Why Does `vector::push_back` Seem to Call the Copy Constructor More Than Expected?

Vector's push_back: Uncovering the Hidden Copy Constructor Calls

In a confusing scenario, the push_back method of a vector seems to invoke the copy constructor more times than anticipated. Investigating this behavior, let's dive into the specifics.

Consider the provided code snippet:

<code class="cpp">class Myint
{
    //...
};

vector<Myint> myints;
Myint x;

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

Initially, x is pushed into the vector. As expected, one copy occurs during initialization. But upon examining the output, we discover that the copy constructor is invoked three times, not the expected two. What's causing this discrepancy?

The vector's internal behavior comes into play. When the vector runs out of space, it must reallocate memory. In our case, the second push_back triggers reallocation. Since no move constructor is implicitly defined for Myint, the copy constructor is utilized. Consequently, the first element is copied into the newly allocated memory, followed by a second copy of x. This additional copy explains the third invocation of the copy constructor, where x has my_int set to 1.

Hence, in total, three copy constructor calls occur. This number may vary depending on the implementation and initial vector capacity, but two calls are a minimum.

To mitigate this, consider reserving more memory in advance using the reserve method. This ensures sufficient capacity, avoiding unnecessary reallocation:

<code class="cpp">myints.reserve(2); // Allows two insertions without reallocation</code>

Additionally, the emplace_back method can be employed to elide copies:

<code class="cpp">myints.emplace_back(0); // Creates a new element directly in the vector</code>

Emplace_back takes arbitrary arguments forwarded to the constructor, bypassing copies or moves.

The above is the detailed content of Why Does `vector::push_back` Seem to Call the Copy Constructor More Than Expected?. For more information, please follow other related articles on the PHP Chinese website!