methods. In case of normal program flow that works just fine. But in case of calling methods of nil objects – they return… just garbage, i.e. the stack memory gets allocated but never touched afterwards. No constructors/destructors are called and you’ll get a C++ object in an undefined state with some garbage inside:

@implementation Foo
– (std::string) tellTheTruth
{
    return “42”;
}
[…]
 
void A( Foo *_foo )
{
    auto s = [_foo tellTheTruth]; // _foo != nil: ok, fine  
    std::cout << s << std::end;   // _foo == nil: good luck!
}

This issue might also cause some fun with debugging when C++ lambdas are mixed with ObjC blocks:

void A(std::function<void()> a)
{
    if(a) a();
}

void B(void (^a)())
{
    if(a) a();
}

int main(int argc, const char * argv[])
{
    A( []{} );                // <- ok
    A( nullptr );             // <- ok
    A( ^{} );                 // <- ok
    B( ^{} );                 // <- ok
    B( (void(^)(void)) nil ); // <- ok
    A( (void(^)(void)) nil ); // <- crash

    return 0;
}

Happy debugging!