The C++ standard specifies the entire set of header files that
must be available to all hosted implementations. Actually, the
word "files" is a misnomer, since the contents of the
headers don't necessarily have to be in any kind of external
file. The only rule is that when one #include
s a
header, the contents of that header become available, no matter
how.
That said, in practice files are used.
There are two main types of include files: header files related to a specific version of the ISO C++ standard (called Standard Headers), and all others (TS, TR1, C++ ABI, and Extensions).
Multiple dialects of standard headers are supported, corresponding to the 1998 standard as updated for 2003, the 2011 standard, the 2014 standard, and so on.
Table 3.2, “C++ 1998 Library Headers” and
Table 3.3, “C++ 1998 Library Headers for C Library Facilities” and
Table 3.4, “C++ 1998 Deprecated Library Header”
show the C++98/03 include files.
These are available in the C++98 compilation mode,
i.e. -std=c++98
or -std=gnu++98
.
Unless specified otherwise below, they are also available in later modes
(C++11, C++14 etc).
Table 3.2. C++ 1998 Library Headers
algorithm | bitset | complex | deque | exception |
fstream | functional | iomanip | ios | iosfwd |
iostream | istream | iterator | limits | list |
locale | map | memory | new | numeric |
ostream | queue | set | sstream | stack |
stdexcept | streambuf | string | utility | typeinfo |
valarray | vector |
Table 3.3. C++ 1998 Library Headers for C Library Facilities
cassert | cerrno | cctype | cfloat | ciso646 |
climits | clocale | cmath | csetjmp | csignal |
cstdarg | cstddef | cstdio | cstdlib | cstring |
ctime | cwchar | cwctype |
The following header is deprecated and might be removed from a future C++ standard.
Table 3.5, “C++ 2011 Library Headers” and
Table 3.6, “C++ 2011 Library Headers for C Library Facilities” show the C++11 include files.
These are available in C++11 compilation
mode, i.e. -std=c++11
or -std=gnu++11
.
Including these headers in C++98/03 mode may result in compilation errors.
Unless specified otherwise below, they are also available in later modes
(C++14 etc).
Table 3.5. C++ 2011 Library Headers
array | atomic | chrono | codecvt | condition_variable |
forward_list | future | initalizer_list | mutex | random |
ratio | regex | scoped_allocator | system_error | thread |
tuple | typeindex | type_traits | unordered_map | unordered_set |
Table 3.6. C++ 2011 Library Headers for C Library Facilities
ccomplex | cfenv | cinttypes | cstdalign | cstdbool |
cstdint | ctgmath | cuchar |
Table 3.7, “C++ 2014 Library Header” shows the C++14 include file.
This is available in C++14 compilation
mode, i.e. -std=c++14
or -std=gnu++14
.
Including this header in C++98/03 mode or C++11 will not result in
compilation errors, but will not define anything.
Unless specified otherwise below, it is also available in later modes
(C++17 etc).
Table 3.8, “C++ 2017 Library Headers” shows the C++17 include files.
These are available in C++17 compilation
mode, i.e. -std=c++17
or -std=gnu++17
.
Including these headers in earlier modes will not result in
compilation errors, but will not define anything.
Unless specified otherwise below, they are also available in later modes
(C++20 etc).
Table 3.8. C++ 2017 Library Headers
any | charconv | execution | filesystem | memory_resource |
optional | string_view | variant |
Table 3.9, “C++ 2020 Library Headers”
shows the C++2a include files.
These are available in C++2a compilation
mode, i.e. -std=c++2a
or -std=gnu++2a
.
Including these headers in earlier modes will not result in
compilation errors, but will not define anything.
The following headers have been removed in the C++2a working draft. They are still available when using this implementation, but in future they might start to produce warnings or errors when included in C++2a mode. Programs that intend to be portable should not include them.
Table 3.11, “File System TS Header”, shows the additional include file define by the File System Technical Specification, ISO/IEC TS 18822. This is available in C++11 and later compilation modes. Including this header in earlier modes will not result in compilation errors, but will not define anything.
Table 3.12, “Library Fundamentals TS Headers”, shows the additional include files define by the C++ Extensions for Library Fundamentals Technical Specification, ISO/IEC TS 19568. These are available in C++14 and later compilation modes. Including these headers in earlier modes will not result in compilation errors, but will not define anything.
Table 3.12. Library Fundamentals TS Headers
experimental/algorithm | experimental/any | experimental/array | experimental/chrono | experimental/deque |
experimental/forward_list | experimental/functional | experimental/iterator | experimental/list | experimental/map |
experimental/memory | experimental/memory_resource | experimental/numeric | experimental/optional | experimental/propagate_const |
experimental/random | experimental/ratio | experimental/regex | experimental/set | experimental/source_location |
experimental/string | experimental/string_view | experimental/system_error | experimental/tuple | experimental/type_traits |
experimental/unordered_map | experimental/unordered_set | experimental/utility | experimental/vector |
In addition, TR1 includes as:
Table 3.13. C++ TR 1 Library Headers
tr1/array | tr1/complex | tr1/memory | tr1/functional | tr1/random |
tr1/regex | tr1/tuple | tr1/type_traits | tr1/unordered_map | tr1/unordered_set |
tr1/utility |
Table 3.14. C++ TR 1 Library Headers for C Library Facilities
tr1/ccomplex | tr1/cfenv | tr1/cfloat | tr1/cmath | tr1/cinttypes |
tr1/climits | tr1/cstdarg | tr1/cstdbool | tr1/cstdint | tr1/cstdio |
tr1/cstdlib | tr1/ctgmath | tr1/ctime | tr1/cwchar | tr1/cwctype |
Decimal floating-point arithmetic is available if the C++
compiler supports scalar decimal floating-point types defined via
__attribute__((mode(SD|DD|LD)))
.
