MSVC编译器可能的错误

给定在for循环的条件子句中声明的shared_ptr变量，并且for循环主体包含if / continue语句，Microsoft编译器（自2015版开始）在每次循环迭代中生成额外的析构函数调用（共两个）。这导致销毁应由Holder接口用户无法访问的Item对象。请参阅下面的示例代码

namespace
{
    class Item
    {
    public:
        Item(size_t v)
            : value_(v)
        {
            std::cout << "Item(" << value_ << ")" << std::endl;
        }

        ~Item()
        {
            std::cout << "~Item(" << value_ << ")" << std::endl;
        }

        void print() const
        {
            std::cout << "Item::print(" << value_ << ")" << std::endl;
        }

    private:
        size_t value_;

    };

    typedef std::shared_ptr<const Item> ItemCPtr;

    class Holder
    {
    public:
        Holder(size_t n)
        {
            for (size_t i = 0; i < n; ++i)
                items_.emplace_back(new Item(i));
        }

        ItemCPtr getItem(size_t i) const
        {
            if (i < items_.size())
                return items_[i];
            return ItemCPtr();
        }

    private:
        std::vector<ItemCPtr> items_;
    };
}

TEST(Test, Test)
{
    Holder _holder(5);

    std::cout << "before loop" << std::endl;

    for (size_t i = 0; auto _item = _holder.getItem(i); ++i)
    {
        if (!!(i % 2))
            continue;

        _item->print();
    }

    std::cout << "after loop" << std::endl;

    _holder.getItem(1)->print();
    _holder.getItem(3)->print();
}

该产量低于

||   [ RUN      ] Test.Test
||   Item(0)
||   Item(1)
||   Item(2)
||   Item(3)
||   Item(4)
||   before loop
||   Item::print(0)
||   ~Item(1)
||   Item::print(2)
||   ~Item(3)
||   Item::print(4)
||   after loop
||   Item::print(3722304989)
||   Item::print(3722304989)
||   ~Item(0)
||   ~Item(2)
||   ~Item(4)
||   [       OK ] Test.Test (0 ms)

如果我以这种方式将_item声明移出for循环

    auto _item = ItemCPtr();
    for (size_t i = 0; _item = _holder.getItem(i); ++i)

然后我得到这样的预期输出

||   [ RUN      ] Test.Test
||   Item(0)
||   Item(1)
||   Item(2)
||   Item(3)
||   Item(4)
||   before loop
||   Item::print(0)
||   Item::print(2)
||   Item::print(4)
||   after loop
||   Item::print(1)
||   Item::print(3)
||   ~Item(0)
||   ~Item(1)
||   ~Item(2)
||   ~Item(3)
||   ~Item(4)
||   [       OK ] Test.Test (0 ms)

据我了解，getItem应该产生ItemCPtr的副本，并且不能通过Holder接口修改任何Item。但是用户可以销毁循环中五分之二的东西，在/标记之间查看~Item(1)和~Item(3)析构函数的输出。before loopafter loop

这是暴露问题的简单例子。在现实世界中，这将导致难以跟踪的内存损坏问题。

编译器身份：

cmake -G "Visual Studio 14 2015" ..\
-- Selecting Windows SDK version 10.0.14393.0 to target Windows 10.0.19041.
-- The C compiler identification is MSVC 19.0.24210.0
-- The CXX compiler identification is MSVC 19.0.24210.0

操作系统是64位Windows 10

即使禁用优化（/Od默认选项），错误也会显示出来。