Why Can't I store references in a `std::map` in C++?

Question

I understand that references are not pointers, but an alias to an object. However, I still don\'t understand what exactly this means to me as a programmer, i.e. what are re

Answer 1

You should think of a reference as a 'const pointer to a non-const object':

MyObject& ~~ MyObject * const

Furthermore, a reference can only be built as an alias of something which exists (which is not necessary for a pointer, though advisable apart from NULL). This does not guarantee that the object will stay around (and indeed you might have a core when accessing an object through a reference if it is no more), consider this code:

// Falsifying a reference
MyObject& firstProblem = *((MyObject*)0);
firstProblem.do(); // undefined behavior

// Referencing something that exists no more
MyObject* anObject = new MyObject;
MyObject& secondProblem = *anObject;
delete anObject;
secondProblem.do(); // undefined behavior

Now, there are two requirements for a STL container:

• T must be default constructible (a reference is not)
• T must be assignable (you cannot reset a reference, though you can assign to its referee)

So, in STL containers, you have to use proxys or pointers.

Now, using pointers might prove problematic for memory handling, so you may have to:

• use smart pointers (boost::shared_ptr for example)
• use a specialized container: Boost Pointer Container Library

DO NOT use auto_ptr, there is a problem with assignment since it modifies the right hand operand.

Hope it helps :)