Question

I’m trying to pass a temporary object to another object constructor for the second object to take ownership of the generated object. My code is something like this

class A {
};

class B {
    A a;
    public:
        B(A && _a) : a(_a) {}

        void test(){ }
};

int main(int argc, const char *argv[])
{
    B b(A());
    b.test();
    return 0;
}

but I’m getting this error that I’m not able to understand

$ g++ -std=c++0x main.cpp  
main.cpp: In function 'int main(int, const char**)': 
main.cpp:15:7: error: request for member 'test' in 'b', which is of non-class type 'B(A (*)())'

Perhaps it is just a silly syntax error, but in case it is not, how would you defined such constructor for taking ownership of some created resource?

Thanks

Answer 1

You got caught by what is known as C++’s most vexing parse. Your line

B b(A());

does not define an object b of type B initialized with a temporary object of type A, but declares a function named b taking an unnamed argument of type (pointer to a) function with no arguments and returning A (thus the type of b‘s argument is declared as the function type A() and decays to the pointer-to-function type A(*)() which you can see in the error message), and returning a B.

The simplest way around is to change that line to

B b = B(A());

In C++11, the best option is to use the new initializer syntax and write

B b { A() };