在Windows上编译C ++程序

Question

I am compiling C++ on VS 2005. When and why use #include and when and why use pre-decleration as class XXXX ? What is the benefit of using each option and which one is preffered?

I would also glad for a good tutorial on compiling.

Answer 1

Always prefer forward declaration whenever possible. Changes to the referred class file will not trigger recompilation of cpp files including the class using the pre-declared one. This reduces a bit the dependencies.

On each place where you are effectively using the class XXXX, you will have to include that header. If you derive from class XXXX, you will also have to include the header.

Answer 2

A header file is used to contain the declaration of entities that are defined in separate compilation units. If you did not have a header file, you'd have to enter such declarations in every compilation unit (which is essentially what #include does for you, it inserts the contained text at that point in the file, however if you did not use a header, you'd have to do it multiple times and that is both error prone and difficult to maintain when the code changes.

You'd use a declaration directly in the .cpp file for example if the symbol being defined is only ever used within that compilation unit and therefore did not need global visibility. In the case of data declarations, you also typically declare them static to give them scope limited to the compilation unit.