Difference between decltype and typeof?

Question

Two question regarding decltype and typeof:

• Is there any difference between the decltype and typeof operato

Answer 1

The difference between the two is that decltype always preserves references as part of the information, whereas typeof may not. So...

int a = 1;
int& ra = a;
typeof(a) b = 1;     // int
typeof(ra) b2 = 1;   // int
decltype(a) b3;      // int
decltype(ra) b4 = a; // reference to int

The name typeof was the preferred choice (consistent with sizeof and alignof, and the name already used in extensions), but as you can see in the proposal N1478, concern around compatibility with existing implementations dropping references led them to giving it a distinct name.

"We use the operator name typeof when referring to the mechanism for querying a type of an expression in general. The decltype operator refers to the proposed variant of typeof. ... Some compiler vendors (EDG, Metrowerks, GCC) provide a typeof operator as an extension with reference-dropping semantics. As described in Section 4, this appears to be ideal for expressing the type of variables. On the other hand, the reference-dropping semantics fails to provide a mechanism for exactly expressing the return types of generic functions ... In this proposal, the semantics of the operator that provides information of the type of expressions reflects the declared type. Therefore, we propose the operator to be named decltype."

J. Jarvi, B. Stroustrup, D. Gregor, J. Siek: Decltype and auto. N1478/03-0061.

So it's incorrect to say that decltype completely obviated typeof (if you want reference dropping semantics, then the typeof extension in those compilers still has use), but rather, typeof was largely obviated by it plus auto, which does drop references and replaces uses where typeof was used for variable inference.