How to Use a PI Constant in C++

1. Use M_PI Macro From GNU C Library
2. Use std::numbers::pi Constant From C++20
3. Declare You Own PI Constant Variable

This article will introduce different ways to declare and use PI constant value in C++.

Use M_PI Macro From GNU C Library

It uses predefined macro expression from C standard math library. The library defines multiple common mathematical constants, which are listed in the following table. M_PI macro can be assigned to a floating point variable or used as a literal value in calculations. Notice that we are using the setprecision manipulator function, which can be used to control the precision to which the output number is displayed.

#include <cmath>
#include <iomanip>
#include <iostream>

using std::cout;
using std::endl;

int main() {
  double pi1 = M_PI;
  cout << "pi = " << std::setprecision(16) << M_PI << endl;
  cout << "pi * 2 = " << std::setprecision(16) << pi1 * 2 << endl;
  cout << "M_PI * 2 = " << std::setprecision(16) << M_PI * 2 << endl;

  cout << endl;
  return EXIT_SUCCESS;
}

Output:

pi = 3.141592653589793
pi * 2 = 6.283185307179586
M_PI * 2 = 6.283185307179586

Use std::numbers::pi Constant From C++20

Since the C++20 standard, the language supports the mathematical constants defined in the <numbers> header. These constants are supposed to offer better cross-platform compliance, but it is still in the early days, and various compilers might not support it yet. The full list of the constants can be seen here.

#include <iomanip>
#include <iostream>
#include <numbers>

using std::cout;
using std::endl;

int main() {
  cout << "pi = " << std::setprecision(16) << std::numbers::pi << endl;
  cout << "pi * 2 = " << std::setprecision(16) << std::numbers::pi * 2 << endl;

  cout << endl;
  return EXIT_SUCCESS;
}

pi = 3.141592653589793
pi * 2 = 6.283185307179586

Declare You Own PI Constant Variable

Alternatively, we can declare a custom constant variable with PI value or any other mathematical constant as needed. It can be achieved either using a macro expression or the constexpr specifier for a variable. The following sample code demonstrates the use of both methods.

#include <iomanip>
#include <iostream>

using std::cout;
using std::endl;

#define MY_PI 3.14159265358979323846
constexpr double my_pi = 3.141592653589793238462643383279502884L;

int main() {
  cout << std::setprecision(16) << MY_PI << endl;
  cout << std::setprecision(16) << my_pi << endl;

  return EXIT_SUCCESS;
}