1. Introduction

In C++ programming, const and constexpr are both used to declare constants, but they serve different purposes and have different capabilities. Understanding their differences is key to writing more efficient and reliable C++ code.

2. Key Points

1. const is used to define constants that cannot be changed after initialization.

2. constexpr is used to define constants that are evaluated at compile time.

3. While const can be used for both compile-time and runtime constants, constexpr ensures the constant is evaluated at compile time.

4. constexpr can be used for functions and variables, allowing more complex compile-time calculations.

3. Differences

const	constexpr
Declares a variable as constant.	Declares a variable or function as constant expression.
Can be initialized at runtime.	Must be initialized at compile time.
Used for variables.	Used for both variables and functions.

4. Example

#include <iostream>
using namespace std;

const int constVar = 10; // const example
constexpr int constexprVar = constVar + 5; // constexpr example

int main() {
    cout << "constVar: " << constVar << endl;
    cout << "constexprVar: " << constexprVar << endl;

    return 0;
}

Output:

constVar: 10
constexprVar: 15

Explanation:

1. constVar is declared as a const integer and cannot be modified after its initialization.

2. constexprVar is declared as a constexpr integer, meaning its value is computed at compile time.

5. When to use?

- Use const for variables that need to be constant after their initialization, which can happen at runtime or compile time.

- Use constexpr for expressions that you want to be evaluated at compile time, such as for optimization purposes and to enforce that certain expressions are constant throughout your program.