Does Reserve Ensure the Validity of Vector References During Element Insertion?

Pushing an Element from the Same Vector

In certain vector operations, such as push_back, the internal capacity may need to be reallocated. This can raise concerns about the validity of references to previous elements in the vector.

One such concern arises when pushing an element from the same vector into itself:

<code class="cpp">vector<int> v;
v.push_back(1);
v.push_back(v[0]);</code>

After the second push_back operation, it's possible that the vector's memory allocation has been changed, rendering the reference to the first integer (i.e., v[0]) invalid.

Alternatively, consider the following code:

<code class="cpp">vector<int> v;
v.push_back(1);
v.reserve(v.size() + 1);
v.push_back(v[0]);</code>

By calling reserve before the second push_back, we're ensuring that the vector has sufficient capacity to accommodate the new element without reallocating. This guarantees that the reference to the first integer remains valid even after the operation.

Based on the proposed resolution to LWG defect report 526, it is understood that the first code example is valid because vector::push_back is required to work in such scenarios. However, to maintain the validity of references to previous elements, it is prudent to use reserve or alternative methods to avoid potential reallocations.

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