Multi tool use

C++ downgrade parent pointer to child pointer used as function parameter

I want use child pointer in place of parent parameter in some function.
Here is simplified code

#include <iostream>
#include <string>
using namespace std;
class Parent
{
private:
string value;
public:
void SetValue(const string& in){ value = in; }
const string& GetValue() const { return value; }
virtual ~Parent() {};
};
class Child : public Parent
{
};
void Func(Parent* obj)
{
Child* cobj = dynamic_cast<Child*>(obj);
cobj->SetValue("new value");
delete cobj;
}
int main(int argc, char** argv)
{
Child* cobj = new Child();
Func(cobj);
cout<<"value: "<<cobj->GetValue();
return 0;
}


But it returns value:
Func does not set the value property. How can I fix the code?
UPDATE1:
But with this code:

value:

Func

#include <iostream>
#include <string>
using namespace std;
class Parent
{
private:
string value;
public:
void SetValue(const string& in){ value = in; }
const string& GetValue() const { return value; }
virtual ~Parent() {};
};
class Child : public Parent
{
private:
int number;
public:
void SetNumber(int in){ number = in; }
int GetNumber() const { return number; }
};
void Func(Parent* obj)
{
Child* cobj = dynamic_cast<Child*>(obj);
cobj->SetValue("new value");
cobj->SetNumber(100);
delete cobj;
}
int main(int argc, char** argv)
{
Child* cobj = new Child();
Func(cobj);
cout<<"value: "<<cobj->GetValue()<<endl;
cout<<"number: "<<cobj->GetNumber()<<endl;
return 0;
}


I get value: number: 100
Why SetNumber work but SetValue does not?

value: number: 100

Func

main

dynamic_cast

dynamic_cast

Parent* obj

Func

delete cobj;

cobj

Func

obj

cobj

SetNumber

2 Answers
2

You delete the object and then continue to access it. This causes undefined behaviour, which means anything may happen.

You can fix the code by removing the line delete cobj;. However you should also check that the result of the dynamic_cast is not null before dereferencing it.

delete cobj;

dynamic_cast

In case it is unclear, the delete expression deletes the object being pointed to. There is no such concept as "delete the pointer".

delete

Your problem has nothing to do with inheritance or function parameters. The problematic code can be reduced to:

Child* cobj = new Child();
cobj->SetValue("new value");
delete cobj;
cobj->GetValue();


The last two lines are the problem. delete cobj destroys the object, but cobj->GetValue() still tries to use it. This code has undefined behavior.

delete cobj

cobj->GetValue()

To fix the code, you need to remove either delete cobj or the GetValue() call. In any case, don't try to use an object after it has been destroyed.

delete cobj

GetValue()

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