Question

Could you please check out this piece of code:

#include <vector>
class A
{
public:
    A(int a, int b);
};

class C :public A
{
public:
    C(int a, int b):A(a,b){}
    static C instances;
};

C C::instances;

int main()
{
    return 1;
}

The compilation gives me error as:

$ c++ inheritance.cpp 
inheritance.cpp:16:6: error: no matching function for call to ‘C::C()’
inheritance.cpp:16:6: note: candidates are:
inheritance.cpp:12:2: note: C::C(int, int)
inheritance.cpp:12:2: note:   candidate expects 2 arguments, 0 provided
inheritance.cpp:8:7: note: C::C(const C&)
inheritance.cpp:8:7: note:   candidate expects 1 argument, 0 provided

I need C to inherit from A and I need A to have arguments in its constructor.
Finally, I need the instance’s static variable to be declared and defined with no arguments.
So is there a solution to that? I value your kind comments.

Another point to note:
if the static variable was a container, like:

static std::vector instances;

the code would compile just fine. Why?

EDIT:

Thanks for all the answers,
but, if I modify C C::instances; to C C::instances(0,0); i will get another error:
$ c++ inheritance.cpp
/tmp/cctw6l67.o: In function C::C(int, int)':
inheritance.cpp:(.text._ZN1CC2Eii[_ZN1CC5Eii]+0x1b): undefined reference toA::A(int, int)’
collect2: ld returned 1 exit status

any idea why? and how to fix it?

thanks

Answer 1

If you define a constructor, the compiler no longer generates a default one for you, which you attempt to call with C C::instances;. You can bypass this by calling the available constructor:

C C::instances(0,0);

or provide a default constructor for C.

With

static std::vector<C> instances;

it compiles because no elements are created, and std::vector has a default constructor which initializes an empty vector. But

C::instances.push_back(C());

wouldn’t compile.