Why doesn't the pointer write the address of the class? - c++

Question

I have an abstract class "Mark" and it has a child class "Int_num". I also have a "Subject" class. I want a pointer to the address in the memory of the "Mark" class to be written to the "mark" parameter when calling its constructor. What should I do to make the mark pointer point to the "Mark" class?" occurred, after the compiler complaint about "expression must have class type" or something like that in mark.print_mark()?
class Mark {
private:
int mark;
public:
virtual void change_mark(int);
virtual void print_mark();
virtual int return_mark();
};
class Int_mark : public Mark {
private:
int mark;
public:
Int_mark();
Int_mark(int);
~Int_mark();
void change_mark(int = 0);
void print_mark() const;
int return_mark() const;
};
Int_mark::Int_mark() {
std::string str_mark;
std::cout << "New mark: ";
std::cin.ignore();
std::getline(std::cin, str_mark);
str_mark = ltrim(rtrim(str_mark));
int new_mark;
try {
new_mark = stoi(str_mark);
} catch(...) {
std::cout <<"wq";
mark = 1;
return ;
}
try {
if((new_mark < 1) || (new_mark > 5))
throw 1;
else
mark = new_mark;
} catch(int a) {
std::cout << "qw" << std::endl;
mark = 1;
}
}
void Int_mark::print_mark() const {
std::cout << "Mark: " << mark << std::endl;
}
Subject
#include "Mark.h"
#include <string>
#include <vector>
class Subject {
private:
std::string name_subject;
std::string type_subject;
unsigned hour_subject = 0;
void *mark = nullptr;
public:
Subject();
Subject(std::string, int);
Subject(std::string, bool);
~Subject();
void change_mark(unsigned);
void change_mark(bool);
void rename_subj(std::string);
void add_hour(unsigned);
};
Subject::Subject() {
std::string name_sub;
std::cout << "Введите название предмета: ";
getline(std::cin, name_sub);
name_sub = split_string(name_sub);
name_subject = name_sub;
int select = 2;
if(select == 1) {
type_subject = "Bool";
//mark = new Bool_mark();
} else {
type_subject = "Int";
mark = new Int_mark();
//What should I do to make the mark pointer point to the "Mark" class?
mark.print_mark();
}
}
main
#include "subject/Subject.h"
using namespace std;
int main() {
Subject q;
}
What am I doing wrong? How should I do this?

Answer 1

The pointer mark is of type void *. You could cast it with
static_cast<Int_mark*>(mark)
and call the function with
static_cast<Int_mark*>(mark)->print_mark();
But usually in OOP mark would be a pointer to the base class
Mark *mark = nullptr;
Now you can check for errors with
mark = new Int_mark();
auto *m = dynamic_cast<Int_mark*>(mark);
if (m)
m->print_mark();
Remember the virtual destructor in the base class
virtual ~Mark();
When to use virtual destructors?
Here is a fixed version of your code:
#include <iostream>
#include <string>
#include <vector>
class Mark {
public:
virtual ~Mark() = default;
//virtual void change_mark(int) = 0;
virtual void print_mark() const = 0;
//virtual int return_mark() const = 0;
};
class Int_mark : public Mark {
private:
int mark;
public:
Int_mark();
Int_mark(int);
~Int_mark() override = default;
//void change_mark(int = 0) override;
void print_mark() const override;
//int return_mark() const override;
};
Int_mark::Int_mark() {
std::string str_mark;
std::cout << "New mark: ";
std::cin.ignore();
std::getline(std::cin, str_mark);
//str_mark = ltrim(rtrim(str_mark));
int new_mark;
try {
new_mark = stoi(str_mark);
} catch(...) {
std::cout <<"wq";
mark = 1;
return ;
}
try {
if((new_mark < 1) || (new_mark > 5))
throw 1;
else
mark = new_mark;
} catch(int a) {
std::cout << "qw" << std::endl;
mark = 1;
}
}
void Int_mark::print_mark() const {
std::cout << "Mark: " << mark << std::endl;
}
class Subject {
private:
std::string name_subject;
std::string type_subject;
unsigned hour_subject = 0;
Mark *mark = nullptr;
public:
Subject();
Subject(std::string, int);
Subject(std::string, bool);
~Subject();
void change_mark(unsigned);
void change_mark(bool);
void rename_subj(std::string);
void add_hour(unsigned);
};
Subject::Subject() {
std::string name_sub;
std::cout << "Введите название предмета: ";
getline(std::cin, name_sub);
//name_sub = split_string(name_sub);
name_subject = name_sub;
int select = 2;
if(select == 1) {
type_subject = "Bool";
//mark = new Bool_mark();
} else {
type_subject = "Int";
mark = new Int_mark();
auto *m = dynamic_cast<Int_mark*>(mark);
if (m)
m->print_mark();
}
}
Subject::~Subject() {
delete mark;
}
int main() {
Subject q;
}

Answer 2

Since I did not correctly understand the question in the first place, here a way how you can call the member function of base class Mark by object of derived class Int_Mark:
Int_mark *mark = new Int_mark();
mark->print_mark(); // calls member of the class Int_mark
mark->Mark::print_mark(); // calls member of the class Mark
Make sure that Mark::print_mark() is also defined and not just Int_mark::print_mark()