How Many Times Does the Copy Constructor Get Called During `push_back` Operations in a C Vector?

Understanding Vector's Push_back Copying Behavior

While working with vectors, developers often encounter queries regarding the frequency of copy constructor invocations during push_back operations. Let's delve into this behavior with an example:

Consider the following C code:

<code class="cpp">class Myint {
  int my_int;
public:
  Myint() : my_int(0) {
    cout << "Inside default" << endl;
  }
  Myint(const Myint& x) : my_int(x.my_int) {
    cout << "Inside copy with my_int = " << x.my_int << endl;
  }
};

int main() {
  vector<Myint> myints;
  Myint x;
  myints.push_back(x);
  x.set(1);
  myints.push_back(x);
}</code>

This snippet expectedly triggers the copy constructor twice during the push_back operations. However, upon execution, we observe the following output:

Inside default
Inside copy with my_int = 0
Inside copy with my_int = 0
Inside copy with my_int = 1

Why does the copy constructor appear to be invoked three times?

• Push_back insertion: The first push_back operation inserts x into the vector, invoking the copy constructor to initialize the newly created element.
• Vector reallocation: When the second element is push_back'd, the vector encounters its capacity limit and reallocates memory. As Myint lacks an implicitly defined move constructor, the copy constructor is again employed. This time, it duplicates both the first element (with my_int still at zero) and then x (with my_int updated to one).

Therefore, in total, the copy constructor is invoked three times. To optimize this behavior:

• Reserve memory in advance: By specifying a higher vector capacity, you can reduce the likelihood of reallocation and minimize copying.
• Use emplacement instead of push_back: myints.emplace_back(0) directly constructs a new element within the vector, avoiding extra copies.

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