constexpr

This value/function can be evaluate at compile time.

If the compiler has enough information, it does the computation during compilation, not at runtime.

This gives you:

• Better performance (no runtime cost)

• Stronger correctness checks

• Ability to use results in places that require compile-time constants

constexpr variables

constexpr int size = 10;
int arr[size]; // OK: size is known at compile time

• Comparison

const int x = 10; // maybe compile-time
constexpr int y = 10; // guaranteed compile-time

constexpr functions

A constexpr function:

• Can run at compile time if arguments are known
• Otherwise runs normally at runtime

constexpr int square(int x) {
  return x * x;
}

constexpr int a = square(5); // compile-time
int b = square(7); // runtime
// Same function, two different evaluation times.

Rules or constexpr functions

• C++11 (strict)

  • Single return statement
  • No loops, no branches

• C++14+ (relaxed)

  • if, for, while and local variables

constexpr int factorial(int n) {
  int result = 1;
  for (int i = 1; i <= n; ++i) {
    return *= i;
  }
  return result;
}

constexpr int f = factorial(5); // 120

constexpr constructors & classes

struct Point {
  int x, y;
  constexpr Point(int a, int b) : x(a), y(b) {}
};

constexpr Point p (1, 2); // created at compile time

Useful for:

• Geometry
• Configuration tables
• Embedded systems (flash storage)

When compile-time evaluation Fails

int x;
constexpr int y = x; // error: x not know at compile time

constexpr int foo(int x) {
  return x + rand(); // not allowed
}

Compile-time code must be:

• Deterministic
• No, I/O
• No system calls
• No heap allocation (until C++20 limited support)