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V721. The VARIANT_BOOL type is used incorrectly. The true value (VARIANT_TRUE) is defined as -1.
The analyzer has detected an incorrect use of the VARIANT_BOOL type. The reason is that the value true (VARIANT_TRUE) is designated as -1. Many programmers are unaware of this detail and tend to use this type incorrectly.
This is how the VARIANT_TRUE type and constants denoting "true" and "false" are declared:
typedef short VARIANT_BOOL;
#define VARIANT_TRUE ((VARIANT_BOOL)-1)
#define VARIANT_FALSE ((VARIANT_BOOL)0)Let's take a look at a few examples when the VARIANT_TRUE type is used incorrectly. In all the cases, the programmer expects the condition to be true, while it is actually always false.
Example 1.
VARIANT_BOOL variantBoolTrue = VARIANT_TRUE;
if (variantBoolTrue == true) //falseIf we substitute the value into the expression, we'll get ((short)(-1) == true). When this expression is evaluated, 'true' will turn into '1'. The condition (-1 == 1) is false.
The correct code:
if (variantBoolTrue == VARIANT_TRUE)Example 2.
VARIANT_BOOL variantBoolTrue = TRUE;
if (variantBoolTrue == VARIANT_TRUE) //falseThe programmer made a mistake here and used TRUE instead of VARIANT_TRUE. It will result in the variantBoolTrue variable being assigned the value 1. This value is illegal for variables of the VARIANT_BOOL type.
If we substitute the value into the expression, we will get (1 == (short)(-1)).
The correct code:
VARIANT_BOOL variantBoolTrue = VARIANT_TRUE;Example 3.
bool bTrue = true;
if (bTrue == VARIANT_TRUE) //falseLet's expand the expression: (true == (short)(-1)). When it is evaluated, 'true' will turn into '1'. The condition (1 == -1) is false.
It's not easy to suggest a correct version of this code as it is just fundamentally incorrect. One can't mix variables of the 'bool' type and values of the 'VARIANT_TRUE' type.
There are numbers of other examples like these to be found in code. For instance, when a function's formal argument is of the VARIANT_BOOL type but it is the 'true' value that will be passed as the actual one. Another example is when a function returns an incorrect value. And so on and so forth.
The most important thing you should keep in mind is that you can't mix the VARIANT_BOOL type with the types BOOL, bool, and BOOLEAN.
References:
- MSDN. VARIANT_BOOL.
- The Old New Thing. BOOL vs. VARIANT_BOOL vs. BOOLEAN vs. bool.
This diagnostic is classified as:
You can look at examples of errors detected by the V721 diagnostic. |
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