我们什么时候需要 C++ 中的私有构造函数?
我有一个关于 C++ 中私有构造函数的问题.如果构造函数是私有的，我如何创建类的实例?
I have a question about private constructors in C++. If the constructor is private, how can I create an instance of the class?
我们应该在类内部有一个 getInstance() 方法吗?
Should we have a getInstance() method inside the class?
推荐答案
private 构造函数有几种情况:
There are a few scenarios for having private constructors:
限制除
friend以外的所有对象的创建；在这种情况下，所有构造函数都必须是private
Restricting object creation for all but
friends; in this case all constructors have to beprivate
class A
{
private:
A () {}
public:
// other accessible methods
friend class B;
};
class B
{
public:
A* Create_A () { return new A; } // creation rights only with `B`
};
限制特定类型的构造函数(即复制构造函数、默认构造函数).例如std::fstream 不允许通过这种不可访问的构造函数进行复制
Restricting certain type of constructor (i.e. copy constructor, default constructor). e.g. std::fstream doesn't allow copying by such inaccessible constructor
class A
{
public:
A();
A(int);
private:
A(const A&); // C++03: Even `friend`s can't use this
A(const A&) = delete; // C++11: making `private` doesn't matter
};
要有一个公用的委托构造函数，不应该暴露给外部世界:
To have a common delegate constructor, which is not supposed to be exposed to the outer world:
class A
{
private:
int x_;
A (const int x) : x_(x) {} // common delegate; but within limits of `A`
public:
A (const B& b) : A(b.x_) {}
A (const C& c) : A(c.foo()) {}
};
对于 单例模式，当单例 class 不可继承时(如果是可继承然后使用 protected 构造函数)
For singleton patterns when the singleton class is not inheritible (if it's inheritible then use a protected constructor)
class Singleton
{
public:
static Singleton& getInstance() {
Singleton object; // lazy initialization or use `new` & null-check
return object;
}
private:
Singleton() {} // make `protected` for further inheritance
Singleton(const Singleton&); // inaccessible
Singleton& operator=(const Singleton&); // inaccessible
};