Is Vector Push_Back Safe for Element References?

Vector Push_Back and Reference Validity

When inserting elements into a vector using push_back, it's crucial to consider the potential impact on references to existing elements. Here we examine the safety of push_back under specific conditions.

Consider the following example:

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

In this case, if the second push_back triggers a reallocation, the address of v[0] becomes invalid due to vector movement. This poses a potential safety issue.

To mitigate this, one can employ reserve:

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

In this modified code, the reserve ensures that the allocated memory is sufficient to accommodate the new element without reallocation, preserving the validity of references.

It is worth noting that the C standard has addressed concerns similar to this as potential defects. However, the resolution concluded that these operations are allowed behavior:

v.insert(v.begin(), v[2]);

The rationale is that the standard implicitly permits such operations to succeed, ensuring that vectors remain a reliable data structure for managing and manipulating large collections.

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