reincluding header in implementation

Question

suppose I have a header foo.h like this:

#ifndef FOO_H
#define FOO_H
#include <string>
#include "non_standard_class.h"

std::string foo(MyClass a);
...
#endif

and the implementation foo.cpp will be

#include <vector>

#include "foo.h"

std::string foo(MyClass a)
{
   std::vector<int> x;
   MyClass b;
   ...
}

is it a good pratice to re-include <string> and non_standard_class.h in foo.cpp ? The point is: if I read foo.cpp how can I understand where does MyClass come from? I need to look at foo.h but it will be more difficult.

Answer 1

The practice I follow:

• foo.h should directly include all headers its source code needs (i.e. #include <string> if there is std::string in foo.h)
• foo.cpp should directly include foo.h plus all other headers its own source code needs, except for those already directly included by foo.h

Another formulation of this practice: in foo.*, include only those headers the source code needs. If a header is needed by only foo.cpp (but not by foo.h), then include it from foo.cpp, otherwise include it from foo.h.

So, in your case, I wouldn't #include <string> in foo.cpp, because it's already included by foo.h.

Let's assume that string does an #include <vector>, and foo.h contains std::vector. In this case, I'd #include <vector> in foo.h, because its source code needs it – and it doesn't matter that string already includes it.