C++11智能指针之weak_ptr

含义

Weak shared pointer
比较特殊的函数有lock和expired

函数lock

std::weak_ptr::lockshared_ptr lock() const noexcept;
Lock and restore weak_ptr.
Returns a shared_ptr with the information preserved by the weak_ptr object if it is not expired.
If the weak_ptr object has expired (including if it is empty), the function returns an empty shared_ptr (as if default-constructed).
Because shared_ptr objects count as an owner, this function locks the owned pointer, preventing it from being released (for at least as long as the returned object does not releases it).

函数expired

std::weak_ptr::expiredbool expired() const noexcept;
Check if expired
Returns whether the weak_ptr object is either empty or there are no more shared_ptr in the owner group it belongs to.

Expired pointers act as empty weak_ptr objects when locked, and thus can no longer be used to restore an owning shared_ptr.

This function shall return the same as (use_count()==0), although it may do so in a more efficient way.

举例

例子1

#include 
#include 
using namespace std;

int main()
{
    shared_ptr<int> sp1(new int(11));
    shared_ptr<int> sp2 = sp1;

    weak_ptr<int> wp=sp1;
    cout << "num=" << sp1.use_count() << endl;
    cout << "num=" << sp2.use_count() << endl;
    cout << "num=" << wp.use_count() << endl;

    shared_ptr<int> sp3 = wp.lock();
    cout << "num2=" << sp1.use_count() << endl;
    cout << "num2=" << sp2.use_count() << endl;
    cout << "num2=" << sp3.use_count() << endl;
    cout << "num2=" << wp.use_count() << endl;

    sp1.reset();
    sp2.reset();
    sp3.reset();
    cout << "num3=" << sp1.use_count() << endl;
    cout << "num3=" << sp2.use_count() << endl;
    cout << "num3=" << sp3.use_count() << endl;
    cout << "num3=" << wp.use_count() << endl;
    //当堆空间释放后
    shared_ptr<int> tmp = wp.lock();
    if(tmp == nullptr)
    {
        cout << "heap is null" << endl;
    }

    return 0;
}
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输出结果：
num=2
num=2
num=2
num2=3
num2=3
num2=3
num2=3
num3=0
num3=0
num3=0
num3=0
heap is null

例子2

#include 
#include 
using namespace std;

int main () {
    std::shared_ptr<int> sp1,sp2;
    std::weak_ptr<int> wp;
                                                               // sharing group:
                                                               // --------------
    sp1 = std::make_shared<int> (20);    // sp1
    wp = sp1;                                           // sp1, wp

    std::cout << "num= " << sp1.use_count() << '\n';
    std::cout << "num=" << sp2.use_count() << '\n';
    std::cout << "num=" << wp.use_count() << '\n';

    sp2 = wp.lock();                                // sp1, wp, sp2
    sp1.reset();                                        //      wp, sp2

    std::cout << "num2= " << sp1.use_count() << '\n';
    std::cout << "num2=" << sp2.use_count() << '\n';
    std::cout << "num2=" << wp.use_count() << '\n';

    sp1 = wp.lock();                                // sp1, wp, sp2

    std::cout << "*sp1: " << *sp1 << '\n';
    std::cout << "*sp2: " << *sp2 << '\n';
    std::cout << "num3= " << sp1.use_count() << '\n';
    std::cout << "num3=" << sp2.use_count() << '\n';
    std::cout << "num3=" << wp.use_count() << '\n';

  return 0;
}
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结果如下：
num= 1
num=0
num=1
num2= 0
num2=1
num2=1
*sp1: 20
*sp2: 20
num3= 2
num3=2
num3=2