CODEWITHSUNDEEP
c++vectorlanguage-lawyer
Neil Kirk
Neil Kirk

Reputation: 21773

What does the standard say about how calling clear on a vector changes the capacity?

This website implies that clearing a vector MAY change the capacity:

http://en.cppreference.com/w/cpp/container/vector/clear

Many implementations will not release allocated memory after a call to clear(), effectively leaving the capacity() of the vector unchanged.

But according to @JamesKanze this is wrong and the standard mandates that clear will not change capacity.

What does the standard say?

Upvotes: 63

Views: 13578

Answers (2)

QooBee
QooBee

Reputation: 29

I think the accepted answer is somewhat misleading.

clear() does not change capacity itself, but after clear() any other function call can shrink the capacity to 0, any assumption on capacity would become a trap in future modification.

Upvotes: -1

James Kanze
James Kanze

Reputation: 153909

Depending on the version of the standard you are looking at, clear is defined as the equivalent of erase(begin(), end()), or (in C++11):
"Destroys all elements in a. Invalidates all references, pointers, and iterators referring to the elements of a and may invalidate the past-the-end iterator."

In neither case is it allowed to modify the capacity; the following code is guaranteed safe by the standard:

std::vector<int> v;
for (int i = 0; i != 5; ++ i) {
    v.push_back(i);
}
assert(v.capacity() >= 5);
v.clear();
assert(v.capacity() >= 5);
v.push_back(10);
v.push_back(11);
std::vector<int>::iterator i = v.begin() + 1;
v.push_back(12);
v.push_back(13);
*i = 42;        //  i must still be valid, because none of 
                //  the push_back would have required an
                //  increase of capacity

(The reason for the change in wording in C++11: the committee didn't want to require MoveAssignable for clear, which would have been the case if it were defined in terms of erase.)

Upvotes: 72

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