How to pass instance of one class to another via constructor - c++

Question

I have a simple question. How would you go about passing an instance of a class to a constructor of another class in c++? I have experience in C, however I am struggling with the syntax of c++.
For example say I have Class A and want to pass and Instance of it to Class B.
A.h
class A {
A();
virtual ~A();
public:
B *newb;
}
A.cpp
A::A() {
newb = new B(this);
}
b.h
class B {
B(A *instanceA);
virtual ~B();
}
Could someone please provide me with a simple example? It would be much appreciated.
EDIT: I tried this concept with my current code, but kept getting errors. Sorry I wanted to make sure that I used the right principle. This is a snippet of the code that I am currently working on.
//SentencSelection.h
class SentenceSelection
{
public:
SentenceSelection(TestBenchGui *test);
virtual ~SentenceSelection();
//do stuff
};
//SentencSelection.cpp
#include <iostream>
#include "SentenceSelection.h"
using namespace std;
SentenceSelection::SentenceSelection(TestBenchGui *test)
{
//do stuff
}
SentenceSelection::~SentenceSelection()
{
}
//TestBenchGui.h
#include "SentenceSelection.h"
class TestBenchGui
{
public:
TestBenchGui();
virtual ~TestBenchGui();
private:
SentenceSelection *selection;
};
//TestBenchGui.cpp
#include "TestBenchGui.h"
#include "SentenceSelection.h"
using namespace std;
TestBenchGui::TestBenchGui()
{
selection = new SentenceSelection(this);
}
TestBenchGui::~TestBenchGui()
{
}
When I compile this in eclipse I get the follwing error "expected constructor, destructor, or type conversion before ‘(’ token" for the line - "SentenceSelection::SentenceSelection(TestBenchGui test)" in SentenceSelection.h.

Answer 1

I believe the code you have does pretty much what you are asking, but there are a couple of syntax errors that are probably holding you back from compilation (and thus leading to some confusion). Below is code that I put in a single file (main.cpp) and was able to compile. I added some comments and print statements to highlight the difference between DECLARATIONS and IMPLEMENTATIONS:
#include <iostream>
using std::cout;
using std::endl;
// forward declaration so we can refer to it in A
class B;
// define A's interface
class A {
public:
// member variables
B *newb;
// constructor declaration
A();
// destructor declaration
virtual ~A();
}; // note the semicolon that ends the definition of A as a class
class B {
public:
// B constructor implementation
B(A *instanceA){cout << "I am creating a B with a pointer to an A at " << instanceA << endl;}
// B destructor implementation
virtual ~B(){cout << "Destructor of B" << endl;}
}; // note the semicolon that ends the definition of B as a class
// A constructor implementation
A::A(){
cout << "I am creating an A at " << this << ", allocating a B on the heap..." << endl;
newb = new B(this);
}
// A destructor implementation
A::~A(){
cout << "Destructor of A, deleting my B...." << endl;
delete newb;
}
int main(){
A an_a;
}
Output for my system from running the above program:
I am creating an A at 0x7fff64884ba0, allocating a B on the heap...
I am creating a B with a pointer to an A at 0x7fff64884ba0
Destructor of A, deleting my B....
Destructor of B
Hope this helps.

Answer 2

For example say I have Class A and want to pass and Instance of it to Class B.
If you intend to pass a pointer to the instance, then you have already figured it out. The constructor that you declared takes an A* as an argument:
B(A *instanceA);

Answer 3

#include <iostream>
using namespace std;
class B
{
public:
B(){};
int x;
~B(){};
};
class A
{
public:
A(){}; // default constructor
A(B& b) // constructor taking B
{
y = b.x;
}
~A(){};
int y;
};
int main()
{
B myB;
myB.x = 69;
A myA(myB);
cout << myA.y << endl;
return 0;
}

Answer 4

I answer strictly your question:
If you want to pass a instance on a constructor best practice is pass as a reference:
class CA{
CB &b;
public:
CA( CB &p_b) : b(p_b){};
virtual ~CA();
};
But in your example you have a classes with cross references that is another case