Why does iostream include time.h?

问题:

Consider this code:

#include <iostream>

template<class C>
struct time { };

int main() { }

It produces (GCC 4.5):

error: ‘template<class C> struct time’ redeclared as different kind of symbol
/usr/include/time.h:186:15: error: previous declaration of ‘time_t time(time_t*)’

1. Why does iostream include time_t time(time_t*) ?
2. Why does iostream include time_t time(time_t*) outside std namespace?
3. (unanswered) Why, if I remove template<class C>, do I not get this error?

回答1:

If you run g++ -H -Wall -c testim.cc (where testim.cc is your example) you'll see that

.... /usr/include/c++/4.6/bits/ios_base.h
..... /usr/include/c++/4.6/ext/atomicity.h
...... /usr/include/c++/4.6/x86_64-linux-gnu/./bits/gthr.h
....... /usr/include/c++/4.6/x86_64-linux-gnu/./bits/gthr-default.h
........ /usr/include/pthread.h
......... /usr/include/sched.h
.......... /usr/lib/gcc/x86_64-linux-gnu/4.6/include/stddef.h
.......... /usr/include/time.h

So <time.h> is included for pthread support.

This is with GCC 4.6 on Debian/Sid/AMD64

回答2:

Seems to be for locale support on my box:

$ cat whytime.cc 
#define _TIME_H_
#include <iostream>

int main() {
  std::cout << "hello, world" << std::endl;
  return 0;
}

$ g++ -Wall -Werror -Wextra -ansi -o whytime whytime.cc && ./whytime
In file included from /usr/include/c++/4.2.1/cwchar:52,
                 from /usr/include/c++/4.2.1/bits/postypes.h:46,
                 from /usr/include/c++/4.2.1/iosfwd:49,
                 from /usr/include/c++/4.2.1/ios:43,
                 from /usr/include/c++/4.2.1/ostream:45,
                 from /usr/include/c++/4.2.1/iostream:45,
                 from whytime.cc:2:
/usr/include/c++/4.2.1/ctime:66: error: ‘::clock_t’ has not been declared
/usr/include/c++/4.2.1/ctime:68: error: ‘::tm’ has not been declared
/usr/include/c++/4.2.1/ctime:70: error: ‘::clock’ has not been declared
/usr/include/c++/4.2.1/ctime:71: error: ‘::difftime’ has not been declared
/usr/include/c++/4.2.1/ctime:72: error: ‘::mktime’ has not been declared
/usr/include/c++/4.2.1/ctime:73: error: ‘::time’ has not been declared
/usr/include/c++/4.2.1/ctime:74: error: ‘::asctime’ has not been declared
/usr/include/c++/4.2.1/ctime:75: error: ‘::ctime’ has not been declared
/usr/include/c++/4.2.1/ctime:76: error: ‘::gmtime’ has not been declared
/usr/include/c++/4.2.1/ctime:77: error: ‘::localtime’ has not been declared
/usr/include/c++/4.2.1/ctime:78: error: ‘::strftime’ has not been declared
In file included from /usr/include/c++/4.2.1/locale:46,
                 from /usr/include/c++/4.2.1/bits/ostream.tcc:46,
                 from /usr/include/c++/4.2.1/ostream:572,
                 from /usr/include/c++/4.2.1/iostream:45,
                 from whytime.cc:2:
/usr/include/c++/4.2.1/bits/locale_facets.tcc: In member function ‘_InIter std::time_get<_CharT, _InIter>::_M_extract_via_format(_InIter, _InIter, std::ios_base&, std::_Ios_Iostate&, tm*, const _CharT*) const’:
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1839: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1846: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1854: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1861: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1873: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1880: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1883: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1895: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1900: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1908: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1912: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1932: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1968: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc:1976: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc: In member function ‘virtual _InIter std::time_get<_CharT, _InIter>::do_get_weekday(_InIter, _InIter, std::ios_base&, std::_Ios_Iostate&, tm*) const’:
/usr/include/c++/4.2.1/bits/locale_facets.tcc:2210: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc: In member function ‘virtual _InIter std::time_get<_CharT, _InIter>::do_get_monthname(_InIter, _InIter, std::ios_base&, std::_Ios_Iostate&, tm*) const’:
/usr/include/c++/4.2.1/bits/locale_facets.tcc:2259: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’
/usr/include/c++/4.2.1/bits/locale_facets.tcc: In member function ‘virtual _InIter std::time_get<_CharT, _InIter>::do_get_year(_InIter, _InIter, std::ios_base&, std::_Ios_Iostate&, tm*) const’:
/usr/include/c++/4.2.1/bits/locale_facets.tcc:2288: error: invalid use of incomplete type ‘struct tm’
/usr/include/wchar.h:152: error: forward declaration of ‘struct tm’

回答3:

Apparently GCC's iostream needs something in time for something.

time_t is a C language thing, which doesn't have namespaces, and so for backwards comparability-ish, it is in both the std namespace, and not in the std namespace.

As for why there's no errors, I'm not 100% sure. I know C could have functions and structs with the same name, and structs were qualified with the name struct so that the compiler could tell them apart. C++ compilers thinks it's a function, unless preceeded with the struct keyword. http://ideone.com/XZB2M v http://ideone.com/kWMKE. I guess it also allows that for backwards comparability?