Safe string copy in C/C++

I wrote this string copy routine for C strings. It is supposed to behave like strlcpy, that is - null terminate the destination if size > 0, and return the length of the source string.

However I also want the function to fail if either the source or destination pointer is null, and somehow notify this to the caller. But I can not think 开发者_开发技巧of a very elegant way to do this. Right now I send two negative values as size to denote that the source or the destination pointer points to null. Hence I changed the return type from size_t to a signed integer, and I am not happy with this interface. What would be a better interface?

  #include <cstddef> // size_t
  #include <cstdint> // 32 bit int

  const std::int32_t SRC_NULL = -1;
  const std::int32_t DST_NULL = -2;

  std::int32_t CopyStringn (char *dest, const char *src, std::size_t size) {
     const char* temp (src);
     if (temp == NULL)
         return SRC_NULL;
     if (dest == NULL)
         return DST_NULL;
     while (*temp) {
        if (size > 1) {
           *dest++ = *temp;
           --size;
        }
        ++temp;
     }

     if (size)
         *dest = '\0';

     return static_cast<std::int32_t> (temp - src); // Length does not include null
}

In C++, you can throw an exception.

Magic return values are rarely a good idea. I expect such a function to tell me how many chars would be copied, and that is what should be returned. If src or dest is NULL, you copy 0 characters, return 0.

Alternatively, you can choose to return either 1/true if everything was copied and properly 0-terminated, and 0/false otherwise.