A **[`constexpr` variable](https://www.educative.io/answers/what-is-the-constexpr-keyword-in-cpp)** is used when its value is supposed to be constant throughout a program's execution. The `constexpr` keyword computes the value of such a variable during compilation, and the compiler looks to see if it can detect any changes to that variable.

 

# Manual initialization

A `constexpr` array can be declared and initialized manually through:

```c++
constexpr int arr[3] = {1,2,3};
```

However, manually initializing a large array can become tiresome.

## Using struct

For example, to initialize an integer array of size $n$, with integers from $0$ to $n-1$, a [struct](https://www.educative.io/answers/what-is-a-cpp-struct) is declared that has an `int` array. This array is then initialized using a `constexpr` constructor.

> A [template](https://www.educative.io/answers/what-are-class-templates-in-c) with **non-type parameters** has been used in the code below to declare an array of a particular size.

## Code

#include <iostream>
using namespace std;

template<int size>
struct ConstArray{
  int arr[size];

  // 'constexpr' constructor:
  constexpr ConstArray():arr(){
    for(int i = 0; i < size; i++)
      arr[i] = i;
  }

  // This member function should have 'const':
  void print() const{
    for(int i = 0; i < size; i++)
      cout << arr[i] << endl;
  }
};

int main() {
  constexpr int n = 5;
  constexpr ConstArray arr = ConstArray<n>();
  arr.print();

  return 0;
}

How to create an N-element constexpr array in C++

constexpr initializes constant array values during compilation. Manual initialization can be tedious for large arrays. Structs and templates streamline this process.