Header files in .h

This is question rather than a problem.

I know it is bad to place header files in class definition files, as this means once the header file is included into the .cpp file it will include all the headers in the .h fille that are not needed.

I follow this rule to as much as I can, however, is it ok to do this:

Code:

---

#ifndef FOO_H
#define FOO_H

#include <string> // i need this!

class
{
Foo();
~Foo();

private:
    std::string m_MyString;
};

#endif

---

If I compiled the code without the string header, I would get a compile error winging it string was not a member of std::. So, is this allowed in the sense of "a file should only get what it requries and nothing more"?

That is fine, although another option (not recommended) is that any file which includes this particular header could also #include the string header before you #include this one. This can confuse people who simply move around the order of the #include statements and suddenly they start getting errors. So... what you are doing above is the way to do it.

I'm guessing STL headers are typically guarded by include guards (from what I can see anyway). Then you face no penalty at all by doing the above. In fact, I would recommend it.

In my opinion, all include files should never require that other includes are made before this one.

As a consequence: If one include file relies on any other header file (e.g. a class uses std::string), then it should include the header file(s) necessary for it to compile with only itself being included.

--
Mats

Quote:

---

I know it is bad to place header files in class definition files, as this means once the header file is included into the .cpp file it will include all the headers in the .h fille that are not needed.

---

This is nonsense. By the very act of including the header that requires the other headers, the cpp file requires those other headers. Dependency is a transitive relationship. (If A depends on B and B depends on C, A depends on C.)

You should not include any more headers than strictly necessary, and you never should place using statements on global or namespace scope. (Avoid using declarations too, unless you actually mean to import a symbol into a target namespace.)

If you need certain header files for types in the public section of a class, you should always include that header in the class definition.
If you need certain header files only for types in the private section of a class, you can use the Pimpl idiom:
Chapter 43. Pimpl judiciously