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C++ Constants

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Its a qualifier. Declare const before your variables. example:

const double gravity {9.8};

Const function parameters

void printInt(const int x)
{
    std::cout << x << '\n';
}

You would use thisā€¦ sometimesā€¦

C++ Macros vs Consts

Use constants. macros can be replaced depending on loaded header files.

The opposite to constant is a volatile qualifier

Compile vs Runtime Constants

If a constant has only literal values, then it is a compile-time constant. If a constant has a value of another variable, that will be resolved during runtime

constexpr

constant expression makes sure that the current expression is optimized during compile time. This will only be possible if the circumstances around this is posible. You cannot constexpr a constant which has a volatile value gotten from input or function or other means.

constexpr int sum { 4 + 5 };      // ok: 4 + 5 is a constant expression
int age{};
    std::cin >> age;
 
    constexpr int myAge { age };      // compile error: age is not a constant expression

consteval

consteval is specifically for functions. If a function can be optimized at compile time, that is, if all the times its used is only with other constants as arguments, then consteval is the keyword to use beforehand to ensure it is compiled at compile-time. You can also use constexpr, but it may allow it to compile at runtime.

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