should I include a header that is already included via other headers?

Question

I had only just noticed my programs using the string class were compiling without including the <string> header. It turns out that <iostream> includes <ios_base> which in turn includes <string>.

Is this bad practice and should I explicitly include <string>? Even if it's just a case of clarity?

Is it safe to assume this applies to more than just the <string> header? Perhaps this is implementation specific and or does the standard state the <string> header be included via <ios_base> and <iostream>? Ensuring that any respected and widely used implementation will always include <string> providing the the call to <iostream> exists.

Answer 1

You should explicitly include whatever standard library headers you need.

It is not specified which standard library headers are included by other standard library headers, so such details will differ between compilers.

One case where you can rely on a header being included by another header is if a class in one header derives from a class in another. For example, <iostream> has to include <ios_base> because classes defined in <iostream> are derived from classes defined in <ios_base>.

Answer 2

A good practice is to always include the headers for the classes you'll be using in a given source file, regardless of whether you "know" they're included by already-included files.

If, while refactoring your code, you remove the necessity for one of the higher-level included files (iostream, for example), it could become quite painful to determine why your application no longer compiles.