[英]Why does iostream include time.h?
考慮以下代碼:
#include <iostream>
template<class C>
struct time { };
int main() { }
它產生(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*)’
iostream
包含time_t time(time_t*)
? iostream
在std
命名空間之外包含time_t time(time_t*)
? template<class C>
,我不會收到此錯誤嗎? 如果你運行g++ -H -Wall -c testim.cc
(其中testim.cc
就是你的例子),你會看到
.... /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
所以<time.h>
包含在pthread支持中。
這是在Debian / Sid / AMD64上的GCC 4.6
似乎是我的盒子上的語言環境支持:
$ 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’
顯然GCC的iostream
需要一些time
的東西。
time_t
是一個C
語言的東西,它沒有名稱空間,因此對於向后可比性,它既在std
名稱空間中,也不在std
名稱空間中。
至於為什么沒有錯誤,我不是百分百肯定。 我知道C
可以具有相同名稱的函數和結構,並且struct
使用名稱struct
進行限定,以便編譯器能夠區分它們。 C ++編譯器認為它是一個函數,除非在struct
關鍵字之前。 http://ideone.com/XZB2M v http://ideone.com/kWMKE 。 我想它也可以實現向后兼容性?
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