c++ object member destructor is called and member exist with default values

Question

I have a B class and it has a class C member. B objects are stored in a vector. Full Code and output as follows:

#ifndef B_H
#define B_H

#include "C.h"

class B
{
public:
    B( int = 0 );           // constructor
    B( const B& );  // copy constructor
    ~B();           // destructor
    const B &operator=( const B & );    // assignment operator
    C c();          // return C object
    void val( int );// set val
    int val() const;// return val
private:
    C _c;           // C object
    int _val;       // a value
};

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

using namespace std;

B::B( int val )
{
    _val = val;

    cout << "B constructor called" << endl;
}

B::B( const B &other )
{
    _c = other._c;

    cout << "B copy constructor called" << endl;
}

B::~B()
{
    cout << "B destructor called" << endl;
}

const B &B::operator=( const B &other )
{
    if ( &other != this ) {     // avoid self-assignment

        _c = other._c;
    }

    return *this;
}

C B::c()
{
    return _c;
}

void B::val( int val )
{
    _val = val;
}

int B::val() const
{
    return _val;
}

#ifndef C_H
#define C_H

class C
{
public:
    C();            // constructor
    ~C();           // destructor
    void val( int );// set val
    int val() const;// return val
private:
    int _val;       // a value
};

#endif

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

using namespace std;

C::C()
{
    _val = 0;

    cout << "C constructor called" << endl;
}

C::~C()
{
    cout << "C destructor called" << endl;
}

void C::val( int val )
{
    _val = val;
}

int C::val() const
{
    return _val;
}

#include <vector>
#include <iostream>
#include "B.h"

using namespace std;

const int n = 5;

vector<B> v( n );

int main()
{
    for ( int i = 0; i < n; i++ )
        v[i] = B( i );

    for ( int i = 0; i < n; i++ )
        B &b = v[i];


    for ( int i = 0; i < n; i++ ) {

        B &b = v[i];

        b.c().val( 5 );
    }

    for ( int i = 0; i < n; i++ ) {

        B &b = v[i];

        cout << b.c().val() << endl;
    }

    cin.ignore( 1 );

    return 0;
}

output:

C destructor called
C destructor called
C destructor called
C destructor called
C destructor called
0
C destructor called
0
C destructor called
0
C destructor called
0
C destructor called
0
C destructor called

problem: at each cycle C object is destructed and b.c().val() returns with default ( zero ) value.

question: why?

output is belong to last two for.

And... what problem are you having? Your "question" as it stands is nothing more than a statement — Bojangles, Feb 06 '13 at 14:49
**at each cycle C object is destructed and b.c().val() returns with default ( zero ) value.** — Kun, Feb 06 '13 at 14:50
please provide a minimal example that actually compiles i.e. get rid of the #include "B.h" stuff and provide a link to an [online compiler](http://liveworkspace.org) with your example — TemplateRex, Feb 06 '13 at 14:54
Re-posting question after it got closed will rather harm you than help you. — Tadeusz Kopec for Ukraine, Feb 06 '13 at 15:07
trolling rather than answering question will harm you than help you — Kun, Feb 06 '13 at 15:40

score 2 · Answer 1 · answered Feb 06 '13 at 14:52

2

Your B::c member function returns a C by value. That is, it copies the _c object that is being held by the B object out of the function. You are calling val on that copy and do not affect the member _c.

Consider the line:

b.c().val( 5 );

b.c() returns a temporary copy of b's _c object. You then call val on that temporary. Then the temporary is destroyed at the end of the statement. It is this temporary that is being destroyed in each iteration. That temporary is outputting C destructor called.

If you want to affect the member _c, you'll need to return it by reference:

C& B::c()
{
    return _c;
}

answered Feb 06 '13 at 14:52

Joseph Mansfield

108,238
20
242
324

thank you very much. I'm very new to C++ and sorry for wasting your time – Kun Feb 06 '13 at 14:58
@Kun I don't find posting on Stack Overflow a waste of my time. I enjoy doing it. – Joseph Mansfield Feb 06 '13 at 15:03

score 0 · Answer 2 · answered Feb 06 '13 at 14:52

0

Your problem lies here:

C c();          // return C object

In the B object. Try to understand what this is doing.

answered Feb 06 '13 at 14:52

Nim

33,299
2
62
101

c++ object member destructor is called and member exist with default values

2 Answers2