C ++ Function Learning: c_str ()

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const char* c_str ( ) const;

String: c_strpublic member function
const char* c_str ( ) const;
Get C string equivalent
Generates a null-terminated sequence of characters (c-string) with the same content as the string object and returns it as a pointer to an array of characters.
A terminating null character is automatically appended.
The returned array points to an internal location with the required storage space for this sequence of characters plus its terminating null-character, but the values in this array shoshould not be modified in the program and are only guaranteed to remain unchanged until the next call to a non-constant member function of the string object
 
The standard header file <cstring> contains a function library that operates C-strings. These library functions express almost every string operation we want to use. When a library function is called, the client program provides a string type parameter, while the library function uses a C-string internally. Therefore, the string object must be converted to a char * object, c_str () provides such a method that returns the pointer to the character array of the const char * type (readable and unchangeable. Example:
# Include <iostream>
# Include <string. h>
Using namespace STD;
Int main (void ){
String add_to = "Hello! ";
Const string add_on = "baby ";
Const char * cfirst = add_to.c_str ();
Const char * csecond = add_on.c_str ();
Char * Copy = new char [strlen (cfirst) + strlen (csecond) + 1];
Strcpy (copy, cfirst );
Strcat (copy, csecond );
Add_to = copy;
Cout <"Copy:" <copy <Endl;
Delete [] copy;
Cout <"add_to:" <add_to <Endl;
Return 0;
}
 
Output:
Copy: Hello! Baby
Add_to: Hello! Baby
 
Note:
The _ STR () function returns a pointer to a regular C string with the same content as this string.
This is to be compatible with the C language and has no string type in the C language. Therefore, you must use the member function c_str () of the string class object to convert the string object to the string style in C.
Note: Use the strcpy () function to operate the pointer returned by the c_str () method.
For example, it is best not:
Char * C;
String S = "1234 ";
C = S. c_str ();
// C finally points to the content of spam, because the S object is destructed and its content is processed (correct: the structure of the S object is carried out after the assignment operation is completed on the pointer C, so there is no error)
 
Cause of error prompted in VC ++ 2010: error: the "const char *" type value cannot be allocated to a "char *" type object.
It should be used as follows:
Char C [20];
String S = "1234 ";
Strcpy (C, S. c_str ());
In this way, no error occurs. c_str () returns a temporary pointer and cannot be operated on.