Is there any guaranteed dependency of C++ system headers?

Question

For example, in <algorithm>, the function equal_range returns a pair, so can I assume that if I #include <algorithm>, <utility> is #included?

Answer 1

There is never a guaranteed inclusion of header files that other headers depend on. Fortunately, it's common practice (though not certain 100% of the time) that headers are guarded against multiple inclusion -- meaning you can #include them as many times as you wish without causing harm.

Answer 2

You should always include what you need, you cannot rely on all implementations including the same set of headers in another header.

In your example, if a function returns a pair you can be reasonably sure that the pair class is declared, but nothing requires the implementation to include the rest of <utility>.

In fact, it is impossible to implement the standard library using exactly the headers shown in the standard, because there are some circular references. Implementations must split them in smaller parts, and include those sub-headers in the required <> headers.

The GCC team, for example, is working to minimize the amount of inclusions to speed up compile time.