Question

Couldnt find a relevant answer to my case so i will try explaining my situation:

I have the following code:

enum Flags {
    OnlySpaces      = 1 <<  0,

    valComment      = 1 <<  1,
    valCommentBlock = 1 <<  2,
    valLabelName    = 1 <<  3,
    valVariableName = 1 <<  4,
    valFunctionName = 1 <<  5,

    invSyntax       = 1 << 32,

    fInVarLab       = valLabelName|valVariableName,
    fInIdentifier   = valLabelName|valVariableName|valFunctionName,
    fInCommentBlock = OnlySpaces|valCommentBlock,

    initState       = OnlySpaces|fInIdentifier|valComment,
};

int lexStatus = initState;

for (int i = 0; sc.More(); sc.Forward(), i++) {

    if (sc.atLineStart) {
        if (lexStatus & fInCommentBlock != fInCommentBlock) // Here is the problem
            sc.SetState(DEFAULT);

        lexStatus = initState;
    }
 ... // More code
 }

My code is for lexing a document and I am trying to do an action only when i am NOT in a comment block. The thing is that the above statement returns exactly the opposite when i am not in a comment…

the following statement does the job correctly but seems rather counterintuitive:

if (lexStatus & fInCommentBlock == fInCommentBlock)

So the questions are:

• Why? Why is it acting totally the opposite to what i am expecting?

Due to operator precedence if ((lexStatus & fInCommentBlock) != fInCommentBlock) fixes the issue

• Am i approaching the correct way?
• Is there a better way?
• Is there a way to make sure each flag has a distinct value?

Last question added because for some reason lexStatus & fInCommentBlock because some of the bits ARE set even though i dont set them anywhere in the code…

Thanks in advance for your help!

Answer 1

To address your first question: Your problem is operator precedence and understanding of how bitwise operators work.

if (lexStatus & fInCommentBlock == fInCommentBlock)

This works for you only because == have higher precedence than & so fInCommentBlock == fInCommentBlock is always true, hence the same thing as lexStatus & 1, which is the same thing as lexStatus & OnlySpaces. This will be true when lexStatus == initState because initState includes flag OnlySpaces.

Is there a better way?

You want to abstract the bitwise comparison into one or two helper functions.

int LexStatus_Is(int flags) { return (lexStatus & flags) == flags; }
int LexStatus_IsNot(int flags) { return (lexStatus & flags) != flags; }

Then you can write:

if (LexStatus_IsNot(fInComment))

Which would be more intuitive.