Should I use #define, enum or const?

Question

In a C++ project I'm working on, I have a flag kind of value which can have four values. Those four flags can be combined. Flags describe the records in database and can be:

• new record
• deleted record
• modified record
• existing record

Now, for each record I wish to keep this attribute, so I could use an enum:

enum { xNew, xDeleted, xModified, xExisting }

However, in other places in code, I need to select which records are to be visible to the user, so I'd like to be able to pass that as a single parameter, like:

showRecords(xNew | xDeleted);

So, it seems I have three possible appoaches:

#define X_NEW      0x01
#define X_DELETED  0x02
#define X_MODIFIED 0x04
#define X_EXISTING 0x08

or

typedef enum { xNew = 1, xDeleted, xModified = 4, xExisting = 8 } RecordType;

or

namespace RecordType {
    static const uint8 xNew = 1;
    static const uint8 xDeleted = 2;
    static const uint8 xModified = 4;
    static const uint8 xExisting = 8;
}

Space requirements are important (byte vs int) but not crucial. With defines I lose type safety, and with enum I lose some space (integers) and probably have to cast when I want to do a bitwise operation. With const I think I also lose type safety since a random uint8 could get in by mistake.

Is there some other cleaner way?

If not, what would you use and why?

P.S. The rest of the code is rather clean modern C++ without #defines, and I have used namespaces and templates in few spaces, so those aren't out of question either.

Answer 1

Combine the strategies to reduce the disadvantages of a single approach. I work in embedded systems so the following solution is based on the fact that integer and bitwise operators are fast, low memory & low in flash usage.

Place the enum in a namespace to prevent the constants from polluting the global namespace.

namespace RecordType {

An enum declares and defines a compile time checked typed. Always use compile time type checking to make sure arguments and variables are given the correct type. There is no need for the typedef in C++.

enum TRecordType { xNew = 1, xDeleted = 2, xModified = 4, xExisting = 8,

Create another member for an invalid state. This can be useful as error code; for example, when you want to return the state but the I/O operation fails. It is also useful for debugging; use it in initialisation lists and destructors to know if the variable's value should be used.

xInvalid = 16 };

Consider that you have two purposes for this type. To track the current state of a record and to create a mask to select records in certain states. Create an inline function to test if the value of the type is valid for your purpose; as a state marker vs a state mask. This will catch bugs as the typedef is just an int and a value such as 0xDEADBEEF may be in your variable through uninitialised or mispointed variables.

inline bool IsValidState( TRecordType v) {
    switch(v) { case xNew: case xDeleted: case xModified: case xExisting: return true; }
    return false;
}

 inline bool IsValidMask( TRecordType v) {
    return v >= xNew  && v < xInvalid ;
}

Add a using directive if you want to use the type often.

using RecordType ::TRecordType ;

The value checking functions are useful in asserts to trap bad values as soon as they are used. The quicker you catch a bug when running, the less damage it can do.

Here are some examples to put it all together.

void showRecords(TRecordType mask) {
    assert(RecordType::IsValidMask(mask));
    // do stuff;
}

void wombleRecord(TRecord rec, TRecordType state) {
    assert(RecordType::IsValidState(state));
    if (RecordType ::xNew) {
    // ...
} in runtime

TRecordType updateRecord(TRecord rec, TRecordType newstate) {
    assert(RecordType::IsValidState(newstate));
    //...
    if (! access_was_successful) return RecordType ::xInvalid;
    return newstate;
}

The only way to ensure correct value safety is to use a dedicated class with operator overloads and that is left as an exercise for another reader.