Can I define before usage - c++

Question

I have cpp and h of class file. Class implementation contains wcsncpy function that only might be used if defined _CRT_SECURE_NO_WARNINGS. It works if I place #define _CRT_SECURE_NO_WARNINGS before all header inclusions. But why I can't place this line after headers or event just before wcsncpy usage?
UPD
According to my understanding I can do #define in code:
int _tmain(int argc, _TCHAR* argv[])
{
#define a
#ifdef a
std::wcout << _T("hello") << std::endl;
#endif
}
Why definition below not works?
int main(void)
{
...
#define _CRT_SECURE_NO_WARNINGS
wcsncpy(dest, src, 4);
...
}

Answer 1

You can suppress a warning (even this one is treated as error by default) locally with pragma warning:
#pragma warning(push)
#pragma warning(disable: 4996) // 'wcsncpy': This function or variable may be unsafe.
wcsncpy(dest, src, 4);
#pragma warning(pop)
It is even possible to wrap this in more readable macros with Microsoft-specific __pragma keyword:
#define DISABLE_CRT_SECURE_WARNINGS() \
__pragma(warning(push)) \
__pragma(warning(disable: 4996))
#define RESTORE_CRT_SECURE_WARNINGS() \
__pragma(warning(pop))
...
DISABLE_CRT_SECURE_WARNINGS();
wcsncpy(dst, src, 4);
RESTORE_CRT_SECURE_WARNINGS();