March 11, 2022 diary: Mr. Wang's spring, strange template mode

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---

DEMO1

template <typename T>  
	class Base 
	{
    
	public:
		void asDerived()
		{
    
			T& derived = static_cast<T&>(*this); 
			derived.myfunc(); 
		}
	private:
		Base() {
    };
		friend T; 
	};

	class Derived1 : public Base<Derived1> 
	{
    
	public:
		void myfunc()
		{
    
			cout << "Derived1::myfunc() Yes " << endl;
		}
		//......
	};

Case study 1 It can be noted that ,Base Class T& derived = static_cast<T&>(*this); this It's a little weird , At least it seems strange to me , It is a template T Type variables are placed in Base Class , And then call T Functions of type objects ,

Derived1 myd;
myd.asDerived(); // Call the member function of the base class 

stay main Function is called in this way , It really looks like a monk , It's confusing , It's very strange .

DEMO2

template<typename T>
struct shape
{
    
	// The judgment of whether the derived class objects are equal is moved to the base class ( Using the method of defining friend functions in class templates, the global operator== Put in base class )
	friend bool operator==(const shape<T>& obj1, const shape<T>& obj2) // Define friends in the class template 
	{
    
		const T& objtmp1 = static_cast<const T&>(obj1);// Derived class objects are also base class objects , So this static type conversion is OK 
		const T& objtmp2 = static_cast<const T&>(obj2);
		if (!(objtmp1 < objtmp2) && !(objtmp2 < objtmp1))
			return true;
		return false;
	}
};

struct square : public shape<square>
{
    
	int sidelength; // Side length 
};

// Out of class operator overload 
bool operator<(square const& obj1, square const& obj2)
{
    
	if (obj1.sidelength < obj2.sidelength)
	{
    
		return true;
	}
	return false;
}

Case study 2 It's a case 1 A derivative of , But there is operator Knowledge of overloading and friends . Collect this knowledge first , I know there's such a thing , Maybe you can use it when you have time or have a whim in the future ！