Why do stream operators return references in C++?

Question

I know that indexing operator implementations usually return references so that the values can be set as well as retrieved, but why do streams?

Answer 1

So you can chain them together.

cout << "hello" << "how are you";

Works because cout << "hello" returns a reference to the cout so that << "how are you" knows there to put itself.

Most operators, such as +=, also do this.

Answer 2

Streams do not support copying or assignment, so anything that passes or returns a stream must use either a pointer or reference. You can't use overloaded operators on a pointer (without dereferencing it) because they'd try to apply the built-in operator to the pointer itself.

So, returning a reference is the only choice that supports operator chaining.