编译错误：“stddef.h：没有这样的文件或目录”

每当我尝试编译此代码时，总是会出现以下错误：

    In file included from /usr/include/wchar.h:6:0,
             from /usr/lib/gcc/i686-pc-cygwin/4.9.2/include/c++/cwchar:44,
             from /usr/lib/gcc/i686-pc-cygwin/4.9.2/include/c++/bits/postypes.h:40,
             from /usr/lib/gcc/i686-pc-cygwin/4.9.2/include/c++/iosfwd:40,
             from /usr/lib/gcc/i686-pc-cygwin/4.9.2/include/c++/ios:38,
             from /usr/lib/gcc/i686-pc-cygwin/4.9.2/include/c++/ostream:38,
             from /usr/lib/gcc/i686-pc-cygwin/4.9.2/include/c++/iostream:39,
             from test.cpp:1:
    /usr/include/sys/reent.h:14:20: fatal error: stddef.h: No such file or directory
    #include <stddef.h>
                ^
    compilation terminated.

我试图编译的代码是：

#include <iostream>
using namespace std;

int main()
{
    cout << "Hello World! :D";
    return 0;
}

该错误是因为你的 gcc-core 包和 gcc-g++ 不是同一版本。要么降级其中一个来解决问题，要么更新这两个库。更新这两个库是推荐的方法。