constructor and destructor when initializing sized vector

[jjleto]

When I run this simple program:

#include <vector>
#include <iostream>
using namespace std;

class A {
public:
A() { cout << "[ctor]" << endl; }
~A() { cout << "[dtor]" << endl; }

};

int main()
{
vector<A> v(3);
return 0;
}

I've got the following result:

[ctor]
[dtor]
[dtor]
[dtor]
[dtor]

(gcc 3.3.4)

How to explain that ? Should not the constructor be called 3 times and
the destructor also ?

Thanks.

 

[Ron Natalie]

When I run this simple program:

#include <vector>
#include <iostream>
using namespace std;

class A {
public:
A() { cout << "[ctor]" << endl; }
~A() { cout << "[dtor]" << endl; }

Add:
A(const A&) { cout << "[copy ctor]" << endl; }
A& operator=(const A&) { cout << "[op=]" <<endl; return *this; }

int main()
{
vector<A> v(3)

The constructor you are using above has a second defaulted argument
which is the value to fill the vector with. It is defaulted to a default
constructoed A object.

So you get:
ctor - default constructed object for second constructor parameter
copy ctor - v[0] being filled
copy ctor - v[1] being filled
copy ctor - v[2] being filled.

 

[jjleto]

Ron Natalie a écrit :

When I run this simple program:

#include <vector>
#include <iostream>
using namespace std;

class A {
public:
A() { cout << "[ctor]" << endl; }
~A() { cout << "[dtor]" << endl; }

Add:
A(const A&) { cout << "[copy ctor]" << endl; }
A& operator=(const A&) { cout << "[op=]" <<endl; return *this; }

int main()
{
vector<A> v(3)

The constructor you are using above has a second defaulted argument
which is the value to fill the vector with. It is defaulted to a default
constructoed A object.

So you get:
ctor - default constructed object for second constructor parameter
copy ctor - v[0] being filled
copy ctor - v[1] being filled
copy ctor - v[2] being filled.

Thanks. It is clear now.