Why does my C++ subclass need an explicit constructor?

Question

I have a base class that declares and defines a constructor, but for some reason my publicly derived class is not seeing that constructor, and I therefore have to explicitly declare a forwarding constructor in the derived class:

class WireCount0 {
protected:
    int m;
public:
    WireCount0(const int& rhs) { m = rhs; }
};

class WireCount1 : public WireCount0 {};

class WireCount2 : public WireCount0 {
public: 
  WireCount2(const int& rhs) : WireCount0(rhs) {}
};

int dummy(int argc, char* argv[]) {
    WireCount0 wireCount0(100);
    WireCount1 wireCount1(100);
    WireCount2 wireCount2(100);
    return 0;
}

In the above code, my WireCount1 wireCount1(100) declaration is rejected by the compiler ("No matching function for call to 'WireCount1::WireCount1(int)'"), while my wireCount0 and wireCount2 declarations are fine.

I'm not sure that I understand why I need to provide the explicit constructor shown in WireCount2. Is it because the compiler generates a default constructor for WireCount1, and that constructor hides the WireCount0 constructor?

For reference, the compiler is i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5659).

Answer 1

Constructors are not inherited. You have to create a constructor for the derived class. The derived class's constructor, moreover, must call the base class's constructor.