Question

could somebody please explain why this happens? I have written the following:

1) a WindowManager class which is implemented as a singleton and has an Instance() method, defined like so:

static WindowManager * instance_;
//...
WindowManager * WindowManager::instance_ = 0;
WindowManager & WindowManager::Instance()
{
    if (!instance_)
        instance_ = new WindowManager();
    return *instance_;
}

2) a WindowManager::createWindow method that returns a reference to a newly created window, defined like so:

Window & WindowManager::createWindow()
{
    windows_.push_back(Window());
    return windows_.at(windows_.size() - 1);
}

3) a Window::print method that prints a message inside the window

In my main program, I have written the following:

ui::Window & win1 = ui::WindowManager::Instance().createWindow();
ui::Window & win2 = ui::WindowManager::Instance().createWindow();
win1.print("First window");
win2.print("Second window");

This does not work! Only the second call to print is executed (for win2).
However, if I change the order, like so:

ui::Window & win1 = ui::WindowManager::Instance().createWindow();
win1.print("First window");
ui::Window & win2 = ui::WindowManager::Instance().createWindow();
win2.print("Second window");

then everything works as expected. If anybody could shed some light on this situation, any help would be greatly appreciated.

Answer 1

Here you have quick example which illustrates the problem:

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

struct T
{
    int id;
    T(int id) : id(id)
       { cout << "created " << id << endl; }
    T(T const& t) : id(t.id)
       { cout << "copy: " << t.id << endl; }
    void print(char const* m)
       { cout << id << ": " << m << endl; }
};

vector<T> ts;

T& create(int id)
{
    ts.push_back(T(id));
    return ts.at(ts.size() - 1);
}

int main()
{
    // Uncomment these lines and compare results
    //int const max_windows = 10;
    //ts.reserve(max_windows);

    T& t1 = create(1);
    T& t2 = create(2);
    t1.print("t1");
    t2.print("t2");
}

Compile and run as it is and see what is printed to stdout.
Then uncomment reserve() call, compile and run once again and compare with previous output.

The problem is that std::vector::push_back causes reallocation of the data internally. This invalidates all references, pointers or iterators to vector elements you’ve obtained before the reallocation.