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std::shared_ptr<T>:: swap

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shared_ptr::swap
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void swap ( shared_ptr & r ) noexcept ;
(C++11以降)

格納されたポインタ値と所有権を、 * this と r の間で交換します。参照カウントがある場合でも調整されません。

パラメータ

r - 内容を交換するスマートポインタ

戻り値

(なし)

例

このコードを実行 
#include <iostream>
#include <memory>
#include <string>
struct Foo
{
    Foo(int _val) : val(_val) { std::cout << "Foo...\n"; }
    ~Foo() { std::cout << "~Foo...\n"; }
    std::string print() { return std::to_string(val); }
    int val;
};
int main()
{
    std::shared_ptr<Foo> p1 = std::make_shared<Foo>(100);
    std::shared_ptr<Foo> p2 = std::make_shared<Foo>(200);
    auto print = [&]()
    {
        std::cout << " p1=" << (p1 ? p1->print() : "nullptr");
        std::cout << " p2=" << (p2 ? p2->print() : "nullptr") << '\n';  
    };
    print();
    p1.swap(p2);
    print();
    p1.reset();
    print();
    p1.swap(p2);
    print();   
}

出力: 

Foo...
Foo...
 p1=100 p2=200
 p1=200 p2=100
~Foo...
 p1=nullptr p2=100
 p1=100 p2=nullptr
~Foo...