Difference Between String.H and Cstring

Difference between string and string.h?

• <string.h> contains old functions like strcpy, strlen for C style null-terminated strings.
• <string> primarily contains the std::string, std::wstring and other classes.

Difference between cstring and string

The cstring header provides functions for dealing with C-style strings — null-terminated arrays of characters. This includes functions like strlen and strcpy. It's the C++ version of the classic string.h header from C.

The string header provides the std::string class and related functions and operators.

The headers have similar names, but they're not really related beyond that. They cover separate tasks.

Including standard header files. string.h or cstring? or both?

<cstring> is newer; <string.h> is really there for backwards compatibility (and for C, of course). The difference is that <cstring> puts the string functions in the std namespace, while <string.h> puts them in the global namespace.

In addition, <cstring> changes the types of certain functions to promote type-safety. E.g., the C declaration

char *strchr(char const *, int);

is replaced by the overloads (in the std namespace)

char       *strchr(char       *, int);
char const *strchr(char const *, int);

In the case of <cmath> there are further differences with <math.h> which make <cmath> more idiomatic and less C-like.

Prefer <cstring> for new code and use the std:: prefix on the functions.

Difference between string.h and strings.h

strings.h comes from the BSD branch in the unix evolution. Its content has been standardized by POSIX, but most of it is marked as legacy and can be easily replaced with other functions:

int    bcmp(const void *, const void *, size_t); /* LEGACY, see memcmp */
void   bcopy(const void *, void *, size_t); /* LEGACY, see memcpy, memmove */
void   bzero(void *, size_t); /* LEGACY, see memset */
int    ffs(int);
char  *index(const char *, int); /* LEGACY, see strchr */
char  *rindex(const char *, int); /* LEGACY, see strrchr */
int    strcasecmp(const char *, const char *);
int    strncasecmp(const char *, const char *, size_t);

include string or string.h

<string> is a C++ standard library include, and <string.h> is C standard library include.

The equivalent of <string.h> in C++ is <cstring>, although both will work.

The difference is: <cstring> wraps everything in the std namespace whereas <string.h> puts everything in the global namespace.

Also, expect some stricter type safety rules from <cstring>.

string.h or string?

Those are two different headers.

• <string> is for c++ std::string class
• <string.h> is for c string functions (like strlen(), etc.), which should be <cstring> for c++ project (this is the third, you didn't know of).

How to use functions of strings.h like strcmp() with object of string.h in c++

strcmp operates on c-strings, not std::strings.

To access the internal c-string of an std::string, use the c_str() method.

i.e.

if (strcmp(a.c_str(), b.c_str()) == 0) {
 // ...
}

But you can also do the comparison directly

if (a == b) {
  // ...
}

Difference between strcmp of string.h and my own implementation of strcmp

The implementation of the "real" strcmp on your platform is most likely close to this code:

int strcmp(const char *s, const char *t) {
    for(; *s == *t; s++, t++) {
        if (*s == '\0') {           // are we at the end ?
            return 0;               // yes
        }
    }
    return (*s-*t) > 0 ? 1 : -1;   // return either +1 or -1
}

BTW: it should be int strcmp(const char *s, const char *t) instead of int strcmp(char *s, char *t)

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