为什么 unique_ptr&lt;Derived&gt;隐式转换为 unique_ptr<Base>? [英] Why does unique_ptr&lt;Derived&gt; implicitly cast to unique_ptr&lt;Base&gt;?

问题描述

我编写了以下使用 unique_ptr 的代码，其中需要 unique_ptr

I wrote the following code that uses unique_ptr<Derived> where a unique_ptr<Base> is expected

class Base {
    int i;
 public:
    Base( int i ) : i(i) {}
    int getI() const { return i; }
};

class Derived : public Base {
    float f;
 public:
    Derived( int i, float f ) : Base(i), f(f) {}
    float getF() const { return f; }
};

void printBase( unique_ptr<Base> base )
{
    cout << "f: " << base->getI() << endl;
}

unique_ptr<Base> makeBase()
{
    return make_unique<Derived>( 2, 3.0f );
}

unique_ptr<Derived> makeDerived()
{
    return make_unique<Derived>( 2, 3.0f );
}

int main( int argc, char * argv [] )
{
    unique_ptr<Base> base1 = makeBase();
    unique_ptr<Base> base2 = makeDerived();
    printBase( make_unique<Derived>( 2, 3.0f ) );

    return 0;
}

我预计这段代码不会编译，因为根据我的理解 unique_ptr 和 unique_ptr 是不相关的类型和 unique_ptr; 实际上不是从 unique_ptr 派生的，因此赋值不应该起作用.

and i expected this code to not compile, because according to my understanding unique_ptr<Base> and unique_ptr<Derived> are unrelated types and unique_ptr<Derived> isn't in fact derived from unique_ptr<Base> so the assignment shouldn't work.

但是多亏了一些魔法，它才起作用，我不明白为什么，或者即使这样做是安全的.谁能解释一下?

But thanks to some magic it works, and i don't understand why, or even if it's safe to do so. Can someone explain please?

推荐答案

您正在寻找的神奇之处在于转换构造函数 #6 这里:

The bit of magic you're looking for is the converting constructor #6 here:

template<class U, class E>
unique_ptr(unique_ptr<U, E> &&u) noexcept;

它允许从一个即将到期的 std::unique_ptr if 隐式构造一个 std::unique_ptr删除者为清楚起见):

It enables constructing a std::unique_ptr<T> implicitly from an expiring std::unique_ptr<U> if (glossing over deleters for clarity):

unique_ptr::pointer 可隐式转换为 pointer

也就是说，它模仿隐式原始指针转换，包括派生到基类的转换，并且安全地执行您期望的™(在生命周期方面 - 您仍然需要确保可以多态删除基类型).

Which is to say, it mimicks implicit raw pointer conversions, including derived-to-base conversions, and does what you expect™ safely (in terms of lifetime – you still need to ensure that the base type can be deleted polymorphically).

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