where should "include" be put in C++

While a header file should include only what it needs, "what it needs" is more fluid than you might think, and is dependent on the purpose to which you put the header. What I mean by this is that some headers are actually interface documents for libraries or other code. In those cases, the headers must include (and probably #include) everything another developer will need in order to correctly use your library.

The include files in a header should only be those necessary to support that header. For example, if your header declares a vector, you should include vector, but there's no reason to include string. You should be able to have an empty program that only includes that single header file and will compile.

Within the source code, you need includes for everything you call, of course. If none of your headers required iostream but you needed it for the actual source, it should be included separately.

Include file pollution is, in my opinion, one of the worst forms of code rot.

edit: Heh. Looks like the parser eats the > and < symbols.

You would make all other files including your header file transitively include all the #includes in your header too.

In C++ (as in C) #include is handled by the preprocessor by simply inserting all the text in the #included file in place of the #include statement. So with lots of #includes you can literally boast the size of your compilable file to hundreds of kilobytes - and the compiler needs to parse all this for every single file. Note that the same file included in different places must be reparsed again in every single place where it is #included! This can slow down the compilation to a crawl.

If you need to declare (but not define) things in your header, use forward declaration instead of #includes.

As a rule, put your includes in the .cpp files when you can, and only in the .h files when that is not possible.

You can use forward declarations to remove the need to include headers from other headers in many cases: this can help reduce compilation time which can become a big issue as your project grows. This is a good habit to get into early on because trying to sort it out at a later date (when its already a problem) can be a complete nightmare.

The exception to this rule is templated classes (or functions): in order to use them you need to see the full definition, which usually means putting them in a header file.