Conditional storage class dependent upon template parameter

Question

The following doesn't appear to be valid c++ (-std=c++2a). I want to change the storage class of var based on the template parameter b:

#include <type_traits>

template <bool b>
void f() {
  typename std::conditional<b, static int, int>::type var;
}

Answer 1

The storage class specifier is part of the object's declaration and is not part of the object's type.

Hence it can't be used where a type is expected.

Expanding on cigien's specialization proposal, you can wrap the variable into a helper container class.

template<bool b> struct A;

template<> struct A<true> {
    inline static int var = 0;
};

template<> struct A<false> {
    int var = 0;
};

template <bool b>
void f() {
    A<b> a;
    int& var = a.var;
    do_something_with(var);
}

Beware that the semantics in doing this are quite different - one is created in automatic storage for every invocation, the other is initialized once and shared between invocations. In a threaded environment the former is safe, the latter isn't, etc.