How to call constructor of objects contained in a std::vector?

Question

When I create a std::vector of objects, the constructor of these objects is not always called.

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

struct C {
    int id;
    static int n;
    C() { id = n++; }   // not called
//  C() { id = 3; }     // ok, called
};

int C::n = 0;


int main()
{
    vector<C> vc;

    vc.resize(10);

    cout << "C::n = " << C::n << endl;

    for(int i = 0; i < vc.size(); ++i)
        cout << i << ": " << vc[i].id << endl;  
}

This is the output I get:

C::n = 1
0: 0
1: 0
2: 0
...

This is what I would like:

C::n = 10
0: 0
1: 1
2: 2
...

In this example, am I forced to resize the vector and then initialise its elements "manually"?
Could the reason be that the elements of a vector are not initialised in an ordered way, from the first to the last, and so I cannot obtain a deterministic behaviour?

What I would like to do, is to easily count the number of objects created in a program, in different containers, in different points of the code, and to give a single id to each of them.

Thank's!

Answer 1

the constructor of these objects is not always called.

Yes, it is, but it's not the constructor you think. The member function resize() is actually declared like this:

void resize(size_type sz, T c = T());

The second parameter is the object to copy into each of the newly inserted elements of the vector. If you omit the second parameter, it default constructs an object of type T then copies that object into each of the new elements.

In your code, a temporary C is constructed and the default constructor is called; id is set to 0. The implicitly declared copy constructor is then called ten times (to insert ten elements into the vector), and all of the elements in the vector have the same id.

[Note for those who are interested: in C++03, the second parameter of resize() (c) is taken by value; in C++0x it is taken by const lvalue reference (see LWG Defect 679)].

In this example, am I forced to resize the vector and then initialise its elements "manually"?

You can (and probably should) insert the elements into the vector individually, e.g.,

std::vector<C> vc;
for (unsigned i(0); i < 10; ++i)
    vc.push_back(C());