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In this section modules are introduced using a simple demo example. More
extensive coverage of the module-definition language is provided in subsequent
sections.
To create a module from an existing header file the directory (or its parent
directory) must contain a file ti(module.modulemap). It contains the
specifications for creating a module from one or more available header files.
Assume a directory tt(module) contains the following file header file:
itemization(
itt(module.h):nl()
verbinsert(-a examples/minimal/module/module.h)
)nl()
For the benefit of the example it merely declares a simple tt(struct) and
a function tt(fun) (its content in fact is irrelevant: it could actually be
completely empty).
To create a module from this header file define the following minimal
tt(module.modulemap) and tt(main.cc) files:
verbinsert(-ans4 examples/minimal/module.modulemap)
tt(main.cc):
verbinsert(-as4 examples/minimal/main.cc)
Here the function tt(main) is defined, because that's a requirement for
creating a program, but any other source including a header that's mentioned
in tt(module.modulemap) would also be OK. The above minimal tt(main)
function's source file does exactly that: it includes tt(module/module.h).
The file tt(module.modulemap) itself has a simple organization:
itemization(
it() Line 1 specifies that a module should be constructed, named
`minimal';
it() Line 3 specifies that the header file tt(module/module.h) defines the
module's content.
it() The module's content specification follows the module's header
(line 1), and is surrounded by a pair of curly brackets (lines 2 and
4).
)
To create the module the source file must be compiled using the following
command:
verb( clang++-7 -fmodules --std=c++2a -O2 -c main.cc)
All options are required:
itemization(
itt(-fmodules) is required to inform the compiler that modules should be
used;
it() tt(-)tt(-std=c++2a) is required because modules aren't available
before the C2a standard;
itt(-O2) (or at least tt(-O1)) is required because using modules implies
some form of optimization, and without any optimization request the
compiler won't create the module;
itt(-c) isn't really required, but suffices to create the module.
)
Following this command tt(main.o) and the module are created. By default the
module is created in a system-defined cache location (but see also section
ref(MODULEOPTS)). In my system this system-defined cache location is
tt(/tmp/org.llvm.clang.frank/ModuleCache/), containing
itemization(
itt(modules.timestamp)
it() the module information itself in the subdirectory tt(2TA26R6BHQ19F/).
It contains:
itemization(
itt(minimal-2ORZEBL9H54OZ.pcm) (17,992 bytes)
itt(modules.idx) (552 bytes).
)
)
Once the module is available it will be used if sources including at least one
of its header files are recompiled. The module is rebuilt if one of its
headers have changed.
Compare these sizes to the size of the precompiled header tt(module.h.gch)
(created using tt(clang++ -x c++-header module.h)). On my system the compiled
header (tt(module.h.gch)) is a file of some 195 KB large.
Note that the file tt(module.h) doesn't contain include guards. As
modules handle the administration of which header has already been included,
it's OK if tt(module.h) is included multiple times (e.g., add another
tt(#include "module/module.h") line in tt(main.cc)) and recompile: compilation
succeeds.
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