std::string capacity() 方法

自 C++20 起

// Const version only
constexpr size_type capacity() const noexcept;

自 C++11 起

// Const version only
size_type capacity() const noexcept;

直到 C++11

// Const version only
size_type capacity() const;

返回字符串当前已分配空间的字符数。

参数

(无)

返回值

当前已分配存储的容量。

复杂度

常数 - O(1)。

异常

(无)

备注

从分配器获取但不可用于存储任何元素的内存位置不计入已分配存储。

注意

空终止符不是 std::basic_string 的元素。

示例

Main.cpp

#include <iostream>
#include <iomanip>
#include <string>
 
void show_capacity(std::string const& s)
{
    std::cout << std::quoted(s) << " has capacity " << s.capacity() << ".\n";
}
 
int main()
{
    std::string s{"Exemplar"};
    show_capacity(s);
 
    s += " is an example string.";
    show_capacity(s);
 
    s.clear();
    show_capacity(s);
 
 
    std::cout << "\nDemonstrate the capacity's growth policy."
                 "\nSize:  Capacity:  Ratio:\n" << std::left;
 
    std::string g;
    auto old_cap {g.capacity()};
 
    for(int mark{}; mark != 5; ++mark)
    {
        while (old_cap == g.capacity()) g.push_back('.');
 
        std::cout << std::setw( 7) << g.size()
                  << std::setw(11) << g.capacity()
                  << std::setw(10) << g.capacity() / static_cast<float>(old_cap) << '\n';
 
        old_cap = g.capacity();
    }
}

输出

"Exemplar" has capacity 15.
"Exemplar is an example string." has capacity 30.
"" has capacity 30.
 
Demonstrate the capacity's growth policy.
Size:  Capacity:  Ratio:
16     30         2
31     60         2
61     120        2
121    240        2
241    480        2