<<Home: Back to http://devdoc.net

Headers

Header Files

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 (TR1, C++ ABI, and Extensions).

Two dialects of standard headers are supported, corresponding to the 1998 standard as updated for 2003, and the current 2011 standard.

C++98/03 include files. These are available in the default compilation mode, i.e. -std=c++98 or -std=gnu++98.

Table 3.2. C++ 1998 Library Headers

algorithmbitsetcomplexdequeexception
fstreamfunctionaliomanipiosiosfwd
iostreamistreamiteratorlimitslist
localemapmemorynewnumeric
ostreamqueuesetsstreamstack
stdexceptstreambufstringutilitytypeinfo
valarrayvector   

Table 3.3. C++ 1998 Library Headers for C Library Facilities

cassertcerrnocctypecfloatciso646
climitsclocalecmathcsetjmpcsignal
cstdargcstddefcstdiocstdlibcstring
ctimecwcharcwctype  

C++11 include files. These are only available in C++11 compilation mode, i.e. -std=c++11 or -std=gnu++11.

Table 3.4. C++ 2011 Library Headers

algorithmarraybitsetchronocomplex
condition_variabledequeexceptionforward_listfstream
functionalfutureinitalizer_listiomanipios
iosfwdiostreamistreamiteratorlimits
listlocalemapmemorymutex
newnumericostreamqueuerandom
ratioregexsetsstreamstack
stdexceptstreambufstringsystem_errorthread
tupletype_traitstypeinfounordered_mapunordered_set
utilityvalarrayvector  

Table 3.5. C++ 2011 Library Headers for C Library Facilities

cassertccomplexcctypecerrnocfenv
cfloatcinttypesciso646climitsclocale
cmathcsetjmpcsignalcstdargcstdbool
cstddefcstdintcstdlibcstdiocstring
ctgmathctimecucharcwcharcwctype

In addition, TR1 includes as:

Table 3.6. C++ TR 1 Library Headers

tr1/arraytr1/complextr1/memorytr1/functionaltr1/random
tr1/regextr1/tupletr1/type_traitstr1/unordered_maptr1/unordered_set
tr1/utility    

Table 3.7. C++ TR 1 Library Headers for C Library Facilities

tr1/ccomplextr1/cfenvtr1/cfloattr1/cmathtr1/cinttypes
tr1/climitstr1/cstdargtr1/cstdbooltr1/cstdinttr1/cstdio
tr1/cstdlibtr1/ctgmathtr1/ctimetr1/cwchartr1/cwctype

Decimal floating-point arithmetic is available if the C++ compiler supports scalar decimal floating-point types defined via __attribute__((mode(SD|DD|LD))).

Table 3.8. C++ TR 24733 Decimal Floating-Point Header

decimal/decimal

Also included are files for the C++ ABI interface:

Table 3.9. C++ ABI Headers

cxxabi.hcxxabi_forced.h

And a large variety of extensions.

Table 3.10. Extension Headers

ext/algorithmext/atomicity.hext/array_allocator.hext/bitmap_allocator.hext/cast.h
ext/codecvt_specializations.hext/concurrence.hext/debug_allocator.hext/enc_filebuf.hext/extptr_allocator.h
ext/functionalext/iteratorext/malloc_allocator.hext/memoryext/mt_allocator.h
ext/new_allocator.hext/numericext/numeric_traits.hext/pb_ds/assoc_container.hext/pb_ds/priority_queue.h
ext/pod_char_traits.hext/pool_allocator.hext/rb_treeext/ropeext/slist
ext/stdio_filebuf.hext/stdio_sync_filebuf.hext/throw_allocator.hext/typelist.hext/type_traits.h
ext/vstring.h    

Table 3.11. Extension Debug Headers

debug/bitsetdebug/dequedebug/listdebug/mapdebug/set
debug/stringdebug/unordered_mapdebug/unordered_setdebug/vector 

Table 3.12. Extension Profile Headers

profile/bitsetprofile/dequeprofile/listprofile/map
profile/setprofile/unordered_mapprofile/unordered_setprofile/vector

Table 3.13. Extension Parallel Headers

parallel/algorithmparallel/numeric

Mixing Headers

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 C Headers and namespace std

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.

Precompiled Headers

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.