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C++ Booleans

A boolean is a value that is either true or false. The bool type holds a boolean value.

A bool prints as a number

A bool variable holds true here. C++ prints a bool as 1 or 0, so this value prints as 1.

#include <iostream>

int main() {
  bool ready = true;
  std::cout << ready << "\n";
  return 0;
}

A bool prints as a word

std::boolalpha tells the print to use words for bools. The same value now prints as true.

#include <iostream>

int main() {
  bool ready = true;
  std::cout << std::boolalpha;
  std::cout << ready << "\n";
  return 0;
}

Two comparisons

Each comparison makes a bool from two numbers. The prints show true first, then false.

#include <iostream>

int main() {
  std::cout << std::boolalpha;
  bool first = 7 >= 7;
  bool second = 3 <= 2;
  std::cout << first << "\n";
  std::cout << second << "\n";
  return 0;
}

Joining and flipping

The && sign needs both sides true, and the || sign needs one side true. The ! sign flips the bool after it.

#include <iostream>

int main() {
  std::cout << std::boolalpha;
  std::cout << "And: " << (true && false) << "\n";
  std::cout << "Or: " << (false || true) << "\n";
  std::cout << "Not: " << (!true) << "\n";
  return 0;
}

Example

#include <iostream>

int main() {
  int age = 20;
  bool member = false;
  std::cout << std::boolalpha;
  std::cout << "Adult: " << (age >= 18) << "\n";
  std::cout << "Member price: " << (age >= 18 && member) << "\n";
  return 0;
}

Key points

  • The bool type holds true or false.
  • Use == to test whether two values are equal.
  • Use != to test whether two values differ.
  • The sign >= tests greater than or equal, and <= tests less than or equal.
  • The && sign needs both sides true, and the || sign needs one side true.
  • The ! sign flips a boolean, turning true into false and false into true.

In this example, age holds 20 and member holds false. The test age >= 18 gives true, so the first line prints Adult: true. The joined test also needs member to be true, so the second line prints Member price: false.

Note

Later lessons set std::boolalpha before printing a bool, so each bool prints as a word.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Programiz C++ Compiler

Exercise

A program sets std::boolalpha. What does 5 != 5 print?