This example just illustrates how this keyword can be used.

int a = 10;

// Assume that type of variable 'a' is not known here, or it may
// be changed by programmer (from int to long long, for example).
// Hence we declare another variable, 'b' of the same type using 
// decltype keyword.
decltype(a) b; // 'decltype(a)' evaluates to 'int'

If, for example, someone changes, type of 'a' to:

float a=99.0f;

Then the type of variable b now automatically becomes float.

Let's say we have vector:

std::vector<int> intVector;

And we want to declare an iterator for this vector. An obvious idea is to use auto. However, it may be needed just declare an iterator variable (and not to assign it to anything). We would do:

vector<int>::iterator iter;

However, with decltype it becomes easy and less error prone (if type of intVector changes).

decltype(intVector)::iterator iter;

Alternatively:

decltype(intVector.begin()) iter;

In second example, the return type of begin is used to determine the actual type, which is vector<int>::iterator.

If we need a const_iterator, we just need to use cbegin:

decltype(intVector.cbegin()) iter; // vector<int>::const_iterator