Forwarding Reference is a new feature in C++11.
They are also called as Universal References. It means, it will binds to both lvalues and rvalues. We will see in example further below.
Forwarding reference uses the syntax “T&&” or “auto&&”.
What is the difference between rvalue reference and forward reference ?
rvalue reference looks like “int &&”.
Forward Reference looks like “auto&&” or “T&&”.
So if we use template paramter type or “auto”, then it is a forward reference.
Let us understand both with an example:
We have taken 2 functions, “multiply()” “add()“.
multiply( int&& x ) properties:
1. It is taking a rvalue reference to a non-const as input.
2. x argument has to be always rvalue.
3. x argument is capturing temp values.
template <typename T>
add( T&& x ) properties:
1. “add()” takes lvalue or rvalue reference to const, volatile or both.
2. T argument can be of any type.
3. T argument is forwarding the arguments.
Examples:
int var = 20 // var is alvalue of type int.
multiply(10); // works as multiply accepts rvalue type.
multiply(var); // error. as multiply (int&) is not ok. Because var should be of rvalue type.
add(10); // this is ok
add (var); // this is ok, as var can be of rvalur or lvalue.