Question

There are two implications of using the inline keyword(§ 7.1.3/4):

1. It hints the compiler that substitution of function body at the point of call is preferable over the usual function call mechanism.
2. Even if the inline substitution is omitted, the other rules(especially w.r.t One Definition Rule) for inline are followed.

Usually any mainstream compiler will substitute function body at the point of call if needed, so marking function inline merely for #1 is not really needed.

Further w.r.t #2, As I understand when you declare a function as static inline function,

The static keyword on the function forces the inline function to have an internal linkage(inline functions have external linkage) Each instance of such a function is treated as a separate function(address of each function is different) and each instance of these functions have their own copies of static local variables & string literals(an inline function has only one copy of these)

Thus such a function acts like any other static function and the keyword inline has no importance anymore, it becomes redundant.

So, Practically marking a function static and inline both has no use at all. Either it should be static(not most preferred) or inline(most preferred),
So, Is using static and inline together on a function practically useless?

Answer 1

Your analysis is correct, but doesn’t necessarily imply uselessness. Even if most compilers do automatically inline functions (reason #1), it’s best to declare inline just to describe intent.

Disregarding interaction with inline, static functions should be used sparingly. The static modifier at namespace scope was formerly deprecated in favor of unnamed namespaces (C++03 §D.2). For some obscure reason that I can’t recall it was removed from deprecation in C++11 but you should seldom need it.

So, Practically marking a function static and inline both has no use at all. Either it should be static(not most preferred) or inline(most preferred),

There’s no notion of preference. static implies that different functions with the same signature may exist in different .cpp files (translation units). inline without static means that it’s OK for different translation units to define the same function with identical definitions.

What is preferred is to use an unnamed namespace instead of static:

namespace {
    inline void better(); // give the function a unique name
}

static inline void worse(); // kludge the linker to allowing duplicates