在仅具有参数化的Ctor的另一个类中创建一个类的对象时，“错误：x不是类型”

Question

When I try to create an object of class A having only parametrized Constructor in class B, I get following errors:

A ob(5); error: expected identifier before numeric constant  
A ob(x); error: x is not a type 

class A {
    public:
    int z;
    A(int y){z = y;}
};
class B {
    public:
    int x;
    A ob(5); or A ob(x);//This line is creating a problem
};

I searched for the same and got that we can solve this problem by writing

A ob;
B():ob(5);
OR
int x;
A ob;
B():ob(x);   //here x will be uninitialized though

But I am thinking why it was giving error in the prior case. Can someone explain in detail. Thanks.

Answer 1

You cannot assign default value to class member in C++. You need to define constructor.

class A {
    public:
    int z;
    A(int y){z = y;}
};
class B {
    public:
    int x;
    A ob; //
    B() : x(5), ob(x) {}
};