std::shared_ptr<T>::owner_before

 

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shared_ptr::owner_before

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推导指引(C++17)

 

template< class Y > bool owner_before( const shared_ptr<Y>& other ) const noexcept;
template< class Y > bool owner_before( const std::weak_ptr<Y>& other ) const noexcept;

以实现定义的基于拥有者（与基于值相反）顺序，检查此 shared_ptr 是否先于 other 。二个智能指针仅若都占有同一对象或均为空才比较相等，即使由 get() 获得的指针不同（例如因为它们指向同一对象中的不同子对象）。

此顺序用于令共享和弱指针可用作关联容器中的关键，通常经由 std::owner_less 。

参数

other

-

要比较的 std::shared_ptr 或 std::weak_ptr

返回值

如果 *this 先于 other，那么返回 true，否则返回 false。常见实现会比较控制块的地址。

示例

运行此代码

#include <iostream>
#include <memory>
 
struct Foo
{
    int n1;
    int n2; 
    Foo(int a, int b) : n1(a), n2(b) {}
};
 
int main()
{   
    auto p1 = std::make_shared<Foo>(1, 2);
    std::shared_ptr<int> p2(p1, &p1->n1);
    std::shared_ptr<int> p3(p1, &p1->n2);
 
    std::cout << std::boolalpha
              << "p2 < p3 " << (p2 < p3) << '\n'
              << "p3 < p2 " << (p3 < p2) << '\n'
              << "p2.owner_before(p3) " << p2.owner_before(p3) << '\n'
              << "p3.owner_before(p2) " << p3.owner_before(p2) << '\n';
 
    std::weak_ptr<int> w2(p2);
    std::weak_ptr<int> w3(p3);
    std::cout 
//            << "w2 < w3 " << (w2 < w3) << '\n' // 无法编译
//            << "w3 < w2 " << (w3 < w2) << '\n' // 无法编译
              << "w2.owner_before(w3) " << w2.owner_before(w3) << '\n'
              << "w3.owner_before(w2) " << w3.owner_before(w2) << '\n';
}

输出：

p2 < p3 true
p3 < p2 false
p2.owner_before(p3) false
p3.owner_before(p2) false
w2.owner_before(w3) false
w3.owner_before(w2) false

缺陷报告

下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。

缺陷报告	应用于	出版时的行为	正确行为
LWG 2873	C++11	owner_before 未声明为 noexcept	声明为 noexcept

参阅

owner_less (C++11)

提供基于所有者的，共享指针和弱指针的混合类型的排序 (类模板)