A Candidate For the “Most Important const".

"Warning Generated- Taking address of temporary."

Q1: Is the following code legal C++?

// Example 1
string f() { return "abc"; }
void g() {
const string& s = f();
  cout << s << endl;    // can we still use the "temporary" object?
}

Answer : Yes.

This is a C++ feature… the code is valid and does exactly what it appears to do.
Normally, a temporary object lasts only until the end of the full expression in which it appears. However, C++ deliberately specifies that binding a temporary object to a reference to const on the stack lengthens the lifetime of the temporary to the lifetime of the reference itself, and thus avoids what would otherwise be a common dangling-reference error. In the example above, the temporary returned by f() lives until the closing curly brace. (Note this only applies to stack-based references. It doesn’t work for references that are members of objects.)
Does this work in practice? Yes, it works on all compilers I tried (except Digital Mars 8.50, so I sent a bug report to Walter to rattle his cage :-) and he quickly fixed it for the Digital Mars 8.51.0 beta).

Q2: What if we take out the const… is Example 2 still legal C++?

// Example 2
string f() { return "abc"; }
void g() {
string& s = f();       // still legal?
  cout << s << endl;
}

Answer: No.

The "const" is important. The first line is an error and the code won’t compile portably with this reference to non-const, because f() returns a temporary object (i.e., rvalue) and only lvalues can be bound to references to non-const.
Note: Visual C++ does allow Example 2 but emits a "nonstandard extension used" warning by default. A conforming C++ compiler can always allow otherwise-illegal C++ code to compile and give it some meaning — hey, it could choose to allow inline COBOL if some kooky compiler writer was willing to implement that extension, maybe after a few too many Tequilas. For some kinds of extensions the C++ standard requires that the compiler at least emit some diagnostic to say that the code isn’t valid ISO C++, as this compiler does.

Full Article