Friend函数无法构造类的唯一指针 [英] Friend function is unable to construct a unique pointer of the class

问题描述

我有一种确定的设计策略，其中我班的构造函数是私有的，只能由班上的朋友构造。在朋友函数中，我尝试使用 std :: make_unique 创建我的类的unique_pointer，但是无法编译。我的VC12编译器抱怨

I have a certain design strategy where the constructor of my class is private and can only be constructed by friends of the class. Inside the friend function, I am trying to create a unique_pointer of my class using std::make_uniquebut it doesn't compile. My VC12 compiler complains

c：\program files（x86）\Microsoft Visual Studio
12.0\vc\ include\memory（1639）：错误C2248： Spam :: Spam：无法访问在 Spam类中声明的私有成员

c:\program files (x86)\microsoft visual studio 12.0\vc\include\memory(1639): error C2248: 'Spam::Spam' : cannot access private member declared in class 'Spam'

在编译过程中失败的相关代码如下

The relevant code which fails during compilation is as follows

#include <memory>
class Spam {
public:
    friend void Foo();

private:
    Spam(int mem) :mem(mem) {}
    int mem;
};
void Foo() {
    std::unique_ptr<Spam> spam = std::make_unique<Spam>(10);
}

为什么我不能编译？

推荐答案

在您的情况下，函数 make_unique 试图创建 Spam的实例，该函数不是朋友。从朋友功能内部调用非朋友功能不会使非朋友功能具有朋友状态。

In your case the function make_unique is trying to create an instance of Spam and that function is not a friend. Calling a non-friend function from inside a friend function does not imbue the non-friend function with friend status.

要解决此问题，您可以编写 Foo ：

To solve this you can write in Foo:

std::unique_ptr<Spam> spam(new Spam(10));

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