Details and implementation of string functions --- strcpy () and strncpy ()

1. strcpy () and strncpy ()

Strcpy (): strcpy (DEST, Src); strcpy copies the string pointing to SRC ending with '\ 0' to the array indicated by DEST, and returns the pointer pointing to DeST.

When sizeof (DEST)> = sizeof (SRC), the copy is correct and '\ 0' is added to the end of the Dest string ';

When sizeof (DEST) <sizeof (SRC), an error occurred while copying.

 

Strncpy (): strncpy (DEST, SRC, n); strncpy copies the first n characters of the string that SRC points to ending with '\ 0' to the array specified by DeST, returns the pointer to DeST.

When N> = sizeof (SRC), the copy is correct and '\ 0' is added to the end of the Dest string ';

When n <sizeof (SRC), only the first n-1 strings of SRC are copied to DEST, and '\ 0' is not added to the end of the Dest string ';

 

Sample Code:

# Include <iostream> # include <string> using namespace STD; int main () {char a [] = "lanzhihui is a good boy! "; // The following strcpychar B [30]; char C [30]; char d [30]; strcpy (B, A); // strcpy for simple copying, in addition to copying the string of array A, the '\ 0' following array A is also copied to array B. cout <"B:" <B <Endl; // when the storage space sizeof (B)> = sizeof (A) of array B is copied correctly, add '\ 0' after array B string '. // An error occurred while copying the storage space sizeof (B) <sizeof (A) of array B. Cout <"strlen (c):" <strlen (strcpy (D, strcpy (C, A) <Endl; // copy A to C, strcpy returns the c Address, copies C to D, returns the D address, and returns the length of the string D. // This is the reason why the target string address is returned after the strcpy () function is copied. It supports chained expression. Cout <"C:" <C <Endl; cout <"D:" <D <Endl; // The following strncpychar E [10]; char f [30]; char G [30]; strncpy (F, A, sizeof (f); // sizeof (f)> = sizeof (, copy strncpy (G, A, 10); // copy the first nine characters in A to G, you must manually add '\ 0' G [9] =' \ 0' to the G array; // n <sizeof (, otherwise, the output fails. strncpy (E, A, sizeof (e); e [sizeof (E)-1] = '\ 0'; // sizeof (E) <sizeof (a), this statement must exist, otherwise the output has an error cout <"E:" <e <Endl; cout <"F: "<F <Endl; cout <" G: "<G <Endl; System (" pause "); Return 0 ;}

 

For example, the copy operation between strcpy () and strncpy () ends with '\ 0:

# Include <iostream> # include <string> using namespace STD; int main () {char s [] = "lanzhihui \ 0 is a good boy! "; Char name [30]; strcpy (name, S); // copy the cout ended with '\ 0' <name <Endl; // The output end strncpy (name, S, sizeof (name) with '\ 0'; // The copy end cout with' \ 0' <name <Endl; // The output ends system ("pause") with '\ 0'; return 0 ;}

II. Implementation of strcpy () and strncpy ()

Write the implementation code based on the test of strcpy () and strncpy () functions:

# Include <iostream> # include <string> # include <assert. h> using namespace STD; char * strcpy_m (char * DEST, const char * Str) {assert (DEST! = NULL) & (STR! = NULL); char * CP = DEST; while (* CP ++ = * STR ++ )! = '\ 0') {//} return DEST;} Char * strncpy_m (char * DEST, const char * STR, int N) {assert (DEST! = NULL) & (STR! = NULL); char * CP = DEST; while (N & (* CP ++ = * STR ++ )! = '\ 0') {n --;} If (n) {While (-- N) * CP ++ =' \ 0';} return DEST;} int main () {char a [] = "lanzhihui is a good boy! "; // The following strcpy_mchar B [30]; char C [30]; char d [30]; strcpy_m (B, A); cout <" B: "<B <Endl; cout <" strlen (c): "<strlen (strcpy_m (D, strcpy_m (C, A) <Endl; cout <"C:" <C <Endl; cout <"D:" <D <Endl; // The following strncpy_mchar E [10]; char f [30]; char G [30]; strncpy_m (F, A, sizeof (f); strncpy_m (G, A, 10 ); G [9] = '\ 0'; strncpy_m (E, A, sizeof (e); e [sizeof (E)-1] =' \ 0 '; cout <"E:" <e <Endl; cout <"F:" <F <Endl; cout <"G: "<G <Endl; System (" pause "); Return 0 ;}

It can be seen that the strcpy_m () function and strncpy_m () function are the same as the running result of the library function.

Details and implementation of string functions --- strcpy () and strncpy ()