Iterators are a generalization of pointers that allow a C++ program to work with different data structures (containers) in a uniform manner [ISO/IEC 14882-2014]. Pointers, references, and iterators share a close relationship in which it is required that referencing values be done through a valid iterator, pointer, or reference. Storing an iterator, reference, or pointer to an element within a container for any length of time comes with a risk that the underlying container may be modified such that the stored iterator, pointer, or reference becomes invalid. For instance, when a sequence container such as std::vector requires an underlying reallocation, outstanding iterators, pointers, and references will be invalidated [Kalev 99]. Use only a valid pointer, reference, or iterator to refer to an element of a container.
According to the The C++ Standard, [container.requirements.general], paragraph 12 [ISO/IEC 14882-2014] states the following:
Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container.
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The following container functions can invalidate iterators, references, and pointers under certain circumstances:.
| Class | Function | Iterators | References/Pointers | Notes |
|---|---|---|---|---|
std::deque | ||||
insert(), emplace_front(), emplace_back(),emplace(), push_front(), push_back() | X | X | An insertion in the middle of the deque invalidates all the iterators and references to elements of the deque. An insertion at either end of the deque invalidates all the iterators to the deque but has no effect on the validity of references to elements of the deque. ([deque.modifiers], paragraph 1) | |
erase(), pop_back(), resize() | X | X | An erase operation that erases the last element of a deque invalidates only the past-the-end iterator and all iterators and references to the erased elements. An erase operation that erases the first element of a deque but not the last element invalidates only the erased elements. An erase operation that erases neither the first element nor the last element of a deque invalidates the past-the-end iterator and all iterators and references to all the elements of the deque. ([deque.modifiers], paragraph 4) | |
clear() | X | X | Destroys all elements in the container. Invalidates all references, pointers, and iterators referring to the elements of the container and may invalidate the past-the-end iterator. ([sequence.reqmts], Table 100) | |
std::forward_list | ||||
erase_after(), pop_front(), resize() | X | X | erase_after shall invalidate only iterators and references to the erased elements. ([forwardlist.modifiers], paragraph 1) | |
remove(), unique() | X | X | Invalidates only the iterators and references to the erased elements. ([forwardlist.ops], paragraph 12 & paragraph 16) | |
clear() | X | X | Destroys all elements in the container. Invalidates all references, pointers, and iterators referring to the elements of the container and may invalidate the past-the-end iterator. ([sequence.reqmts], Table 100) | |
std::list | ||||
erase(), pop_front(), pop_back(), clear(), remove(), remove_if(), unique() | X | X | Invalidates only the iterators and references to the erased elements. ([list.modifiers], paragraph 3 and [list.ops], paragraph 15 & paragraph 19) | |
clear() | X | X | Destroys all elements in the container. Invalidates all references, pointers, and iterators referring to the elements of the container and may invalidate the past-the-end iterator. ([sequence.reqmts], Table 100) | |
std::vector | ||||
reserve() | X | X | After | |
insert(), emplace_back(), emplace(), push_back() | X | X | Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. ([vector.modifiers], paragraph 1). All iterators and references after the insertion point are invalidated. | |
erase(), pop_back(), resize() | X | X | Invalidates iterators and references at or after the point of the erase. ([vector.modifiers], paragraph 3) | |
clear() | X | X | Destroys all elements in the container. Invalidates all references, pointers, and iterators referring to the elements of the container and may invalidate the past-the-end iterator. ([sequence.reqmts], Table 100) | |
std::set, std::multiset, std::map, std::multimap | ||||
erase(), clear() | X | X | Invalidates only iterators and references to the erased elements. ([associative.reqmts], paragraph 9) | |
| ||||
erase(), clear() | X | X | Invalidates only iterators and references to the erased elements. ([unord.req], paragraph 14) | |
insert(), emplace() | X | The | ||
rehash(), reserve() | X | Rehashing invalidates iterators, changes ordering between elements, and changes which buckets the elements appear in but does not invalidate pointers or references to elements. ([unord.req], paragraph 9) | ||
std::valarray | resize() | X | Resizing invalidates all pointers and references to elements in the array. ([valarray.members], paragraph 12) |
A std::basic_string object is also a container for to which this rule applies. For more specific information pertaining to std::basic_string containers, see STR52-CPP. Use valid references, pointers, and iterators to reference elements of a basic_string.
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In this noncompliant code example, pos is invalidated after the first call to insert(), and subsequent loop iterations have undefined behavior:.
| Code Block | ||||
|---|---|---|---|---|
| ||||
#include <deque>
void f(const double *items, std::size_t count) {
std::deque<double> d;
auto pos = d.begin();
for (std::size_t i = 0; i < count; ++i, ++pos) {
d.insert(pos, items[i] + 41.0);
}
} |
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In this compliant solution, pos is assigned a valid iterator on each insertion, preventing undefined behavior:.
| Code Block | ||||
|---|---|---|---|---|
| ||||
#include <deque>
void f(const double *items, std::size_t count) {
std::deque<double> d;
auto pos = d.begin();
for (std::size_t i = 0; i < count; ++i, ++pos) {
pos = d.insert(pos, items[i] + 41.0);
}
}
|
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| [ISO/IEC 14882-2014] | Clause 23, "Containers Library" |
| [Kalev 99] | ANSI/ISO C++ Professional Programmer's Handbook |
| [Meyers 01] | Item 43, "Prefer Algorithm Calls to Hand-written Handwritten Loops" |
| [Sutter 04] | Item 84, "Prefer Algorithm Calls to Handwritten Loops" |
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