How do you copy a object? - c++

Question

I am trying to make a deep copy of the object but for some reason I keep getting an error thrown at me.
Invalid address specified to RtlValidateHeap( 000002081EF60000, 000002081EF755C0 )
What does this mean? I have commented out the last two lines as this is not what causes the error but is what I would like to achieve.
#include "pch.h"
#include <iostream>
class Constant
{
public:
int Value;
Constant(int value) {
Value = value;
}
void set_value(int new_value) {
Value = new_value;
}
int get_value() {
return Value;
}
};
int main() {
std::shared_ptr<Constant> foo = std::make_shared<Constant>(1);
std::shared_ptr<Constant> bar(foo.get());
//bar->set_value(4);
//std::cout << (foo->get_value()) << " should be differnt than " << (bar->get_value());
return 0;
}
I expect the output "1 should be different from 4".

Answer 1

You are getting heap corruption due to a double-free problem. Your second shared_ptr is constructed with a raw pointer, which is already managed by the first shared_ptr. So both try to manage the same object, which leads to a double-free at the end of the scope.
To make a copy of an object in C++, just use copy construction:
#include "pch.h"
#include <iostream>
class Constant
{
public:
int Value;
Constant(int value) {
Value = value;
}
void set_value(int new_value) {
Value = new_value;
}
int get_value() {
return Value;
}
};
int main() {
Constant foo(1);
Constant bar(foo); // copy-construction
bar.set_value(4);
std::cout << (foo.get_value()) << " should be differnt than " << (bar.get_value());
return 0;
}
Note: in this example it is sufficient to rely on the default compiler-generated copy constructor.

Answer 2

Phil1970 got it right in the comments.
#include "pch.h"
#include <iostream>
class Constant
{
public:
int Value;
Constant(int value) {
Value = value;
}
void set_value(int new_value) {
Value = new_value;
}
int get_value() {
return Value;
}
};
int main() {
std::shared_ptr<Constant> foo = std::make_shared<Constant>(1);
std::shared_ptr<Constant> bar = std::make_shared<Constant>(*foo);
bar->set_value(4);
std::cout << (foo->get_value()) << " should be differnt than " << (bar->get_value());
return 0;
}
His solution allows you to use shared pointers and still obtain the desired result. Thanks Phil!

Answer 3

When you call this function：
std::shared_ptr<Constant> bar(foo.get());
You are directly taking the data pointer of the ptr internal and adding an additional controller(bar) to it. So don't pass the data ptr instead of the share ptr itself to the constructor.It should be like:
std::shared_ptr<Constant> bar(foo);