Classes C++ (I)
User-Defined Data Types
Some programs can be written using only the built-in C++ data types Most programs create new data types that more effectively model the problem being solved Structs, enums, typedefs can be used to define new data types Classes can be used to create new data types that are fully integrated into the C++ language
User-Defined Data Types
// BigInteger supports arbitrary-precision integers class BigInteger { }; int main() { BigInteger x("123451234512345"); BigInteger y(789L); BigInteger z = -((x + y)/2); if (z < x) { ++z; } cout << z << endl; }
Data Type Operations
Before learning how to create new types with classes, let's look at how the built-in types are used Declare with no initial value
float f;
Initialize with value of the same type
float f = 123.45f;
Initialize with value of a different type
float f = 789;
Assign to value of the same type
f = 123.45f;
Data Type Operations
Assign to value of a different type
f = 789;
Convert to a different type
double d = f;
Apply operators
(f * g)
Print to output stream
cout << f ;
Deinitialize Goes out of scope Deallocated with delete Defining a new class involves specifying how all of these operations work on objects of the new class
String Class Example
Let's define a class named String that is similar to the standard C++ string class
class String { }; void main() { String byu("BYU"); String gatech; gatech.Append("Georgia Tech"); String msg = gatech.Concat(" threw an interception."); msg.Write(cout); msg = byu.Concat(" kicked a field goal."); msg.Write(cout); for (int x=0; x < msg.Length(); ++x) { cout << msg.GetChar(x); } }
class String { private: char * str; };
Instance Variables
class String {
Constructors
public: String() { Init(""); } String(const char * s) { Init(s); } String(const String & s) { Init(s.str); } private: void Init(const char * s) { str = new char[strlen(s) + 1]; strcpy(str, s); } };
class String { public: ~String() { Free(); } private: void Free() { delete [] str; str = 0; } };
Destructor
class String {
Assignment Operators
public: String & operator =(const char * s) { Free(); Init(s); return *this; } String & operator =(const String & s) { if (&s this) != { Free(); Init(s.str); } return *this; } };
class String {
Instance Operations
public: bool IsEmpty() const { return (Length() == 0); } const char * CString() const { return str; } int Length() const { return strlen(str); } };
class String {
Instance Operations
public: char GetChar(int index) const { return str[index]; } void SetChar(int index, char c) { str[index] = c; } };
class String {
Instance Operations
public: void Append(const char * s) { char * newStr = new char[strlen(str) + strlen(s) + 1]; strcpy(newStr, str); strcat(newStr, s); Free(); str = newStr; } void Append(const String & s) { Append(s.str); } };
class String {
Instance Operations
public: String Concat(const char * s) const { String result(str); result.Append(s); return result; } String Concat(const String & s) const { return Concat(s.str); } };
class String {
Instance Operations
public: void Write(ostream & s) const { s << str; } };
Tracking the Number of String Objects
String * UseStrings() { String a[5]; String b("a string"); cout << String::ObjectCount() << endl; String * c = new String("another string"); cout << String::ObjectCount() << endl; return c; } void main() { cout << String::ObjectCount() << endl; String * p = UseStrings(); cout << String::ObjectCount() << endl; delete ...