evaluation strategy.cpp

/* by Dmitry A. Soshnikov */

#include "stdafx.h"
#include <iostream>

struct Object {
    int x;
    int y;
};

// call "by reference"
void call_by_reference(Object& oAlias) {
    oAlias.x = 100;
    oAlias.y = 200;

    // but if we assign a new value to oAlias
    // object outside will be modified also
    // Object tmp;
    // tmp.x = 1000;
    // tmp.y = 2000;
    // oAlias = tmp;
}

// call by pointer as a version of "by sharing"
void call_by_pointer(Object* oRefCopy) {

    // change properties, dereferencing the pointer
    (*oRefCopy).x = 100;

    // or via convinient "sugar", overloaded "->" operator
    oRefCopy->y = 200;

    // bind oRefCopy to the *new* memory block
    // assigning new address
    // it can be done also as:
    // Object tmp;
    // tmp.x = 1000;
    // tmp.y = 2000;
    // oRefCopy = &tmp;
    oRefCopy = new Object;
    
    // the cnages won't touch object outside
    // because oRefCopy references already other object
    oRefCopy->x = 1000;
    oRefCopy->y = 2000;

    // but if we would dereferenced oRefCopy pointer
    // and assign a new value to it (and *before* oRefCopy was bound to new address):
    // (*oRefCopy) = tmp;
    // then object outside will be changed
}

using namespace std;

int main()
{
    // test object
    Object o;

    o.x = 10;
    o.y = 20;

    // --------------call by reference ----------------

    // reference (alias)
    Object& oLocalAlias = o;

    // before (referencing to o) - 10, 20
    cout << oLocalAlias.x << endl << oLocalAlias.y << endl;

    // call by reference
    call_by_reference(o);
     
    // after:

    // via o directly - 100, 200
    cout << o.x << endl << o.y << endl;

    // and via reference alias - 100, 200
    cout << oLocalAlias.x << endl << oLocalAlias.y << endl;
    
    // --------------call by sharing ----------------

    // call by sharing (by pointer) - pass copy of address, see &
    call_by_pointer(&o);

    // 100, 200
    cout << o.x << endl << o.y << endl;
    // 100, 200
    cout << oLocalAlias.x << endl << oLocalAlias.y << endl;
    
    return 0;
}