Overloaded operators

Overloaded operators

Restrictions

• Only existing operators can be overloaded.
• Operators must be overloaded on a class or enumeration type

• Overloaded operators must

  • Preserve number of operands
  • Preserve precedence
• Just a function with an operator name!
  • Use the operator keyword as a prefix to name operator *(...)

Member Functions

• Implicit first argument
• Developer must have access to class definition
• Members have full access to all data in class
• No type conversion performed on receiver(The number before the operator is receiver)

The prototypes of operators

• +-*/%^&|~
const T operatorX(const T& l, const T& r) const;
• ! && || < <= == >= >
bool operatorX(const T& l, const T& r) const;
• []
T& T::operator[](int index)
  .........

operator ++ and --

• postfix forms take an int argument -- compiler will pass in 0 as that int

class Integer{
public:
    const Integer& operator++(); //prefix++
    const Integer operator++(int); //postfix++
    const Integer& operator--(); //prefix--
    const Integer operator--(int); //postfix--
};

const Integer& Integer::operator++(){
    *this += 1; 
    return *this;
}

//int argument not used so leave unnamed so won't get compiler warnings
//int参数未使用，因此保留未命名，因此不会收到编译器警告
const Integer Integer::operator++(int){
    Integer old(*this);
    ++(*this);
    return old;
}

Relational operators

• implement != in terms of ==
• implement >, >=, <= in terms of <

class Integer{
public:
    
    bool Integer::operator==( const Integer& rhs ) const;
    bool Integer::operator!=( const Integer& rhs ) const;
    bool Integer::operator< ( const Integer& rhs ) const;
    bool Integer::operator> ( const Integer& rhs ) const;
    bool Integer::operator>=( const Integer& rhs ) const;
    bool Integer::operator<=( const Integer& rhs ) const;
};

• Function prototype：

bool Integer::operator==( const Integer& rhs ) const{
    return i == rhs.i;
}

//implement lhs != rhs in terms of !(lhs == rhs)
bool Integer::operator!=( const Integer& rhs ) const{
    return !(*this == rhs);
}

bool Integer::operator< ( const Integer& rhs ) const{
    return i < rhs.i;
}

//implement lhs > rhs in terms of lhs < rhs
bool Integer::operator> ( const Integer& rhs ) const{
    return rhs < *this;
}

//implement lsh <= rhs in terms of !(lhs < rhs)
bool Integer::operator<=( const Integer& rhs ) const{
    return !(rhs < *this);
}

//implement lsh >= rhs in terms of !(lhs < rhs)
bool Integer::operator>=( const Integer& rhs ) const{
    return !(*this < rhs);
}

从函数原型可知，6个关系运算符都由小于和等于演变而成，若要修改较为方便。

operator []

• Must be a member function
• Single argument
• Implies that the object it is being called for acts like an array, so it should return a reference

operator =

T& T::operator=(const T& rhs){
    //check for self assignment
    if(this != &rhs){
        //perform assignment
    }
    return *this;
}

• For classes with dynamically allocated memory declare an assignment operator (and a copy constructor)
• To prevent assignment, explicitly declare operator= as private

Value classes

• new key word : explicit

class One{
public:
    One(){}
};

class Two{
public:
    //Two(const One&) {}
    explicit Two(const One&) {}
};

void f(Two) {}

int main()
{
    One one;
    //f(one); No auto conversion allowed
    f(Two(one)); //OK -- user performs conversion
}

examples:

实现自己的MyString类（源码）https://github.com/Mered1th-Wang/Cpp-Learning/tree/master/20190520/MyString

Reference:

面向对象程序设计-C++