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shared_ptr 的 reset() 有 4 个重载版本，如下：

void reset() noexcept; 							(1)	(since C++11)
template< class Y >
void reset( Y* ptr ); 							(2)	(since C++11)
template< class Y, class Deleter >
void reset( Y* ptr, Deleter d ); 				(3)	(since C++11)
template< class Y, class Deleter, class Alloc >
void reset( Y* ptr, Deleter d, Alloc alloc ); 	(4)	(since C++11)

常用到的应该是前 3 个版本，这里不举例第 4 个版本，因为我也没用过^_^ 

1，void reset() 版本

这个函数是最简单的，其作用：释放托管对象的所有权（如果有），调用后，该智能指针不再管理任何对象。注意，这里的释放并不是真正地去释放，只是引用计数减 1，而释放是引用计数为 0 时自行进行的。

#include <memory>
#include <iostream>
 
struct Foo {
    Foo(int n = 0) noexcept : bar(n) {
        std::cout << "Foo::Foo(), bar = " << bar << " @ " << this << '\n';
    }
    ~Foo() {
        std::cout << "Foo::~Foo(), bar = " << bar << " @ " << this << "\n";
    }
    int getBar() const noexcept { return bar; }
private:
    int bar;
};

void deleter(Foo *f)
{
    delete []f;
}


int main()
{
    std::cout << "1) unique ownership\n";
    {
        std::shared_ptr<Foo> sptr = std::make_shared<Foo>(100);
 
        std::cout << "Foo::bar = " << sptr->getBar() << ", use_count() = "
                  << sptr.use_count() << '\n';
 
        // Reset the shared_ptr without handing it a fresh instance of Foo.
        // The old instance will be destroyed after this call.
        std::cout << "call sptr.reset()...\n";
        sptr.reset(); // calls Foo's destructor here
        std::cout << "After reset(): use_count() = " << sptr.use_count()
                  << ", sptr = " << sptr << '\n';
    }   // No call to Foo's destructor, it was done earlier in reset().

    return 0;
}

可以看到在调用 reset() 后，调用了对象的析构函数进行释放。我们看 reset() 的另一种情况：

#include <memory>
#include <iostream>
 
struct Foo {
    Foo(int n = 0) noexcept : bar(n) {
        std::cout << "Foo::Foo(), bar = " << bar << " @ " << this << '\n';
    }
    ~Foo() {
        std::cout << "Foo::~Foo(), bar = " << bar << " @ " << this << "\n";
    }
    int getBar() const noexcept { return bar; }
private:
    int bar;
};


int main()
{
    std::cout << "1) unique ownership\n";
    {
        std::shared_ptr<Foo> sptr = std::make_shared<Foo>(100);
 
        std::cout << "Foo::bar = " << sptr->getBar() << ", use_count() = "
                  << sptr.use_count() << '\n';
 
        // Reset the shared_ptr without handing it a fresh instance of Foo.
        // The old instance will be destroyed after this call.
        std::cout << "call sptr.reset()...\n";
        sptr.reset(); // calls Foo's destructor here
        std::cout << "After reset(): use_count() = " << sptr.use_count()
                  << ", sptr = " << sptr << '\n';
    }   // No call to Foo's destructor, it was done earlier in reset().

    std::cout << "1) unique ownership\n";
    {
        std::shared_ptr<Foo> sptr = std::make_shared<Foo>(100);
 
        std::cout << "Foo::bar = " << sptr->getBar() << ", use_count() = "
                  << sptr.use_count() << '\n';

        std::shared_ptr<Foo> sptr1 = sptr;
        // Reset the shared_ptr without handing it a fresh instance of Foo.
        // The old instance will be destroyed after this call.
        std::cout << "call sptr1.reset()...\n";
        sptr1.reset(); // not calls Foo's destructor here
        std::cout << "After reset(): use_count() = " << sptr.use_count()
                  << ", sptr = " << sptr << '\n';
    }   // No call to Foo's destructor, it was done earlier in reset().
    return 0;
}

可以看到 Foo::bar = 100, use_count() = 2，此时两个智能指针都管理着 0x1029028 的资源，当调用 sptr1.reset(); 后，这个资源就少了一个管理者，所以 After reset(): use_count() = 1, sptr = 0x1029028，当离开作用域后，此资源才被释放。

2，模板版本带一个参数

template< class Y >
void reset( Y* ptr );  用ptr指向的对象替换托管对象

#include <memory>
#include <iostream>
 
struct Foo {
    Foo(int n = 0) noexcept : bar(n) {
        std::cout << "Foo::Foo(), bar = " << bar << " @ " << this << '\n';
    }
    ~Foo() {
        std::cout << "Foo::~Foo(), bar = " << bar << " @ " << this << "\n";
    }
    int getBar() const noexcept { return bar; }
private:
    int bar;
};

int main()
{
    std::cout << "\n1) unique ownership\n";
    {
        std::shared_ptr<Foo> sptr = std::make_shared<Foo>(200);
 
        std::cout << "Foo::bar = " << sptr->getBar() << ", use_count() = "
                  << sptr.use_count() << '\n';
 
