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    <p>The interesting question is really what does it mean for an iterator to be invalidated. And I truly don't have a good answer from the standard. What I do know is that to some extent the standard considers an iterator not as a pointer to a location inside the container, but rather as a proxy to a particular element that lives within the container.</p> <p>With that in mind, after erasing of a single element in the middle of a vector, all iterators after the point of removal become <em>invalidated</em> as they no longer refer to the same element that they referred to before.</p> <p>Support for this line of reasoning comes from the iterator invalidation clauses of other operations in the container. For example, on <code>insert</code>, the standard guarantees that if there is no reallocation the iterators <em>before</em> the point of insertion remain valid. <em>Exceptio probat regulam in casibus non exceptis</em>, it invalidates all iterators <em>after</em> the point of insertion.</p> <p>If the validity of iterators was only related to the fact that there is an element of the container where the iterator points, then <em>none</em> of the iterators would be invalidated with that operation (again, in the absence of reallocations). </p> <p>Going even further in that line of reasoning, if you consider iterator validity as <em>pointer validity</em>, then none of the iterators into a vector would be invalidated during an <code>erase</code> operation. The <code>end()-1</code> iterator would become non-dereferencable, but it could remain valid, which is not the case.</p>
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    1. POWhy does vector::pop_back invalidate the iterator (end() - 1)?
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    1. USDavid Rodríguez - dribeas
    1. USDavid Rodríguez - dribeas
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    1. POWhy does vector::pop_back invalidate the iterator (end() - 1)?
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    1. COWell, if the meaning of "invalidation" is unclear, here's another way to approach the question: if I have `i2 = v.end() - 1;` `i5 = v.end();` `v.pop_back();`, would subsequently saying `i2 == v.end()` produce undefined behavior, or would it be `true`? What about saying `i5 == v.end()` -- is that undefined or is it `false`?
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      1. USMehrdad
    2. COJust for my understanding: As we know, the Standard defines what it means when an iterator is _invalid_. Your point is, while _invalid_ is defined, the meaning of the verb _invalidate_ is not explicitly defined. Correct?
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      1. USjogojapan
    3. CO@jogojapan: Can you point me to a definition of what a *valid* or *invalid* iterator is in the standard? As far as I understand it, there is no such definition, only a some conditions that must hold true for *valid* iterators (which does not mean that they cannot also hold for invalid ones, but the standard makes no such claim)
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      1. USDavid Rodríguez - dribeas
 

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