Why including <string.h> in .cpp file causes compilation errors (while including it .h file is ok)

Question

I have a strange compilation error when using:

#include <string.h>

in .cpp file:

/usr/include/string.h:47:8: error: ‘size_t’ has not been declared
        size_t __n) __THROW __nonnull ((1, 2));
        ^
/usr/include/string.h:50:56: error: ‘size_t’ has not been declared
 extern void *memmove (void *__dest, const void *__src, size_t __n)
                                                        ^
/usr/include/string.h:59:18: error: ‘size_t’ has not been declared
         int __c, size_t __n)
                  ^
/usr/include/string.h:66:42: error: ‘size_t’ has not been declared
 extern void *memset (void *__s, int __c, size_t __n) __THROW __nonnull ((1));    /usr/include/string.h:47:8: error: ‘size_t’ has not been declared
        size_t __n) __THROW __nonnull ((1, 2));
        ^
/usr/include/string.h:50:56: error: ‘size_t’ has not been declared
 extern void *memmove (void *__dest, const void *__src, size_t __n)
                                                        ^
/usr/include/string.h:59:18: error: ‘size_t’ has not been declared
         int __c, size_t __n)
                  ^
/usr/include/string.h:66:42: error: ‘size_t’ has not been declared
 extern void *memset (void *__s, int __c, size_t __n) __THROW __nonnull ((1));
                                          ^
/usr/include/string.h:69:56: error: ‘size_t’ has not been declared
 extern int memcmp (const void *__s1, const void *__s2, size_t __n)
                                          ^
/usr/include/string.h:69:56: error: ‘size_t’ has not been declared
 extern int memcmp (const void *__s1, const void *__s2, size_t __n)

while putting the same line code in .h file, does not give any errors.

I'm confused, what is the problem ?

Answer 1

It is impossible for there to be a difference here between including the header from a .h and from a .cpp . Literally impossible. Next time, post a testcase (I cannot emphasise how important this is).

However, you are probably including some other header in your .cpp , which defines size_t , masking the bug. Yes, this is a bug. Standard headers should work in isolation and, apparently, on your implementation, this one doesn't.

However, since you should be including C++ <cstring> , not C <string.h> , it's moot.

If you do that and still have a problem, then you messed something up. And we cannot tell what that is without a testcase.

Answer 2

I don't know exactly why you'd be getting that error, but in general the use of <string.h> is deprecated for C++, so although it often works, you can't expect it to. <string> is the C++ header that contains the string class defined in the std namespace.

<string.h> contains the C string functions and definitions. In C++, if you want those older functions you should use <cstring> . I think if you mix things around in C++ you'll get namespace issues and such, which may be causing your error.