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7.4 #pragma interface and implementation

#pragma interface and #pragma implementation provide the user with a way of explicitly directing the compiler to emit entities with vague linkage (and debugging information) in a particular translation unit.

Note: As of GCC 2.7.2, these #pragmas are not useful in most cases, because of COMDAT support and the “key method” heuristic mentioned in Vague Linkage. Using them can actually cause your program to grow due to unnecessary out-of-line copies of inline functions. Currently (3.4) the only benefit of these #pragmas is reduced duplication of debugging information, and that should be addressed soon on DWARF 2 targets with the use of COMDAT groups.

#pragma interface
#pragma interface "subdir/objects.h"
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 is not generated (unless the main input source file itself uses ‘#pragma implementation’). Instead, the object files contain references to be resolved at link time.

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 implementation’.

#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.

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, 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 even if you never specified ‘#pragma implementation’. This was deemed to be more trouble than it was worth, however, and disabled.

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 an inline 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). If all calls to the function can be inlined, you can avoid emitting the function by compiling with -fno-implement-inlines. If any calls are not inlined, you will get linker errors.


[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’.