The “enum hack” in old code
In older versions of C++, static
const was not supported inside
classes. This meant that
const was useless for constant expressions inside classes. However,
people still wanted to do this so a typical solution (usually referred to as the
“enum hack”) was to use an untagged
enum
with no instances. An
enumeration must have all its values established at compile time, it’s
local to the class, and its values are available for constant expressions. Thus,
you will commonly see:
//: C08:EnumHack.cpp
#include <iostream>
using namespace std;
class Bunch {
enum { size = 1000 };
int i[size];
};
int main() {
cout << "sizeof(Bunch) = " << sizeof(Bunch)
<< ", sizeof(i[1000]) = "
<< sizeof(int[1000]) << endl;
} ///:~
The use of enum here is guaranteed
to occupy no storage in the object, and the enumerators are all evaluated at
compile time. You can also explicitly establish the values of the
enumerators:
enum { one = 1, two = 2, three };
With integral enum types, the
compiler will continue counting from the last value, so the enumerator
three will get the value 3.
In the StringStack.cpp example
above, the line:
static const int size = 100;
enum { size = 100 };
Although you’ll often see the
enum technique in legacy code, the static const feature was added
to the language to solve just this problem. However, there is no overwhelming
reason that you must choose static const over the enum
hack, and in this book the enum hack is used because it is supported by
more compilers at the time this book was
written.
