C++: When and why is the destructor called?

Question

I have a simple program to test whether my cross product of two 3D-Vectors works.

#include <iostream>
#include "math\Vec3.h"

using namespace std;

int main(int argc, char **argv)
{
    Vec3 v1(1, 23, 5);
    Vec3 v2(7, 3, 4);
    cout << "Crossing v1 and v2" << endl;
    Vec3 v3 = v1.cross(v2);
    cout << "crossed" << endl;
    return 0;
}

Why was the destructor called just after creating the variable?
Here is what it printed out:

Created: Vec3[1, 23, 5]
Destroy: Vec3[1, 23, 5]     // Why is the vector destroyed here?
Created: Vec3[7, 3, 4]
Destroy: Vec3[7, 3, 4]      // And here?
Crossing v1 and v2
Created: Vec3[77, 31, -158]
Destroy: Vec3[77, 31, -158] //And here??
crossed
Destroy: Vec3[77, 31, -158]
Destroy: Vec3[7, 3, 4]
Destroy: Vec3[1, 23, 5]

Process returned 0 (0x0)   execution time : 0.090 s
Press any key to continue.

Here is the Vec3.h:

#include <iostream>
#include <string>

struct Vec3
{
    float x, y, z;

    Vec3():
        x(0), y(0), z(0) { std::cout << "Created: " << *this << std::endl; };
    Vec3(float i, float j, float k):
        x(i), y(j), z(k) { std::cout << "Created: " << *this << std::endl; };

    //...

    double dot(const Vec3&);
    Vec3 cross(const Vec3&);


    friend std::ostream& operator<<(std::ostream&, const Vec3);

    //...

    ~Vec3();
};

Vec.cpp:

Vec3 Vec3::cross(const Vec3& v)
{
    return Vec3(y * v.z - z * v.y,
                z * v.x - x * v.z,
                x * v.y - y * v.x);
}

std::ostream& operator<<(std::ostream& out, const Vec3 v)
{
    out << "Vec3[" << v.x << ", " << v.y << ", " << v.z << "]";
    return out;
}

Vec3::~Vec3()
{
    std::cout << "Destroy: "
        << "Vec3[" << x << ", " << y << ", " << z << "]"
        << std::endl;
}

Answer 1

Your debug output (using operator<< ) causes a copy (because it takes 'Vec3' by value) and an additional destruction.

You don't provide a copy constructor, so you can't see that. But if you would, you would see that you do in fact not have more destructions than constructions.

Answer 2

Look at the output - you create a Vec3, then it gets destroyed, andf then right at the end the Vec3 gets destroyed... hmm, its obvious you have another Vec3 being created in the middle, and it is a copy of the Vec3 you wanted to create.

So that looks like the problem, you're confusing the output of Vec3s with a single object when something in your code is making a copy which in turn is destroyed. The reason this confusion happens is because you do not have a copy constructor defined that also prints out the 'created' line.

So, first adopt best-practice by adding a copy constructor to your code (hint: if you have 1 of constructor, destructor or copy-ctor, you should implement all 3. If you miss any out, the compiler will put one in for you).