Is STL Vector calling a destructor of a not-allocated object?

Question

The folowing code shows an output not expected:

class test
{
    public:
    test()
    {
        std::cout << "Created" << (long)this << std::endl;
    }
    ~test()
    {
        std::cout << "Destroyed" << (long)this << std::endl;
    }
};

int main(int argc, char** argv)
{
    std::vector<test> v;
    test t;
    v.push_back(t);

    return EXIT_SUCCESS;
}

When executed it shows:

Created-1077942161
Destroyed-1077942161
Destroyed674242816

I think the second "Destroyed" output should not be there. When I don't use the vector the result is one Created and one Destroyed line as expected. Is this behavior normal?

(This is compiled with GCC on a FreeBSD system)

Answer 1

Everything is as it should be: there's the local variable t, which gets created and then destroyed at the end of main(), and there's v[0], which gets created and destroyed at the end of main().

You don't see the creation of v[0] because that happens by copy or move constructor, which your test class doesn't provide. (So the compiler provides one for you, but without output.)

---

For testing purposes it's handy to write for yourself once and for all a test class that contains all the possible constructors, destructors, assignment and swap operators and prints a diagnostic line in each, so you can witness how objects behave when used in containers and algorithms.

Answer 2

#include <cstdlib>
#include <vector>
#include <iostream>

class test
{
    public:
    test()
    {
        std::cout << "Created " << (long)this << std::endl;
    }
    test( const test& )
    {
        std::cout << "Copied " << (long)this << std::endl;
    }
    ~test()
    {
        std::cout << "Destroyed " << (long)this << std::endl;
    }
};

int main(int argc, char** argv)
{
    std::vector<test> v;
    test t;
    v.push_back(t);

    return EXIT_SUCCESS;
}

Output:

Created -1076546929
Copied 147865608
Destroyed -1076546929
Destroyed 147865608

std::vector::push_back copies the t object, you can see the copy constructor being invoked by the above code.