求问下边这段程序是怎么个逻辑,要表达什么意思?
using namespace std;
class global {
public:
static global* a();
protected:
static global* _a;
};
global*global::_a = 0;
int main() {
global::a();
return 0;
}
global *global::a(){
if (!_a)
_a = new global();
return _a;
}