Why Does `push_back` Invoke the Copy Constructor Multiple Times in a Vector?

Multiple Invocations of Copy Constructor in Vector Push-Back

In the given snippet, the push_back method of the vector object myints invokes the copy constructor more than twice, contradicting the initial expectation. This raises questions about the vector's internal behavior.

Explanation:

First Push-Back:

• A new element is inserted into the vector using x. This triggers one copy operation: The new element's initialization using the argument. Because x's default constructor initializes my_int to zero, the copy constructor reports zero.

Second Push-Back and Reallocation:

• The second element is push_back'd. However, the vector's capacity is exceeded, necessitating reallocation. Since Myint lacks an implicit move constructor, the copy constructor is used instead.
• The first element is copied into the new memory, retaining its zero value. Subsequently, x is copied to initialize the second element with my_int set to one, as reflected in the copy constructor's output.
• This results in three total copy constructor invocations.

The Number of Invocations:

• The number of copy constructor calls can vary with different implementations due to initial vector capacity. However, two calls are the minimum.
• Reallocation can be avoided, hence reducing copy operations, by increasing the vector's capacity in advance using reserve(2).

Reducing Copy Operations:

• Alternatively, the emplace_back method can be used for element insertion. It doesn't necessitate copies or moves, directly forwarding arguments to the element's constructor.

In summary, the push_back method can result in multiple copy constructor invocations due to internal vector restructuring. To avoid this, reserve() can be used to increase capacity or emplace_back can be employed for efficient element insertion without copies.

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