Question

In C++ specifically, what are the semantic differences between for example:

static const int x = 0 ;

and

const int x = 0 ;

for both static as a linkage and a storage class specifier (i.e. inside and outside a function).

Answer 1

At file scope, no difference in C++. const makes internal linkage the default, and all global variables have static lifetime. But the first variant has the same behavior in C, so that may be a good reason to use it.

Within a function, the second version can be computed from parameters. In C or C++ it doesn’t have to be a compile-time constant like some other languages require.

Within a class, basically the same thing as for functions. An instance const value can be computed in the ctor-initializer-list. A static const is set during startup initialization and remains unchanged for the rest of the program. (Note: the code for static members looks a little different because declaration and initialization are separated.)

Remember, in C++, const means read-only, not constant. If you have a pointer-to-const then other parts of the program may change the value while you’re not looking. If the variable was defined with const, then no one can change it after initialization but initialization can still be arbitrarily complex.