Implicit sharing iterator problem of Qt

README.md

Implicit sharing iterator problem of Qt

• Example is based on Qt online help.

implicit-sharing-iterator-problem-of-qt.cpp

// implicit-sharing-iterator-problem-of-qt.cpp
// 
// codehubgist from j2doll

#include <QDebug>
#include <QtGlobal>
#include <QCoreApplication>
#include <QVector>

#include <cstdio>
#include <iostream>
#include <vector>

#define p1(param1) printf( "[%d] "#param1" = %d\n", __LINE__, param1 );
#define p2(param1, param2) printf( "[%d] "#param1" = %d, "#param2" = %d\n", __LINE__, param1, param2 );

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    // implicit sharing iterator problem of Qt

    {   
        // STL
        using namespace std;

        vector<int> d, e;
        d.resize(10);
        vector<int>::iterator j = d.begin();

        e = d; // operator=

        p2( d[0] , e[0] ); // d[0] = 0, e[0] = 0

        (*j) = 4; // d[0] is 4. but e[0] is not 4.(it's 0.)
        p2( d[0] , e[0] ); // d[0] = 4, e[0] = 0

        d[0] = 5;
        e.clear();

        int f = *j;
        p1( f ); // f = 5
    }

    {   
        // Qt
        QVector<int> a, b;
        a.resize(10);
        QVector<int>::iterator i = a.begin();

        b = a; // operator=

        (*i) = 4; // a[0] and b[0] is four. they are shared data.
        p2( a[0] , b[0] ); // a[0] = 4, b[0] = 4

        a[0] = 5; // container 'a' is detached from shared data.
        p2( a[0] , b[0] ); // a[0] = 5, b[0] = 4

        (*i) = 6; // iterator 'i' only point to container 'a'
        p2( a[0] , b[0] ); // a[0] = 6, b[0] = 4

        b.clear();

        int c = *i;
        p1( c ); // c = 6
    }    

    return 0;
}