Why include a header and forward declare the class contained in the same cpp file?

Question

I\'ve been looking at the Fear SDK for my university project, but have noticed some code like so:

Foo.h

class Foo
{
    public:
        int iSomethin         


        

Answer 1

It's more than simply redundant, it's potentially problematic. Say Foo.h changes so Foo becomes a typedef to some particular instantiation of a generic, templatised equivalent - the kind of thing that can be anticipated as part of normal software evolution. Then Bar.cpp's "class X" will needlessly cause a compilation error ala:

--- fwd.h ---
template 
class XT
{
  public:
    int n_;
};

typedef XT X;

--- fwd.cc ---
#include "fwd.h"

class X;

int main()
{
    X x;
    x.n_ = 0;
    return x.n_;
}

--- compilation attempt ---
~/dev  .../gcc/4.1.1/exec/bin/g++ fwd.cc -o fwd
fwd.cc:3: error: using typedef-name 'X' after 'class'
fwd.h:8: error: 'X' has a previous declaration here

This is one reason I always recommend using dedicated forward-declaration headers ala , maintained with and included by the main header to ensure ongoing consistency. I never put "class X;" in an implementation file unless the class is defined in there too. Remember that the seeming benefits of "class X;" forward declarations is not so much that they avoid an #include, and more that the files they include can be large and in turn include a lot of other files: dedicated forward-declaration headers typically avoid the overwhelming majority of that anyway.