C++ reference

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C++ reference

C++ references
Reference examples
References themselves can't be changed
Reference to a member

1. C++ references

A C++ reference is like a pointer, except:
- Usage syntax is like an object
- References always refer to something
- References can't be reassigned to something else
This is enforced by:
- Requiring initialization in case of variables
- The implicit reference initialization in case of function parameters
No syntax exists to changing the referent (the object that a reference is pointing to)
Once instantiated, the reference becomes eternal synonym for an object or a variable

2. Reference examples


void swap( int& left, int& right )
{
    int temp = left;
    left = right;
    right = temp;
}

void f()
{
    int x = 8;
    int i = 4;
    int& ri = i;
    ++ri;          // now i == 5
    ri = x;        // now i == 8
    int& ref2 = 2; // error, RHS must be an lvalue
}

3. References themselves can't be changed

Given:


    int i = 4;
    int x = 8;
    int& r = i;

Silly attempts to get r to refer to x instead of i:


    r = x;      // NO: now i == 8
    int& r = x; // NO: variable redefinitions are illegal
    r = &x;     // NO: type mismatch, r not a pointer-type
    r& = x;     // NO: plain syntax error
    &r = &x;    // NO: left operand must be l-value

Conclusion: r refers to i for r's entire life.

4. Reference to a member


// point.h
class Point
{
public:
    int& refx();
private:
    int m_x;
    int m_y;
};

// point.cpp
#include "point.h"
int& Point::refx()
{
    return m_x;
}

// main.cpp
#include "point.h"
int main()
{
    Point pt;
    pt.refx() = 14;
    return 0;
}

Returning a reference to a class member creates a "trapdoor" or "backdoor" that allows the data to be modified outside of the class regardless whether it is public or private.
Trapdoor is a secret way of gaining access to a program, online service, or other resource.