constexpr variables

Learn compile-time constants with constexpr variables.

constexpr variables

The constexpr keyword declares a variable whose value can be computed at compile time. Example: constexpr int monthsPerYear = 12;. The compiler knows this value at compile time and bakes it into the executable as a ready constant.

You can use these constants in other constexpr expressions: constexpr int monthsInDecade = monthsPerYear * 10;. Both values are known at compile time, so the product is computed at compile time and the result 120 is ready at run time.

constexpr versus const

const says a variable will not be modified later; but its value may be computed at run time: const int n = getValue(); is valid.

constexpr is stricter: the value must be computable at compile time. So constexpr int n = getValue(); compiles only if getValue is constexpr. constexpr is used where a compile-time constant is required (array size, template parameter).

Exercise Define the constexpr variables

Define constexpr int monthsPerYear = 12; and constexpr int monthsInDecade = monthsPerYear * 10;. In main write std::cout << monthsInDecade;.

Show example solution

#include <iostream>

constexpr int monthsPerYear = 12;
constexpr int monthsInDecade = monthsPerYear * 10;

int main() {
    std::cout << monthsInDecade;
    return 0;
}

Test yourself

What does constexpr int x = 5; guarantee?
```
constexpr int x = 5;
```
- It only works with float
- It is computed only at run time
- The variable can be modified later
- The value is known at compile time
Show answer

The value is known at compile time — constexpr declares a constant known at compile time.

Complete the constexpr constant

constexpr int monthsPerYear = ____;

Show answer

constexpr int monthsPerYear = 12;

Match each declaration to its trait

constexpr int x = 5;

Compile-time constant
const int y = getValue();

Run-time constant
constexpr int z = x * 2;

Computable constant
int w = 7;

Mutable variable

Next lesson: constexpr functions

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