enums.md

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Enumeration types

Today:

• What are enumeration types
• How to use them
• How not to use them

📺 Watch the related YouTube video!

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Special symbols used in slides

• 🎨 - Style recommendation
• 🎓 - Software design recommendation
• 😱 - Not a good practice! Avoid in real life!
• ✅ - Good practice!
• ❌ - Whatever is marked with this is wrong
• 🚨 - Alert! Important information!
• 💡 - Hint or a useful exercise
• 🔼1️⃣7️⃣ - Holds for this version of C++(here, 17) and above
• 🔽1️⃣1️⃣ - Holds for versions until this one C++(here, 11)

Style (🎨) and software design (🎓) recommendations mostly come from Google Style Sheet and the CppCoreGuidelines

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🔼1️⃣1️⃣ Enumeration classes

• Store an enumeration of options

  enum class FoodOptions : BaseType { kPizza, kPasta, kSushi };
  enum class ChoicesEnum { kYay, kNay };

• By default BaseType is int, can be any other integral type
• Enums can have any number of options
• The options are fixed upon enum definition
• Options are assigned consequent numbers
• Mostly used to pick a path in a switch statement
• Use values as: FoodOptions::kPizza, ChoicesEnum::kYay, ...
• 🎨 Name enum types in CamelCase
• 🎨 Name enum values as constants: kSomeConstant

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Example

#include <iostream>
#include <string>

enum class OutputChannel { kStdOut, kStdErr };

// Note that output_channel is passed by value!
void Print(OutputChannel output_channel, const std::string& msg) {
  switch (output_channel) {
    case OutputChannel::kStdOut:
      std::cout << msg << std::endl;
      break;
    case OutputChannel::kStdErr:
      std::cerr << msg << std::endl;
      break;
  }
}
int main() {
  Print(OutputChannel::kStdOut, "hello");
  Print(OutputChannel::kStdErr, "world");
  return 0;
}

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Explicit values

• By default enum values start from 0
• We can specify custom values if needed

  enum class EnumType {
    kOption1 = 10,      // Decimal
    kOption2 = 0x2,     // Hexadecimal
    kOption3 = 0b101010 // Binary
  };

• 💡 Most of the times enums are used with default values

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😱 The olden days enum

• Before C++11 you would define an enum without class:

  enum MyEnum{kValue1, kValue2};  // 😱 Don't do this in C++11

• Old enums can be implicitly converted to int:
• These enums are not scoped like the class ones

  enum OldEnum{kValue1, kValue2};  // Really, don't use it.
  enum class NewEnum{kValue1, kValue2};
  int main() {
    const OldEnum old_value = kValue1; // Note no OldEnum:: prefix
    const int some_value = kValue1;
    const NewEnum new_value = NewEnum::kValue1;
    return 0;
  }

• ❓😱 Do you see an issue with this?

  enum EnumDecision{kRight, kWrong};
  enum EnumSide{kLeft, kRight};

  
  Try to compile this!

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