How do I instantiate an object inside of a C++ class? - c++

Question

For C++ learning purposes, I have the files class1.h, class1.cpp, class2.h and class2.cpp. I would like to instantiate an object named class1Obj inside class2. Where and how do I instantiate this object? Do I instantiate classObj inside the class2 constructor?
In the past I have created a pointer to a class, which worked well for that time, but I think a pointer is not the route I should take this time because the classObj will only be used inside class2.

Answer 1

class class1
{
//...
};
class class2
{
class1 member;
//...
};
In class2 ctor, you can initialize member in the constructor initialization list.
class2::class2(...)
: member(...)
{
//...
}

Answer 2

Well how did you create a pointer in the past? Presumably, you did something like this:
class class2
{
public:
class2()
{
class1Pointer = new class1();
}
// Destructor, copy constructor/assignment, etc...
private:
class1* class1Pointer;
};
Now you want to do exactly the same but this time you don't want a pointer to class1, you want a class1 itself:
class class2
{
public:
class2() {}
// Destructor, copy constructor/assignment, etc...
private:
class1 class1Obj;
};
The object will be default initialized when your class2 object is created. If your class1 constructor should take some arguments, use an initialization list:
class class2
{
public:
class2() : class1Obj(1, 2, 3) {}
// Destructor, copy constructor/assignment, etc...
private:
class1 class1Obj;
};

Answer 3

Instantiate a class inside a class :
#include <iostream>
using namespace std;
class Foo
{
public:
Foo(int i)
{
}
};
class Bar
{
Foo i; //<--- instantiate a class inside a class ----
public:
Bar() : i(1) //<--- instantiate a class inside a class ----
{
}
};
int main(void)
{
Bar b;
cout<<" \nPress any key to continue\n";
cin.ignore();
cin.get();
return 0;
}

Answer 4

It depends on your Class1. If its constructor accepts some parameters, then you must initialize it explicitly in Class2 constructor or in initialization list.
Class2 {
public:
class2() {
//Here m_class1Obj will be instantiated
m_class1Obj = Class1(/*some params*/);
}
private:
Class1 m_class1Obj;
};
Or
Class2 {
public:
class2() : m_class1Obj() {}
private:
Class1 m_class1Obj;
};