Using and Porting GNU CC

6.4: Declarations and Definitions in One Header

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 #pragma commands; in a future release of GNU C++, an alternative mechanism will make these #pragma commands unnecessary.

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

Use this directive in header files that define object classes, to save space in most of the object files that use those classes. Normally, local copies of certain information (backup copies of inline member functions, debugging information, and the internal tables that implement virtual functions) must be kept in each object file that includes class definitions. You can use this pragma to avoid such duplication. When a header file containing `#pragma interface' is included in a compilation, this auxiliary information will not be generated (unless the main input source file itself uses `#pragma implementation'). Instead, the object files will contain references to be resolved at link time.

#pragma implementation

#pragma implementation "objects.h"

Use this pragma in a main input file, when you want full output from included header files to be generated (and made globally visible). The included header file, in turn, should use `#pragma interface'. Backup copies of inline member functions, debugging information, and the internal tables used to implement virtual functions are all generated in implementation files.

`#pragma implementation' is implied whenever the basename[1] of your source file matches the basename of a header file it includes. There is no way to turn this off (other than using a different name for one of the two files). In the same vein, if you use `#pragma implementation' with no argument, it applies to an include file with the same basename as your source file. For example, in `allclass.cc', `#pragma implementation' by itself is equivalent to `#pragma implementation "allclass.h"'; but even if you do not say `#pragma implementation' at all, `allclass.h' is treated as an implementation file whenever you include it from `allclass.cc'.

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 an explicit 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).

[1] A file's basename is the name stripped of all leading path information and of trailing suffixes, such as `.h' or `.C' or `.cc'.