Strange behaviour of << (as at least seems to me)

Question

I can't realize how could it be possible to print a string this way without any complaint by the compiler:

std::cout << "Hello " "World!";

In fact, the above line works exactly like:

std::cout << "Hello " << "World!";

Is there an explanation for this behaviour?

Answer 1

Adjacent literal tokens are concatenated automatically, it's part of the standard.

2.1 Phases of translation [lex.phases]

6) Adjacent ordinary string literal tokens are concatenated. Adjacent wide string literal tokens are concatenated.

(C++03)

Answer 2

In C++, literals tokens can be concatenated thusly:

const char* thingy = "Hello" "World";

"Hello" and "World" are each a literal token.