Question

http://www.learncpp.com/cpp-tutorial/19-header-files/

It mentions the following as another solution to “forward declaration”:

A header file only has to be written once, and it can be included in as many files as needed. This also helps with maintenance by minimizing the number of changes that need to be made if a function prototype ever changes (eg. by adding a new parameter).

But, cannot this also be made with “forward declaration”? Since we are defining the function int add(int x, int y) for example in “add.cpp”, and using this function in “main.cpp” by typing:

int add(int x, int y);

?
Thanks.

Answer 1

That is certainly possible. But for a realistically-sized program, there will be a large number of functions that a large number of other files will need to declare. If you put a forward declaration in every file that needs to access another function, you have a multitude of problems:

1. You’ve just copy-pasted the same declaration into many different files. If you ever change the function signature, you have to change every place you’ve pasted its forward declaration.
2. The forward declaration itself does not naturally tell you what file the actual function is defined in. If you use a sane method of organizing your header files and your source files (for instance, every function defined in a .cpp file is declared in a .h file with the same name), then the place that the function is defined is implied by the place that it is declared.
3. Your code will be less readable to other programmers, who are very used to using header files for everything (for good reason), even if all you need from a header is one specific function and you could easily forward-declare it yourself.