Question

While reading Wikipedia’s page on decltype, I was curious about the statement,

Its [decltype’s] primary intended use is in generic
programming, where it is often
difficult, or even impossible, to name
types that depend on template
parameters.

While I can understand the difficulty part of that statement, what is an example where there is a need to name a type that cannot be named under C++03?

EDIT: My point is that since everything in C++ has a declaration of types. Why would there ever be a case where it is impossible to name a type? Furthermore, aren’t trait classes designed to yield type informations? Could trait classes be an alternative to decltype?

Answer 1

The wikipedia page you link has a perfect example:

int& foo(int& i);
float foo(float& f);

template <class T> auto transparent_forwarder(T& t) −> decltype(foo(t)) {
  return foo(t);
}

Note that foo(int&) returns int& (a reference type) while foo(float&) returns float (a nonreference type). Without decltype, it’s impossible within the template to specify a type which represents “the return type of the function foo which takes an argument t of type T“.

In this example, it’s not a particular concrete type which is impossible to express — either int& or float are individually expressible — but a higher level generic class of types.

EDIT: and to answer your comment to another answer, this example is inexpressible in C++03. You cannot have a function template which will wrap any function T1 foo(T2) and match both argument and return type of the wrapped function.