C ++ - creating a derived class and using the base class constructor

Question

I have a base class with a constructor that requires one parameter (string). Then I have a derived class that also has its own constructor. I want to instantiate a derived class and also set the constructor parameter of the base class.

class BaseClass { public: BaseClass (string a); }; class DerivedClass : public BaseClass { public: DerivedClass (string b); }; int main() { DerivedClass abc ("Hello"); } 

I'm not sure how to set the constructor parameter of a base class when calling a derived class.

Answer 1

You have two options: inline:

 class DerivedClass : public BaseClass { public: DerivedClass (string b) : BaseClass(b) {} }; 

or off line:

 class DerivedClass : public BaseClass { public: DerivedClass (string b); }; /* ... */ DerivedClass::DerivedClass(string b) : BaseClass(b) {} 

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more examples:

 class DerivedClass : public BaseClass { public: DerivedClass(int a, string b, string c); private: int x; }; DerivedClass::DerivedClass(int a, string b, string c) : BaseClass(b + c), x(a) {} 

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in the initializer lists:

 class MyType { public: MyType(int val) { myVal = val; } // needs int private: int myVal; }; class DerivedClass : public BaseClass { public: DerivedClass(int a, string b) : BaseClass(b) { x = a; } // error, this tries to assign 'a' to default-constructed 'x' // but MyType doesn't have default constructor DerivedClass(int a, string b) : BaseClass(b), x(a) {} // this is the way to do it private: MyType x; }; 

Answer 2

If all you want to do is build an instance of the derived class from one parameter that you pass to the constructor of the base class, you can do this:

C ++ 03 (I added an explicit and passed a const link):

 class DerivedClass : public BaseClass { public: explicit DerivedClass (const std::string& b) : BaseClass(b) {} }; 

C ++ 11 (gets all base class constructors):

 class DerivedClass : public BaseClass { public: using BaseClass::BaseClass; }; 

If you want to call different DerivedClass constructors and call the BaseClass constructor for some other value, you can also do this:

 class DerivedClass : public BaseClass { public: explicit DerivedClass () : BaseClass("Hello, World!") {} }; 

Answer 3

Use this

  DerivedClass::DerivedClass(string b) : BaseClass(b) { } 

just pass the parameter directly to the base class constructor