Question

Im working on an exercise in C++ but im getting unexpected output I hope someone can explain. The exercise asked that I make a class called rock which has a default constructor, a copy constructor and a destructor all of which announce themselves to cout.

In the main method I am to try and add members of this class to a vector by value:

vector<Rock> byValue;
Rock r1, r2, r3;
byValue.push_back(r1);
byValue.push_back(r2);
byValue.push_back(r3);
cout << "byValue populated\n\n";

The output I expected (and shown in the exercise solutions) is:

Rock()
Rock()
Rock()
Rock(const Rock&)
Rock(const Rock&)
Rock(const Rock&)
byValue populated

~Rock()
~Rock()
~Rock()
~Rock()
~Rock()
~Rock()

However the output I get is:

Rock()
Rock()
Rock()
Rock(const Rock&)
Rock(const Rock&)
Rock(const Rock&)
~Rock()
Rock(const Rock&)
Rock(const Rock&)
Rock(const Rock&)
~Rock()
~Rock()
byValue populated

~Rock()
~Rock()
~Rock()
~Rock()
~Rock()
~Rock()

Can anyone explain why there seems to be extra calls to the copy constructor and destructor?

Answer 1

When the vector gets resized, the elements have to be moved to their new location.

This is normal.

If you call

byValue.reserve(10);

before any calls to push_back, the extra copies should disappear.