如何确保特定的类只能创建另一个类的实例？

Question

如何仅在特定类中限制类的实例化？

我不想在单个文件中限制它,因此匿名命名空间不适合我.

请注意,我想让整个世界都可以看到要限制的类的声明,只是来自整个世界的一个特定候选者只能实例化它.

我怎样才能做到这一点？

Answer 1

用friends!使一个班级成为班级Foo的朋友Bar意味着Foo可以访问Bar私人会员.如果您将构造函数Foo设为私有,则只能创建一个实例Bar.

struct restricted_t {
    friend struct allowed_t; // allow 'allowed_t' to access my private members
private:
    // nobody can construct me (except for my friends of course)
    restricted_t() = default;
};

struct allowed_t {
    restricted_t yes; // ok
};

struct not_allowed_t {
    restricted_t no; // not ok
};

int main() {
    allowed_t a; // ok, can access constructor
    not_allowed_t b; // error, constructor is private
}

Answer 2

你可以采用密钥模式(借用Rakete1111的例子):

class pass_key { friend class allowed_t; pass_key() {} };

struct restricted_t
{
  restricted_t(pass_key);
};

class allowed_t
{
public:
  allowed_t() : yes(pass_key()) {}  // ok, allowed_t is a friend of pass_key

private:
  restricted_t yes;
};

class not_allowed_t
{
public:
  not_allowed_t() : no(pass_key()) {}  // not ok, pass_key() is private

private:
  restricted_t no;
};

int main()
{
  allowed_t a;     // OK, can access constructor
  not_allowed_t b; // ERROR, only a friend of the key
                   // class has access to the restricted
                   // constructor
}

该模式允许比制作restricted_t朋友更精细的访问控制,allowed_t并避免复杂的代理模式.