std::vector<T,Allocator>::resize

 

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std::vector

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vector::vector vector::~vector vector::operator= vector::assign vector::assign_range (C++23)   vector::get_allocator Element access vector::operator[] vector::at vector::data vector::front vector::back Iterators vector::beginvector::cbegin (C++11) vector::endvector::cend (C++11) vector::rbeginvector::crbegin (C++11) vector::rendvector::crend (C++11)	Capacity vector::empty vector::size vector::max_size vector::reserve vector::capacity vector::shrink_to_fit (DR*) Modifiers vector::clear vector::erase vector::insert vector::insert_range (C++23) vector::append_range (C++23) vector::emplace (C++11) vector::emplace_back (C++11) vector::push_back vector::pop_back vector::resize vector::swap
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operator==operator<=> (C++20) swap(std::vector) erase(std::vector)erase_if(std::vector) (C++20)(C++20)	operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides(C++17)

 

void resize( size_type count );	(1)	(constexpr since C++20)
void resize( size_type count, const value_type& value );	(2)	(constexpr since C++20)

Resizes the container to contain count elements, does nothing if count == size().

If the current size is greater than count, the container is reduced to its first count elements.

If the current size is less than count, then:

1) Additional default-inserted elements are appended.

2) Additional copies of value are appended.

Parameters

count	-	new size of the container
value	-	the value to initialize the new elements with
Type requirements
- T must meet the requirements of MoveInsertable and DefaultInsertable in order to use overload (1).
- T must meet the requirements of CopyInsertable in order to use overload (2).

Complexity

Linear in the difference between the current size and count. Additional complexity possible due to reallocation if capacity is less than count.

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 new vector would exceed max_size().

In overload (1), if T's move constructor is not noexcept and T is not CopyInsertable into *this, vector will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.

(since C++11)

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 a custom Allocator::construct.
Vector capacity is never reduced when resizing to smaller size because that would invalidate all iterators, rather than only the ones that would be invalidated by the equivalent sequence of pop_back() calls.

Example

Run this code

#include <vector>
#include <iostream>
 
void print(auto rem, const std::vector<int>& c)
{
    for (std::cout << rem; const int 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);
}

Output:

The vector holds: 1 2 3
After resize up to 5: 1 2 3 0 0
After resize down to 2: 1 2
After 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.

DR	Applied to	Behavior as published	Correct behavior
LWG 679	C++98	resize() passed value by value	passes by const reference
LWG 1525	C++98	the behavior of resize(size()) was not specified	specified
LWG 2033	C++11	1. elements were removed by using erase() 2. the type requirements of T were incorrect	1. uses pop_back() 2. corrected
LWG 2066	C++11	overload (1) did not have the exception safety guarantee of overload (2)	added
LWG 2160	C++11	elements were removed by using pop_back() due to the resolution of LWG 2033	does not specify the method of removing elements

See also

max_size	returns the maximum possible number of elements (public member function)
size	returns the number of elements (public member function)
capacity	returns the number of elements that can be held in currently allocated storage (public member function)
empty	checks whether the container is empty (public member function)