Sams Teach Yourself C++ in 21 Days

Advanced Inheritance 581

16

What you could not do, however, was create a C-style string (a character string) and add

to it using a string object, as shown in this example:

char cString[] = {“Hello”};

String sString(“ World”);

String sStringTwo = cString + sString; //error!

C-style strings don’t have an overloaded operator+. As discussed on Day 10, “Working

with Advanced Functions,” when you say cString + sString;what you are really call-

ing is cString.operator+(sString). Because you can’t call operator+()on a C-style

string, this causes a compile-time error.

You can solve this problem by declaring a friendfunction in String, which overloads

operator+but takes two string objects. The C-style string is converted to a string object

by the appropriate constructor, and then operator+is called using the two string objects.

To clarify this, take a look at Listing 16.8.

LISTING16.8 Friendly operator+

0: //Listing 16.8 - friendly operators

1:

2: #include <iostream>

3: #include <string.h>

4: using namespace std;

5:

6: // Rudimentary string class

7: class String

8: {

9: public:

10: // constructors

11: String();

12: String(const char *const);

13: String(const String &);

14: ~String();

15:

16: // overloaded operators

17: char & operator[](int offset);

18: char operator[](int offset) const;

19: String operator+(const String&);

20: friend String operator+(const String&, const String&);

21: void operator+=(const String&);

22: String & operator= (const String &);

23:

24: // General accessors

25: int GetLen()const { return itsLen; }

26: const char * GetString() const { return itsString; }

27:

28: private: