std::weak_ptr<T>::use_count

 

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std::weak_ptr

成员函数

weak_ptr::weak_ptr

weak_ptr::~weak_ptr

weak_ptr::operator=

修改器

weak_ptr::reset

weak_ptr::swap

观察器

weak_ptr::use_count

weak_ptr::expired

weak_ptr::lock

weak_ptr::owner_before

非成员函数

std::swap

辅助类

std::atomic<std::weak_ptr> (C++20)

推导指引(C++17)

 

long use_count() const noexcept;		(C++11 起)

返回共享被管理对象所有权的 shared_ptr 实例数量，或 ​0​ ，若被管理对象已被删除，即 *this 为空。

参数

（无）

返回值

在调用的瞬间共享被管理对象所有权的 shared_ptr 实例数量。

注意

expired() 可能快于 use_count() 。此函数固有地有不稳，若被管理对象在可能创建并销毁 shared_ptr 副本的线程间共享：则结果仅若匹配调用方线程所独占的副本数或零才可靠；任何其他值可能在能使用前就变得过时了。

示例

运行此代码

#include <iostream>
#include <memory>
 
std::weak_ptr<int> gwp;
 
void observe_gwp() {
    std::cout << "use_count(): " << gwp.use_count() << "\t id: ";
    if (auto sp = gwp.lock())
        std::cout << *sp << '\n';
    else
        std::cout << "??\n";
}
 
void share_recursively(std::shared_ptr<int> sp, int depth) {
    observe_gwp(); // : 2 3 4
    if (1 < depth)
        share_recursively(sp, depth - 1);
    observe_gwp(); // : 4 3 2
}
 
int main() {
    observe_gwp();
    {
        auto sp = std::make_shared<int>(42);
        gwp = sp;
        observe_gwp(); // : 1
        share_recursively(sp, 3); // : 2 3 4 4 3 2
        observe_gwp(); // : 1
    }
    observe_gwp(); // : 0
}

输出：

use_count(): 0   id: ??
use_count(): 1   id: 42
use_count(): 2   id: 42
use_count(): 3   id: 42
use_count(): 4   id: 42
use_count(): 4   id: 42
use_count(): 3   id: 42
use_count(): 2   id: 42
use_count(): 1   id: 42
use_count(): 0   id: ??

参阅

expired

检查被引用的对象是否已删除 (公开成员函数)