        // Reset the shared_ptr, hand it a fresh instance of Foo.
        // The old instance will be destroyed after this call.
        std::cout << "call sptr.reset()...\n";
        sptr.reset(new Foo{222});
        std::cout << "After reset(): use_count() = " << sptr.use_count()
                  << ", sptr = " << sptr << "\nLeaving the scope...\n";
    }   // Calls Foo's destructor.

    return 0;
}

可以看到在  sptr.reset(new Foo{222}); 后，释放了 0x17d1028 指向的资源，当前指向的资源为 0x17d1040，离开作用域后自动释放。

3， 模板版本还两个参数

template<class Y, class Deleter>

void reset(Y *ptr, Deleter d)

带删除器的版本只会用在数组对象释放的情况，如果是数组对象的情况下不用删除器，则行为是未定义的。如：

#include <memory>
#include <iostream>
 
struct Foo {
    Foo(int n = 0) noexcept : bar(n) {
        std::cout << "Foo::Foo(), bar = " << bar << " @ " << this << '\n';
    }
    ~Foo() {
        std::cout << "Foo::~Foo(), bar = " << bar << " @ " << this << "\n";
    }
    int getBar() const noexcept { return bar; }
private:
    int bar;
};

int main()
{
    std::cout << "1) unique ownership\n";
    {
        std::shared_ptr<Foo> sptr = std::make_shared<Foo>(100);
 
        std::cout << "Foo::bar = " << sptr->getBar() << ", use_count() = "
                  << sptr.use_count() << '\n';
 
        // Reset the shared_ptr without handing it a fresh instance of Foo.
        // The old instance will be destroyed after this call.
        std::cout << "call sptr.reset()...\n";
        sptr.reset(new Foo[3]); // calls Foo's destructor here
        std::cout << "After reset(): use_count() = " << sptr.use_count()
                  << ", sptr = " << sptr << '\n';
    }   // No call to Foo's destructor, it was done earlier in reset().
 
    return 0;
}

正确用法可以是这样：

#include <memory>
#include <iostream>
 
struct Foo {
    Foo(int n = 0) noexcept : bar(n) {
        std::cout << "Foo::Foo(), bar = " << bar << " @ " << this << '\n';
    }
    ~Foo() {
        std::cout << "Foo::~Foo(), bar = " << bar << " @ " << this << "\n";
    }
    int getBar() const noexcept { return bar; }
private:
    int bar;
};

void customDeleter(Foo *f)
{
    if(f)
    {
        delete []f;
    }
}

int main()
{
    std::cout << "1) unique ownership\n";
    {
        std::shared_ptr<Foo> sptr = std::make_shared<Foo>(100);
 
        std::cout << "Foo::bar = " << sptr->getBar() << ", use_count() = "
                  << sptr.use_count() << '\n';
 
        // Reset the shared_ptr without handing it a fresh instance of Foo.
        // The old instance will be destroyed after this call.
        std::cout << "call sptr.reset()...\n";
        sptr.reset(new Foo[3], customDeleter); // calls Foo's destructor here
        std::cout << "After reset(): use_count() = " << sptr.use_count()
                  << ", sptr = " << sptr << '\n';
    }   // No call to Foo's destructor, it was done earlier in reset().
 
    return 0;
}

 添加删除函数：

void customDeleter(Foo *f)
{
    if(f)
    {
        delete []f;
    }
}

reset 调用时指定删除器： sptr.reset(new Foo[3], customDeleter);

因为只有释放数组对象时才需要指定删除器，所以实现就是 delete []，我们看结果：

最后调用的 3 个析构函数。

还可以使用 std::default_delete 模板，如：sptr.reset(new Foo[3], std::default_delete<Foo[]>());

#include <memory>
#include <iostream>
 
struct Foo {
    Foo(int n = 0) noexcept : bar(n) {
        std::cout << "Foo::Foo(), bar = " << bar << " @ " << this << '\n';
    }
    ~Foo() {
        std::cout << "Foo::~Foo(), bar = " << bar << " @ " << this << "\n";
    }
    int getBar() const noexcept { return bar; }
private:
    int bar;
};


int main()
{
    std::cout << "1) unique ownership\n";
    {
        std::shared_ptr<Foo> sptr = std::make_shared<Foo>(100);
 
        std::cout << "Foo::bar = " << sptr->getBar() << ", use_count() = "
                  << sptr.use_count() << '\n';
 
        // Reset the shared_ptr without handing it a fresh instance of Foo.
        // The old instance will be destroyed after this call.
        std::cout << "call sptr.reset()...\n";
      
        sptr.reset(new Foo[3], std::default_delete<Foo[]>());
        std::cout << "After reset(): use_count() = " << sptr.use_count()
                  << ", sptr = " << sptr << '\n';
    }   // No call to Foo's destructor, it was done earlier in reset().
 
    return 0;
}

同样释放正常。