在shared_ptr的自定义删除器中检查nullptr是否有意义？ [英] Does it make sense to check for nullptr in custom deleter of shared_ptr?

问题描述

我看过一些使用 std :: shared_ptr 和自定义删除器的代码，这些删除器测试nullptr的参数，例如 MyClass 具有 close（）方法，并使用某些 CreateMyClass 构造：

I've seen some code that uses std::shared_ptr with a custom deleter that test the argument for nullptr, for example, MyClass which has a close() method and is constructed with some CreateMyClass:

auto pMyClass = std::shared_ptr<MyClass>(CreateMyClass(), 
                                        [](MyClass* ptr)
                                        { 
                                            if(ptr) 
                                                ptr->close(); 
                                        });

测试 ptr 是否有意义删除器中是否为空？
这会发生吗？

Does it make sense to test ptr for null-ness in the deleter? Can this happen? how?

推荐答案

构造函数 std :: shared_ptr< T> :: shared_ptr（Y * p） 要求删除p 是有效操作。当 p 等于 nullptr 时，此操作有效。

The constructor std::shared_ptr<T>::shared_ptr(Y*p) has the requirement that delete p is a valid operation. This is a valid operation when p equals nullptr.

构造函数 std :: shared_ptr< T> :: shared_ptr（Y * p，Del del）要求 del（p）是有效操作。

The constructor std::shared_ptr<T>::shared_ptr(Y*p, Del del) has the requirement that del(p) is a valid operation.

如果自定义删除器无法处理 p 等于 nullptr ，则在 shared_ptr p 是无效的$ c>。

If your custom deleter cannot handle p being equal to nullptr then it is not valid to pass a null p in the constructor of shared_ptr.

您提供的示例构造函数可以更好地呈现，因此：

The constructor you offer as an example can be better presented, thus:

#include <memory>

struct MyClass {
    void open() {
        // note - may throw
    };

    void close() noexcept {
        // pre - is open
    }
};

struct Closer
{
    void operator()(MyClass* p) const noexcept
    {
        p->close();
        delete p;  // or return to pool, etc
    }
};

auto CreateMyClass() -> std::unique_ptr<MyClass, Closer>
{
    // first construct with normal deleter
    auto p1 = std::make_unique<MyClass>();

    // in case this throws an exception.
    p1->open();

    // now it's open, we need a more comprehensive deleter
    auto p = std::unique_ptr<MyClass, Closer> { p1.release(), Closer() };
    return p;
}

int main()
{
    auto sp = std::shared_ptr<MyClass>(CreateMyClass());
}

请注意，shared_ptr现在不可能拥有空对象。

Note that it is now not possible for the shared_ptr to own a null object.

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