C++11新增预定义的宏

C++11中新增了4个预定义的宏，他们分别为__STDC__，__STDC_HOSTED__，__STDC_VERSION__和__STDC_ISO_10646__。

1. __STDC__用于指示编译器是否支持ISO标准C语言，如果支持ISO标准C语言则_STDC__定义为1，否为定义为0。这个宏在不同的编译器上可能有不同的定义，甚至有未定义的情况。例如在GCC上，编译并输出该宏的值为1，而在Visual Studio C++上，默认情况下该宏处于未定义状态。

2. __STDC_HOSTED__用于指示宿主环境是否具有标准C库的完整功能，如果具有标准C库的完整功能则__STDC_HOSTED__定义为1，否为定义为0。

3. __STDC_VERSION__用于定义C标准的版本号，但是标准文档中并没有明确规定其实现，所以在很多编译器中这个宏处于未定义状态。

4. __STDC_ISO_10646__用于指示wchar_t是否使用Unicode，如果使用Unicode那么wchar_t展开为yyyymmL的形式。

编译运行下面这段代码用于检测这些宏的定义状态：

#include <iostream>
using namespace std;
int main(int argc, char *argv[])
{
    cout << "__cplusplus is " << __cplusplus << endl;
    cout << "__DATE__ is " << __DATE__ << endl;
    cout << "__FILE__ is " << __FILE__ << endl;
    cout << "__LINE__ is " << __LINE__ << endl;
#ifdef __STDC_HOSTED__
    cout << "__STDC_HOSTED__ is " << __STDC_HOSTED__ << endl;
#else
    cout << "__STDC_HOSTED__ is not defined" << endl;
#endif
    cout << "__TIME__ is " << __TIME__ << endl;
#ifdef __STDC__
    cout << "__STDC__ is " << __STDC__ << endl;
#else
    cout << "__STDC__ is not defined" << endl;
#endif
#ifdef __STDC_MB_MIGHT_NEQ_WC__
    cout << "__STDC_MB_MIGHT_NEQ_WC__ is " << __STDC_MB_MIGHT_NEQ_WC__ << endl;
#else
    cout << "__STDC_MB_MIGHT_NEQ_WC__ is not defined" << endl;
#endif
#ifdef __STDC_VERSION__
    cout << "__STDC_VERSION__ is " << __STDC_VERSION__ << endl;
#else
    cout << "__STDC_VERSION__ is not defined" << endl;
#endif
#ifdef __STDC_ISO_10646__
    cout << "__STDC_ISO_10646__ is " << __STDC_ISO_10646__ << endl;
#else
    cout << "__STDC_ISO_10646__ is not defined" << endl;
#endif
#ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
    cout << "__STDCPP_DEFAULT_NEW_ALIGNMENT__ is " << __STDCPP_DEFAULT_NEW_ALIGNMENT__ << endl;
#else
    cout << "__STDCPP_DEFAULT_NEW_ALIGNMENT__ is not defined" << endl;
#endif
#ifdef __STDCPP_STRICT_POINTER_SAFETY__
    cout << "__STDCPP_STRICT_POINTER_SAFETY__ is " << __STDCPP_STRICT_POINTER_SAFETY__ << endl;
#else
    cout << "__STDCPP_STRICT_POINTER_SAFETY__ is not defined" << endl;
#endif
#ifdef __STDCPP_THREADS__
    cout << "__STDCPP_THREADS__ is " << __STDCPP_THREADS__ << endl;
#else
    cout << "__STDCPP_THREADS__ is not defined" << endl;
#endif
}