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V648. Priority of the '&&' operation is…
Analyzer Diagnostics
General Analysis (C++)
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Contents

V648. Priority of the '&&' operation is higher than that of the '||' operation.

Nov 12 2012

The analyzer has detected a potential error: the priority of the '&&' logical operation is higher than that of the '||' operation. Programmers often forget this, which causes the result of a logical expression using these operations to be quite different from what was expected.

Consider the following sample of incorrect code:

if ( c == 'l' || c == 'L' &&
    !( token->subtype & TT_LONG ) )
{ .... }

The programmer most likely expected that equality of the 'c' variable and the value 'l' or 'L' would be checked first, and only then the '&&' operation would be executed. But according to the Operation priorities in C/C++, the '&&' operation is executed first, and only then, the '||' operation.

We recommend that you add parentheses in every expression that contains operators you use rarely, or whenever you're not sure about the priorities. Even if parentheses appear to be unnecessary, it's ok. At the same time, you code will become easier to comprehend and less error-prone.

This is the fixed code:

if ( ( c == 'l' || c == 'L' ) &&
    !( token->subtype & TT_LONG ) )

How to get rid of a false warning in case it was this very sequence you actually intended: first '&&', then '||'?

There are several ways:

1) Bad way. You may add the "//-V648" comment into the corresponding line to suppress the warning.

if ( c == 'l' || c == 'L' && //-V648
    !( token->subtype & TT_LONG ) )

2) Good way. You may write additional parentheses:

if ( c == 'l' || ( c == 'L' &&
    !( token->subtype & TT_LONG ) ) )

These will help other programmers understand that the code is correct.

This diagnostic is classified as:

You can look at examples of errors detected by the V648 diagnostic.

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