What is the purpose of constexpr in C++?

In C++, constexpr is a keyword used to indicate to the compiler to calculate the value of an expression at compile time rather than at runtime. It allows for the calculation of constant expressions at compile time, leading to more efficient code and better optimization.

Functions or variables decorated with the constexpr keyword can be evaluated at compile time, allowing the compiler to optimize the code and improve program efficiency without the need for runtime calculations.

Functions marked with the constexpr keyword must meet the following requirements:

1. The return type of the function and all parameter types must be literal types.
2. The function body must only contain statements that can be calculated at compile time, such as assignments, conditional statements, and loops are not allowed.
3. The statements within the function body must be able to be evaluated by the compiler, meaning they cannot include runtime input and output.

Variables decorated with the constexpr specifier must meet the following conditions:

1. The variable type must be a literal type.
2. Variables must be initialized with a constant expression.

Using constexpr allows for the calculation of constant expressions at compile time, preventing redundant calculations at runtime and improving program performance. Additionally, constexpr can also be used to generate compile-time constants, such as for array sizes, template parameters, and so on.