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 ?

You need to update your question to show a minimal test case. — Keith Thompson
– Keith Thompson, Commented Jul 20, 2015 at 3:42
It'll be due to the ordering of dependent headers but we can only guess because you're not showing any of your source. — Andy Brown
– Andy Brown, Commented Jul 20, 2015 at 8:36

Community · Accepted Answer · 2017-05-23 12:29:48Z

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.

edited May 23, 2017 at 12:29

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answered Jul 20, 2015 at 3:36

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Comments

Keith Thompson · Accepted Answer · 2015-07-20 03:42:15Z

0

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.

edited Jul 20, 2015 at 3:42

Keith Thompson

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answered Jul 20, 2015 at 3:27

mock_blatt

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1 Comment

mock_blatt Over a year ago

Hm, size_t should definitely be defined in cstring: en.cppreference.com/w/cpp/types/size_t - what compiler are you using? Is this relevant? stackoverflow.com/questions/24407193/…

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