inherited constructors

Inherited constructors

Inherited constructors cannot initialize members of derived classes , You can use class initialization , Or append constructor

• using A declaration statement simply makes a name visible in the current scope . When used as a constructor , Will generate code .
  using Base::Base;

1. For each constructor of the base class , Compilers generate a corresponding derived class constructor
2. The control permission of constructor will not be changed , For example, when the constructor in the base class private, Inheritance in derived classes is also private
3. because using Only name can be specified , So if the constructor in the base class exists explicit and constexpr The key and will also be inherited .
4. If the constructor of a base class contains default arguments , Multiple constructors will be generated , Each constructor omits a formal parameter with a default argument . For example, the base class has a constructor with two parameters , The second parameter is the default argument , Then a constructor with two parameters ( Are not default arguments ) And a parameter ( The first parameter ) Constructor for
5. Default 、 Copy 、 The move constructor is not inherited , These constructors are synthesized according to normal rules

C++11 The standard stipulates , Inherit constructors and some default functions of classes （ Default structure 、 destructor 、 Copy constructors, etc ） equally , Is an implicit declaration , If an inheritance constructor is not used by related code , The compiler doesn't generate real function code for it . This is better than using derived class constructors “ Pass through constructor parameters ” To complete the initialization of the base class , You always need to define various constructors of derived classes to save more object code space .

In the case of multiple inheritance ,

The inheritance constructor will appear “ Conflict ” The situation of , Because some constructors in multiple base classes may cause the function name of the inherited constructor in the derived class to be the same as the parameter

class A {
    
public:
	A(int i){
    }
};

class B {
    
public:
	B(int i){
    }
};

class C : public A, public B {
    
public:
	// using A::A; // Compilation error , Repeat the definition C(int)
	// using B::B; // Compilation error , Repeat the definition C(int) 
	
	//  Display definition inheritance constructor  C(int)
	C(int i):A(i),B(i){
    }
};