从没有默认构造函数的类继承的类

Question

Right now I have a class A that inherits from class B , and B does not have a default constructor. I am trying the create a constructor for A that has the exact same parameters for B 's constructor

struct B {
  int n;
  B(int i) : n(i) {}
};

struct A : B {
  A(int i) {
    // ...
  }
}; 

but I get:

error: no matching function for call to ‘B::B()’
note: candidates are: B::B(int)

How would I fix this error?

Answer 1

The constructor should look like this:

A(int i) : B(i) {}

The bit after the colon means, "initialize the B base class sub object of this object using its int constructor, with the value i".

I guess that you didn't provide an initializer for B, and hence by default the compiler attempts to initialize it with the non-existent no-args constructor.

Answer 2

You need to invoke the base constructor via your class' initializer list.

Example:

class C : public B
{
public:
    C(int x) : B(x)
    {
    }

};

When you don't initialize B explicitly it will try to use the default constructor which has no parameters.