Have a C++ variable able to be either Class Foo or Class Bar

Question

I am interested to write a printer to output two different formats. Let's call them txt and csv.

Can something like below be implemented in C++?

class Printer {
public:
  virtual void header() = 0;
};

class CSV : Printer {
public:
 void header() { printf("csv\n"); }
};

class TXT : Printer {
public:
 void header() { printf("txt\n"); }
};

int decider(int type) {
  auto prt;
  if (type == 1) {
     prt = new CSV;
  } else {
     prt = new TXT;
  }
  prt.header();
}

If not, what's an alternative to doing something similar?

Operator `new` returns a pointer. Each call to `new` must have a matching call to `delete`!! — Nathan Xabedi, Apr 08 '21 at 18:57
In order for this to work correctly (without either a leak or undefined behavior), you will need a [virtual destructor](https://stackoverflow.com/q/461203/10077) in `Printer`. — Fred Larson, Apr 08 '21 at 18:59

Cory Kramer · Accepted Answer · 2021-04-08T19:51:16.707

5

Instead of auto you'd want a pointer to the base class

int decider(int type) {
  Printer* prt;
  if (type == 1) {
     prt = new CSV;
  } else {
     prt = new TXT;
  }
  prt->header();
}

but instead of raw pointers and new (since you are leaking memory from a missing delete) I'd instead use smart pointers here

#include <memory>

int decider(int type) {
  std::unique_ptr<Printer> prt;
  if (type == 1) {
     prt = std::make_unique<CSV>();
  } else {
     prt = std::make_unique<TXT>();
  }
  prt->header();
}

As noted by @FredLarson you also need a virtual destructor in your base class

class Printer {
public:
  virtual ~Printer() = default;
  virtual void header() = 0;
};

you should also inherit publicly from the base class

class CSV : public Printer { 
  ...
};

class TXT : public Printer {
  ...
};

edited Apr 08 '21 at 19:51

answered Apr 08 '21 at 18:50

Cory Kramer

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You can't say `prt.header` in this code. Either change `prt` to `prt_p` and after the if statement make a `Printer& prt`, or change the last line to `prt->header`. – Daniel H Apr 08 '21 at 18:52
Thanks to both of you. – Laurenţiu Terţan Apr 08 '21 at 18:55
4

I believe `Printer` will need a virtual destructor to avoid UB here. – Fred Larson Apr 08 '21 at 18:55

score 2 · Answer 2 · answered Apr 08 '21 at 18:54

2

This is the right way:

void decider(int type) {
  std::unique_ptr<Printer> prt{};
  if (type == 1) {
     prt = std::make_unique<CSV>();
  } else {
     prt = std::make_unique<TXT>();
  }
  prt->header();
}

answered Apr 08 '21 at 18:54

Nathan Xabedi

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I believe `Printer` will need a virtual destructor to avoid UB here. – Fred Larson Apr 08 '21 at 18:58
1

Extensions: Consider returning the `unique_ptr` rather than calling the `header` function. This will allow you to reuse `decider` for similar tasks in the future. Also consider using an `enum` for the type rather than an `int`. – user4581301 Apr 08 '21 at 19:12

score 2 · Answer 3 · answered Apr 08 '21 at 19:02

Yes, this kind of polymorphism is possible. Here is a runnable example based on yours:

#include <stdio.h>

class Printer {
public:
  virtual void header() {};
};

class CSV : public Printer {
public:
 void header() { printf("csv\n"); }
};

class TXT : public Printer {
public:
 void header() { printf("txt\n"); }
};

int decider(int type) {
  Printer* prt;
  if (type == 1) {
     prt = new CSV();
  } else {
     prt = new TXT();
  }
  prt->header();
}

int main() {

    int type = 1;

    decider(type);

    type = 0;

    decider(type);
}

max66 · Answer 4 · 2021-04-08T19:00:25.123

0

What about passing the required type as template parameter?

I mean something as

template <typename T>
void decider ()
 {
   auto prt = new T;
   prt.header();

   delete prt;
 }

that you can call as follows

decider<CSV>();
decider<TXT>();

As pointed by Daniel H (thanks!) this only works if you know the type compile-time; if the type (CSV or TXT) is decided run-time, this solution is impracticable.

edited Apr 08 '21 at 19:00

answered Apr 08 '21 at 18:56

max66

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That decides at compile time not runtime. – Daniel H Apr 08 '21 at 18:57
@DanielH - Good point. – max66 Apr 08 '21 at 18:58

Have a C++ variable able to be either Class Foo or Class Bar

4 Answers4