`const` vs. `constexpr` in C : When Should I Use Which?

const vs constexpr on Variables

In C , the const and constexpr keywords are used to define constants. Both modifiers prevent the value of a variable from being changed after it is initialized. However, there are subtle differences between the two that may affect your choice when defining constants.

const vs constexpr: Compile-Time vs Run-Time Initialization

The primary difference between const and constexpr lies in when the constant is initialized. const variables can be initialized at compile time or run time, while constexpr variables must be initialized at compile time.

This has several implications:

• Compile-Time Constants: constexpr variables are guaranteed to be constant at compile time, allowing them to be used in contexts that require constants known at that time, such as array sizes or switch case values.
• Run-Time Constants: const variables can be initialized at run time, enabling dynamic constant values. This can be useful when you need to determine the value of a constant based on user input or runtime conditions.

Example

Consider the following code snippets:

const double PI1 = 3.141592653589793; // compile-time or run-time initialization
constexpr double PI2 = 3.141592653589793; // compile-time initialization only

PI1 can be initialized at compile time or run time, depending on when the value is known. PI2, on the other hand, must be initialized at compile time.

Performance Considerations

constexpr variables have the advantage of being fully evaluated at compile time, eliminating any overhead associated with run-time initialization. This can lead to slightly improved performance, but it's usually insignificant in practice.

Which One to Use?

The choice between const and constexpr depends on your specific requirements:

• Use constexpr when you need a compile-time constant that can be used in contexts that require it.
• Use const when you need a constant that can be initialized at either compile time or run time.

The above is the detailed content of `const` vs. `constexpr` in C : When Should I Use Which?. For more information, please follow other related articles on the PHP Chinese website!