Question

what is wrong with this code, why do I get wrong answer:

class X
{
private:
        const int a;
        const int& b;
public:
        X(): a(10) , b(20)
        {
        //      std::cout << "constructor : a " << a << std::endl;
        //      std::cout << "constructor : b " << b << std::endl;
        }

        void display()
        {
            std::cout << "display():a:" << a << std::endl;
            std::cout << "display():b:" << b << std::endl;

        }
};


int
main(void)
{
        X x;
        x.display();
return 0;
}

The above code will give me the result as

display():a:10
display():b:1104441332

But If I remove the commented 2 lines inside the default constructor it gives me proper result which is

constructor : a 10
constructor : b 20
display():a:10
display():b:20

please help, Thank you

Answer 1

You are initializing b as a reference to a temporary.

The value 20 is created and exists only for the scope of the constructor.

The behavior of the code after this is very interesting – on my machine, I get different values from the ones you posted, but the fundamental behavior is still nondeterministic.

This is because when the value to which the reference points falls out of scope, it begins to reference garbage memory instead, giving unpredictable behavior.

See Does a const reference prolong the life of a temporary?; the answer https://stackoverflow.com/a/2784304/383402 links to the relevant section of the C++ standard, specifically the below text:

A temporary bound to a reference member in a constructor’s ctor-initializer
(12.6.2) persists until the constructor exits.

This is why you always get the right value in the print within the constructor, and rarely (but possibly sometimes!) after. When the constructor exits, the reference dangles and all bets are off.