static global vs static member

a.h

#ifndef A_H
#define A_H

template <class T>
struct A {
    static int a;
};

template <class T>
int A<T>::a;

static int sa;

#endif

hello.cpp

#include <iostream>
#include "a.h"

int show_a_1();
int show_a_2();

int show_sa_1();
int show_sa_2();

int main() {
    A<int>::a = 42;
    sa = 42;

    show_a_1();
    show_a_2();

    show_sa_1();
    show_sa_2();
}

result.txt

$ g++ -o hello hello.cpp s1.cpp s2.cpp
$ ./hello
A::a(1): 42
A::a(2): 42
sa(1): 0
sa(2): 0

s1.cpp

#include <iostream>
#include "a.h"

int show_a_1() {
    std::cout << "A::a(1): " << A<int>::a << std::endl;
}

int show_sa_1() {
    std::cout << "sa(1): " << sa << std::endl;
}

s2.cpp

#include <iostream>
#include "a.h"

int show_a_2() {
    std::cout << "A::a(2): " << A<int>::a << std::endl;
}

int show_sa_2() {
    std::cout << "sa(2): " << sa << std::endl;
}