c++ - Why it seems not necessary to include some STL headers

Question

I think some headers need to be included to compile with gcc(4.9),

#include <algorithm> // for std::transform
#include <numeric>   // for std::adjacent_difference

However, I found it's not necessary include them at all, I can still call for example, the following functions

std::adjacent_difference (V1.begin(), V1.end(), V2.begin());
std::transform(V2.begin(), V2.end(), V3.begin(), V4.begin(), std::divides<double>());

Maybe I have misunderstood the mechanism of including header files...Any hint?

Answer 1

It is quite typical for a header to include other headers. The headers included by header x will be included in any file that includes x. That should be trivial to understand once you grasp what including a file in another means. In this case, one of the standard headers happened to be included in another.

Files included by headers can change between versions. If you don't include a required header, your program may break under another (version of) standard library even though it may appear to work in the current implementation. Of course, this applies to other libraries as well.