How to pass const pointer values in classes in C++ - c++

Question

I'm new to the world of C++ (and OOP).
I'm learning about classes and pointers at this moment, but I'm stuck and hope someone can explain to me what I'm missing or should dig deeper into to broaden my understanding.
Example 1 works:
#include <iostream>
#include <string.h>
using namespace std;
class MyClass
{
private:
char* age;
public:
MyClass(const char* initData)
{
age = NULL;
cout << "In default constructor working on pointers" << endl;
age = new char [strlen(initData)+1];
strcpy(age,initData);
//age = initData;
}
~MyClass()
{
cout << "In destructor working on pointers" << endl;
delete [] age;
}
const char* GetAge()
{
return age;
}
};
int main()
{
//MyClass firstClass(10);
//cout << "First attempt: " ;
//cout << firstClass.GetAge() << endl;
MyClass secondClass("A test from B");
cout << "Second attempt: ";
cout << secondClass.GetAge() << endl;
return 0;
}
However This does not work:
#include <iostream>
#include <string.h>
using namespace std;
class MyClass
{
private:
int* age;
public:
MyClass(const int* initData)
{
cout << "In default constructor working on pointers" << endl;
age = new int;
//strcpy(age,initData);
age = initData;
}
~MyClass()
{
cout << "In destructor working on pointers" << endl;
delete age;
}
const int* GetAge()
{
return age;
}
};
int main()
{
MyClass firstClass(10);
cout << "First attempt: " ;
cout << firstClass.GetAge() << endl;
//MyClass secondClass("B");
//cout << "Second attempt: ";
//cout << secondClass.GetAge() << endl;
return 0;
}
I feel that it has to do with the fact that I pass an int to a constant pointer.
And then try to assign a constant from r-value to l-value with the = operator.
Although it think this is permitted, since I say 'const int* initData' and this tells the compiler to keep the data from changing but memory address can change?
So in my understanding, I pass value 10 to the class that makes a pointer in the default constructor, this makes a memory address and stores it in initData.
I then pass the r-value InitData (mem address) to the l-value age pointer, that accepts memory addresses.
If I try with the exact same code, but use char and copy data over in my first example it works...
Can anyone explain to me what I'm missing, Thanks in advance!
Edit:
I Think I found my solution and understand it better.
Could anyone verify that this is correct, or which is the (more) correct form:
Scenario 1:
#include <iostream>
using namespace std;
class MyClass
{
private:
int* age;
public:
MyClass(const int *initData)
{
cout << "In default constructor working on pointers" << endl;
age = new int;
(*age) = (*initData);
}
~MyClass()
{
cout << "In destructor working on pointers" << endl;
delete age;
}
const int* GetAge()
{
return age;
}
};
int main()
{
int aNum = 10;
MyClass firstClass(&aNum);
cout << "First attempt: " ;
cout << *firstClass.GetAge() << endl;
return 0;
}
Or Scenario 2:
#include <iostream>
using namespace std;
class MyClass
{
private:
int* age;
public:
MyClass(const int &initData)
{
cout << "In default constructor working on pointers" << endl;
age = new int;
(*age) = (initData);
}
~MyClass()
{
cout << "In destructor working on pointers" << endl;
delete age;
}
const int* GetAge()
{
return age;
}
};
int main()
{
MyClass firstClass(10);
cout << "First attempt: " ;
cout << *firstClass.GetAge() << endl;
return 0;
}
I Would think solution 2, as that uses reference to memory and thus speeds up the execution?
Thanks for the assistance already, and sorry for the long post...

Answer 1

First: you can not set a const pointer to a non-const pointer, you can circumvent this using a const_cast<int*>(initData) which gets rid of the const-ness of initData when assigning to age.
But i do not think this is what you want to achieve.
So first of all a pointer is just pointing to memory - that memory must therefore be managed by someone else than the pointer. Usually you only need a pointer if you want to reference something or iterate - if you intend to be the owner of that memory you might not want to use a pointer.
Secondly: if you start learning C++ now, try to learnt 'modern' C++ and utilize smart_pointers (see smart pointers form cppreference).
A rule of thumb: if you do not necessarily need a pointer (no need for iteration, direct memory access) use by reference instead.
But bottom line: your example looks like it is not meant to be used with pointers at all. Just store the int value.
class MyClass
{
private:
int age;
public:
// pass by const reference, this is cheap and good practice
MyClass(const int& initData)
{
cout << "In default constructor working on pointers" << endl;
// here the value of initData is actually copied into age
age = initData;
}
And if you really want to try pointers: start with the std::shared_ptr which is a ref-counting pointer easy to use.
Example:
#include <memory>
#include <iostream>
class test
{
private:
std::shared_ptr<int> age;
public:
// cheap and quick const ref
test(const std::shared_ptr<int>& data)
{
age = data;
}
void print()
{
std::cout << *age << std::endl;
std::cout << "Use count: " << age.use_count() << std::endl;
}
};
int main()
{
// here we store the data
int i = 17;
// here we have the shared_ptr
std::shared_ptr<int> p = std::make_shared<int>(i);
// here we pass it to the object
test t(p);
t.print();
return 0;
}