C++ object definitions can be quite complex. In principle, your source code will need two kinds of things for each object that you use across more than one source file. First, you need an interface specification, describing its structure with type declarations and function prototypes. Second, you need the implementation itself. It can be tedious to maintain a separate interface description in a header file, in parallel to the actual implementation. It is also dangerous, since separate interface and implementation definitions may not remain parallel.
With GNU C++, you can use a single header file for both purposes.
Warning: The mechanism to specify this is in transition. For the nonce, you must use one of two
#pragmacommands; in a future release of GNU C++, an alternative mechanism will make these
The header file contains the full definitions, but is marked with
`#pragma interface' in the source code. This allows the compiler
to use the header file only as an interface specification when ordinary
source files incorporate it with
#include. In the single source
file where the full implementation belongs, you can use either a naming
convention or `#pragma implementation' to indicate this alternate
use of the header file.
#pragma interface "subdir
The second form of this directive is useful for the case where you have
multiple headers with the same name in different directories. If you
use this form, you must specify the same string to `#pragma
#pragma implementation "objects
If you use `#pragma implementation' with no argument, it applies to an include file with the same basename1 as your source file. For example, in allclass.cc, giving just `#pragma implementation' by itself is equivalent to `#pragma implementation "allclass.h"'.
In versions of GNU C++ prior to 2.6.0 allclass.h was treated as an implementation file whenever you would include it from allclass.cc even if you never specified `#pragma implementation'. This was deemed to be more trouble than it was worth, however, and disabled.
If you use an explicit `#pragma implementation', it must appear in your source file before you include the affected header files.
Use the string argument if you want a single implementation file to include code from multiple header files. (You must also use `#include' to include the header file; `#pragma implementation' only specifies how to use the file—it doesn't actually include it.)
There is no way to split up the contents of a single header file into multiple implementation files.
`#pragma implementation' and `#pragma interface' also have an effect on function inlining.
If you define a class in a header file marked with `#pragma
interface', the effect on a function defined in that class is similar to
extern declaration—the compiler emits no code at
all to define an independent version of the function. Its definition
is used only for inlining with its callers.
Conversely, when you include the same header file in a main source file that declares it as `#pragma implementation', the compiler emits code for the function itself; this defines a version of the function that can be found via pointers (or by callers compiled without inlining). If all calls to the function can be inlined, you can avoid emitting the function by compiling with -fno-implement-inlines. If any calls were not inlined, you will get linker errors.
 A file's basename was the name stripped of all leading path information and of trailing suffixes, such as `.h' or `.C' or `.cc'.