Also included are files for the C++ ABI interface:
And a large variety of extensions.
Table 3.17. Extension Headers
ext/algorithm | ext/atomicity.h | ext/bitmap_allocator.h | ext/cast.h | |
ext/codecvt_specializations.h | ext/concurrence.h | ext/debug_allocator.h | ext/enc_filebuf.h | ext/extptr_allocator.h |
ext/functional | ext/iterator | ext/malloc_allocator.h | ext/memory | ext/mt_allocator.h |
ext/new_allocator.h | ext/numeric | ext/numeric_traits.h | ext/pb_ds/assoc_container.h | ext/pb_ds/priority_queue.h |
ext/pod_char_traits.h | ext/pool_allocator.h | ext/rb_tree | ext/rope | ext/slist |
ext/stdio_filebuf.h | ext/stdio_sync_filebuf.h | ext/throw_allocator.h | ext/typelist.h | ext/type_traits.h |
ext/vstring.h |
Table 3.18. Extension Debug Headers
debug/array | debug/bitset | debug/deque | debug/forward_list | debug/list |
debug/map | debug/set | debug/string | debug/unordered_map | debug/unordered_set |
debug/vector |
A few simple rules.
First, mixing different dialects of the standard headers is not possible. It's an all-or-nothing affair. Thus, code like
#include <array> #include <functional>
Implies C++11 mode. To use the entities in <array>, the C++11 compilation mode must be used, which implies the C++11 functionality (and deprecations) in <functional> will be present.
Second, the other headers can be included with either dialect of
the standard headers, although features and types specific to C++11
are still only enabled when in C++11 compilation mode. So, to use
rvalue references with __gnu_cxx::vstring
, or to use the
debug-mode versions of std::unordered_map
, one must use
the std=gnu++11
compiler flag. (Or std=c++11
, of course.)
A special case of the second rule is the mixing of TR1 and C++11 facilities. It is possible (although not especially prudent) to include both the TR1 version and the C++11 version of header in the same translation unit:
#include <tr1/type_traits> #include <type_traits>
Several parts of C++11 diverge quite substantially from TR1 predecessors.
The standard specifies that if one includes the C-style header
(<math.h> in this case), the symbols will be available
in the global namespace and perhaps in
namespace std::
(but this is no longer a firm
requirement.) On the other hand, including the C++-style
header (<cmath>) guarantees that the entities will be
found in namespace std and perhaps in the global namespace.
Usage of C++-style headers is recommended, as then
C-linkage names can be disambiguated by explicit qualification, such
as by std::abort
. In addition, the C++-style headers can
use function overloading to provide a simpler interface to certain
families of C-functions. For instance in <cmath>, the
function std::sin
has overloads for all the builtin
floating-point types. This means that std::sin
can be
used uniformly, instead of a combination
of std::sinf
, std::sin
,
and std::sinl
.
There are three base header files that are provided. They can be used to precompile the standard headers and extensions into binary files that may then be used to speed up compilations that use these headers.
stdc++.h
Includes all standard headers. Actual content varies depending on language dialect.
stdtr1c++.h
Includes all of <stdc++.h>, and adds all the TR1 headers.
extc++.h
Includes all of <stdc++.h>, and adds all the Extension headers (and in C++98 mode also adds all the TR1 headers by including all of <stdtr1c++.h>).
To construct a .gch file from one of these base header files, first find the include directory for the compiler. One way to do this is:
g++ -v hello.cc #include <...> search starts here: /mnt/share/bld/H-x86-gcc.20071201/include/c++/4.3.0 ... End of search list.
Then, create a precompiled header file with the same flags that will be used to compile other projects.
g++ -Winvalid-pch -x c++-header -g -O2 -o ./stdc++.h.gch /mnt/share/bld/H-x86-gcc.20071201/include/c++/4.3.0/x86_64-unknown-linux-gnu/bits/stdc++.h
The resulting file will be quite large: the current size is around thirty megabytes.
How to use the resulting file.
g++ -I. -include stdc++.h -H -g -O2 hello.cc
Verification that the PCH file is being used is easy:
g++ -Winvalid-pch -I. -include stdc++.h -H -g -O2 hello.cc -o test.exe ! ./stdc++.h.gch . /mnt/share/bld/H-x86-gcc.20071201/include/c++/4.3.0/iostream . /mnt/share/bld/H-x86-gcc.20071201include/c++/4.3.0/string
The exclamation point to the left of the stdc++.h.gch
listing means that the generated PCH file was used.
Detailed information about creating precompiled header files can be found in the GCC documentation.