In C++17 and later, the "inline" keyword can also be used for variables, in addition
to functions. This allows for variables to be defined in header files and have their
definition "inlined" into each translation unit that includes the header. This can
be useful for constexpr variables, which can be used at compile-time and in constant
expressions, and for inline initialization of global variables.

Here's an example of using inline variables in C++17:

cpp
Copy code
//header.h
inline constexpr int x = 5;

//main.cpp
#include "header.h"
int main()
{
    int y = x; // y is 5
    return 0;
}
In this example, the value of x is defined in the header file, but because it is 
declared as an inline variable, the definition is effectively copied into main.cpp, 
so the value of x can be used directly in main.