In languages like C# and Java there is no need to declare (for example) a class before using it. If I understand it correctly this is because the compiler does two passes on the code. In the first it just "collects the information available" and in the second one it checks that the code is correct.

In C and C++ the compiler does only one pass so everything needs to be available at that time.

So my question basically is why isn't it done this way in C and C++. Wouldn't it eliminate the needs for header files?