Resizes the container to containcount elements:

• Ifcount is equal to the current size, does nothing.
• If the current size is greater thancount, the container is reduced to its firstcount elements.
• If the current size is less thancount, then:

[edit]Parameters

[edit]Complexity

Linear in the difference between the current size andcount. Additional complexity possible due to reallocation if capacity is less thancount.

Exceptions

If an exception is thrown for any reason, these functions have no effect (strong exception safety guarantee).Although not explicitly specified,std::length_error is thrown if the capacity required by the newvector would exceedmax_size().

Notes

If value-initialization in overload(1) is undesirable, for example, if the elements are of non-class type and zeroing out is not needed, it can be avoided by providing acustomAllocator::construct.
Vector capacity is never reduced when resizing to smaller size because that would invalidate all iterators, while the specification only invalidates the iterators to/after the erased elements.

[edit]Example

#include <vector>#include <iostream> void print(auto rem,conststd::vector<int>& c){for(std::cout<< rem;constint el: c)std::cout<< el<<' ';std::cout<<'\n';} int main(){std::vector<int> c={1,2,3};    print("The vector holds: ", c);     c.resize(5);    print("After resize up to 5: ", c);     c.resize(2);    print("After resize down to 2: ", c);     c.resize(6,4);    print("After resize up to 6 (initializer = 4): ", c);}

The vector holds: 1 2 3After resize up to 5: 1 2 3 0 0After resize down to 2: 1 2After resize up to 6 (initializer = 4): 1 2 4 4 4 4

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

1. ↑erase() can remove elements in the middle of avector, so the value type is required to beMoveAssignable so that the elements following the removed section can be moved forward to fill the gap. However,resize() can only remove elements at the end of thevector, making theMoveAssignable unnecessary.
2. ↑Removing elements by usingpop_back() implies that the elements are required to be removed from back to front.

[edit]See also

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