make an object of one class in other class

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C++

hi. oh i am sorry . i have question again.

class A
{
public:
    int data;
    A(int d):data(d){}
};
class B
{

public:
    A s;
    B(A e):s(e){}
};
int _tmain(int argc, _TCHAR* argv[])
{
    A ins(12);
    B ins1(ins);
    return 0;
}

in above code i make an object of class A in class B. when an object want to make . the constructor should call. in the line

A s;

the constructor of class A should call . yes? but there isn't any default constructor to call. i think this line is false and i should write

A s(0);

because the constructor of class A have an argument . but this is false and the

A s;

is true. but i don't know why ?
please help me
thanks