With the anticipated inclusion of modules in the C++20 langauge, we need a natural and portable mechanism to import the standard library into a modules-enabled compilation.
This provides us with a unique opportunity: when converting the standard
library into a set of named modules (universally assumed to be given names
), we can reconsider which pieces of the library
belong together and which can be split apart, with few backwards compatibility
constraints. (There are techniques that permit standard library vendors to
maintain ABI across such a reorganization, and we trust the them to use those
techniques as they see fit.)
Indeed, various parties are investigating such reorganizations of the standard (see [P0581R1], [P1212R0], [P1453R0]), and we anticipate such investigation will lead to improvements in the overall structure of the standard library as exposed to modern modular C++. If such a solution is ready in time for C++20, we would welcome such a development.
However, it is not clear that such a solution will be ready in time, and the worst outcome is looming: that different implementations might expose the C++ standard library in different ways, fracturing the language and ecosystem. While modules gives us an opportunity to improve upon the status quo in this area, we can decouple such improvements from the addition of the modules language feature and basic support for the standard library therein, and doing so is good engineering practice.
We seek to solve three problems:
1) Provide a minimal, uninventive mechanism to expose the existing standard library organization to modular compilations.
2) Do not get in the way of, or create any impediment to, a proper reorganization of the standard library into named modules.
3) Reserve space for such a future reorgnization.
3.1. Standard library header units
We propose to guarantee that the C++ standard library headers can be imported as header units:
// C++17 or C++20 #include<vector> #include<tuple>
// C++20 import < vector > ; import < tuple > ;
This has a number of consequences, such as:
No reorganization of any library contents nor ABI changes
s of standard library headers transparently turn into module imports in C++20
Additional names can "leak" out of standard library header units, just as they can "leak" out of standard library headers today (for example, it’s unspecified whether
< vector >
< utility >
Some situations that were formerly undefined behavior now become ill-formed or well-defined:
Defining a keyword or library identifier then including or importing a library header need not be undefined, as the
does not affect the standard library header unit.
Including a standard library header inside a namespace becomes ill-formed rather than undefined behavior.
We propose that only the C++ standard library headers (excluding both the
headers and the deprecated
headers) are affected. The
headers are included from shared C / C++ header files, frequently in
blocks, for which
translation would create trouble. In
addition, the underlying C standard library sometimes gives additional
semantics to macros defined before the
headers are included
and the behavior of the
differs based on which macros are
defined beforehand. Per 126.96.36.199 [using.headers]/2:
A translation unit may include library headers in any order (Clause 5). Each may be included more than once, with no effect different from being included exactly once, except that the effect of including either
< cassert >
depends each time on the lexically current definition of
< assert . h >
We propose to reserve modules whose names begin with a
followed by a sequence of digits.
If any module name component is a reserved identifier, the module name is reserved
for implementations (eg,
all other names are reserved for future standardization (eg,
4.1. §10.1 Module units and purviews [module.unit]
Change in 10.1 [module.unit] paragraph 1:
A module unit is a translation unit that contains a module-declaration. A named module is the collection of module units with the same module-name. The identifiers
shall not appear as identifiers in a module-name or module-partition. The optional attribute-specifier-seq appertains to the module-declaration.
4.2. §10.3 Import declaration [module.import]
Change in 10.3 [module.import] paragraph 3:
[...] An importable header is a member of an implementation-defined set of headers . [...]
4.3. §188.8.131.52 Headers [headers]
Add a new paragraph after 184.108.40.206 [headers] paragraph 3:
4.4. §220.127.116.11 Headers [using.headers]
Change in 18.104.22.168 [using.headers] paragraph 1:
The entities in the C++ standard library are defined in headers, whose contents are made available to a translation unit when it contains the appropriate
preprocessing directive (15.2)
Change in 22.214.171.124 [using.headers] paragraph 3:
A translation unit shall include a header only outside of any declaration or definition , and shall include the header lexically before the first reference in that translation unit to any of the entities declared in that header. No diagnostic is required.
4.5. Feature test macro
No feature test macro is proposed. Code that wants to work in the absence of
this feature can use