Question

Does including the same header files multiple times increase the compilation time?

For example, suppose every file in my project uses <iostream> <string> <vector> and <algorithm>. And if I include a lot of files in my source code, then does that increase the compile time?

I always thought that the guard headers served important purpose of avoiding double definitions but as a by product also eliminates double code.

Actually, someone I know proposed some ideas to remove such multiple inclusions. However, I consider them to be completely against the good design practices in c++. But was still wondering what might be the reasons of him to suggest the changes?

Answer 1

Most of these answers are wrong… For modern compilers, there is zero overhead for including the same file multiple times, assuming the header uses the usual “include guard” idiom.

The GCC preprocessor, for example, has special code to recognize the include guard idiom. It will not even open the header file (never mind reading it) for the second and subsequent #include directives.

I am not sure about other compilers, but I would be very surprised if most of them did not implement the same optimization.