Runtime and Compile-time constants in C++

Run-time Constant:

These are the constants whose respective values can only be known or computed at the time of running of source code. Run time Constants are a bit slower than compile-time constants but are more flexible than compile-time constants. However, once it is initialized, the value of these constants can’t be changed.

Below is the program for illustration of Runtime constants:

C++

// C++ program to illustrate
// Run-time Constants
#include <iostream>
using namespace std;

// Driver Code
int main()
{
    // Input a variable
    double electonmass;
    cin >> electonmass;

    // Define a constant
    // and initialize it at
    // run-time
    const double electon_mass{ electonmass };

    // Known to the compiler
    // at the run-time
    cout << electon_mass << endl;

    return 0;
}

Output:

2.07335e-317

Compile-time Constant:

These are the constants whose respective value is known or computed at the time of compilation of source code. Compile-time constants are faster than run-time constants but are less flexible than run-time constants.

Below is the program for illustration of Compile-time Constant:

C++

// C++ program to illustrate
// compile-time constants
#include <iostream>
using namespace std;

// Driver Code
int main()
{
    // Declare and initialize
    // compile time constant
    const double electron_q{ 1.6e-19 };

    // Value known to compiler
    // at compile-time
    cout << electron_q << endl;

    return 0;
}

Output:

1.6e-19

Difference between Run-time and Compile-time constants

	Compile-time constants	Run-time constants
1.	A compile-time constant is a value that is computed at the compilation-time.	Whereas, A runtime constant is a value that is computed only at the time when the program is running.
2.	A compile-time constant will have the same value each time when the source code is run.	A runtime constant can have a different value each time the source code is run.
3	It is generally used while declaring an array size.	It is not preferred for declaring an array size.
4	If you use const int size = 5 for defining a case expression it would run smoothly and won’t produce any compile-time error.	Here, if you use run-time constant in your source code for defining case expression then it will yield a compile-time error.
5	It does not produces any compile time error when used for initializing an enumerator.	Same compilation error, if runtime constant is used for initializing an enumerator.