Why does the copy constructor of a `Myint` object get invoked multiple times when using `push_back()` on a `vector`?

Multiple Invocations of Copy Constructor during Vector's Push_back Operation

In C , when an element is added to a vector using the push_back() method, the copy constructor of that element type is typically employed. However, in certain scenarios, multiple invocations of the copy constructor may be observed.

Consider the following code snippet:

<code class="cpp">class Myint {
  private:
    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;
    }

    void set(const int& x) {
        my_int = x;
    }
};

vector<Myint> myints;
Myint x;

myints.push_back(x);
x.set(1);
myints.push_back(x);</code>

When this code is executed, the following output is generated:

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

This output indicates that the copy constructor is invoked four times, not just twice as expected. The reason for this behavior lies in the internal workings of vector's memory management.

When push_back() is called on a vector that has reached its internal capacity, the vector must be reallocated to accommodate the new element. During this reallocation, the existing elements are copied to the new memory location. This process results in an additional invocation of the copy constructor for each element.

To avoid this multiple copying behavior, some measures can be taken:

• Reserving sufficient capacity: By calling the reserve() method on the vector, the internal capacity can be increased in advance, preventing the need for reallocation.
• Using emplace_back(): This method allows for the direct construction of an element within the vector, eliminating the need for copying.

By employing such techniques, the unnecessary invocations of the copy constructor can be minimized, improving the efficiency of vector operations.

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