6.4 #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
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
#pragma interface "subdir
- 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
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 "objects
- 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
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.
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.
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 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.