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)

调整容器大小以容纳 count 元素，如果 count == size() 则不执行任何操作。

如果当前大小大于 count，则容器将缩减为其前 count 个元素。

如果当前大小小于 count，则

1) 追加额外的 默认插入 元素。

2) 追加 value 的额外副本。

Parameters

count	-	容器的新大小
value	-	用于初始化新元素的值
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

与当前大小和 count 之间的差值呈线性关系。如果容量小于 count，则可能由于重新分配而增加复杂度。

Exceptions

如果由于任何原因抛出异常，这些函数都不会产生影响（强异常安全保证）。虽然未明确指定，但如果新 vector 所需的容量超过 max_size()，则会抛出 std::length_error。

在重载 (1) 中，如果 T 的移动构造函数不是 noexcept 并且 T 不能 CopyInsertable 到 *this 中，则 vector 将使用抛出异常的移动构造函数。如果它抛出异常，则保证被放弃，并且效果未指定。

(since C++11)

Notes

如果在重载 (1) 中的值初始化是不 желаしい 的，例如，如果元素是非类类型且不需要归零，则可以通过提供 自定义 Allocator::construct 来避免。
当调整为较小尺寸时，Vector 容量永远不会减少，因为这会使所有迭代器失效，而规范仅使指向/在擦除元素之后的迭代器失效。

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	返回最大可能的元素数量 (public member function)
size	返回元素数量 (public member function)
capacity	返回当前已分配存储中可以容纳的元素数量 (public member function)
empty	检查容器是否为空 (public member function)