c++ - Virtual baseclass calls empty constructor in C++ (C++11)

Question

Lets look at the following code:

class A{
protected:
  int _val;
public:
  A(){printf("calling A empty constructor\n");}
  A(int val):_val(val){printf("calling A constructor (%d)\n", val);}
};

class B: virtual public A{
public:
  B(){printf("calling B empty constructor\n");}
  B(int val):A(val){printf("calling B constructor (%d)\n", val);}
};

class C: public B{
public:
  C(){printf("calling C empty constructor\n");}
  C(int val):B(val){printf("calling C constructor (%d)\n", val);}
};

int main(void) {
  C test(2);
}

The output is:

calling A empty constructor
calling B constructor (2)
calling C constructor (2)

Could somebody explain to me why the A class constructor is called without any arguments? Additional how can I "fix" this behaviour if I want the B class to inherit virtually from A? (If the inheritance is not virtual - the sample works fine)

Answer 1

In c++03 it'd be the same.

Virtual base constructors are always called from the final leaf class. If you want something else than the default constructor for A when instantiating a C, you have to specify it in the constructor of class C too.

C(int val): A(val), B(val) {printf("calling C constructor (%d)\n", val);}