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For an enumeration whose underlying type is fixed, the values of the enumeration are the values of the
underlying type. Otherwise, for an enumeration where emin is the smallest enumerator and emax is the
largest, the values of the enumeration are the values in the range bmin to bmax, defined as follows: Let K
be 1 for a twoâs complement representation and 0 for a oneâs complement or sign-magnitude representation.
bmax is the smallest value greater than or equal to max(|emin| ? K, |emax|) and equal to 2^M ? 1, where
M is a non-negative integer. bmin is zero if emin is non-negative and ?(bmax + K) otherwise. The size of
the smallest bit-field large enough to hold all the values of the enumeration type is max(M, 1) if bmin is
zero and M + 1 otherwise. It is possible to define an enumeration that has values not defined by any of its
enumerators. If the enumerator-list is empty, the values of the enumeration are as if the enumeration had a
single enumerator with value 0.
According to the same standard, when When converting an arithmetic or enumeration type to an enumeration type, the value will be is unchanged if it is in the range of enumeration values of the enumeration type. Otherwise, the value is unspecified.
Therefore, to avoid unexpected behavior, the value being converted must be inside of the range of enumeration values. FuthermoreFurthermore, if it is necessary to check for out-of-range values dynamically, it must be done before the conversion.
Noncompliant Code Example (Bounds checking)
This noncompliant code exmample example is trying attempts to check for an out-of-bounds condition. However, it is doing so after the conversion, so the result of the conversion is unspecified and the statement may have no effect.
Code Block | ||
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enum et1 {
E_A,
E_B
};
int i1 = -1;
et1 e1 = static_cast<et1>(i1);
if (e1 < 0) {
/* error condition */
}
|
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Code Block | ||
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enum et1 {
E_A,
E_B
};
int i1 = -1;
if (i1 < 0 || i1 > E_C) {
/* error condition */
}
et1 e1 = static_cast<et1>(i1);
|
...
This noncompliant code may result in a truncation of the value of i1
when converted to type et1
, thus causing the program to execute resulting in execution of case E_A.
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enum et1 { E_A, E_B }; int i1 = 5; switch(static_cast<et1>(i1)) { case E_A: /* some action A */ case E_B: /* some action B */ } |
GCC 4.4.3 will execute executes case E_A, and Microsoft VS2008 will execute executes neither case, in this noncompliant example.
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Code Block | ||
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enum et1 {
E_A,
E_B
};
int i1 = 5;
if (i1 < 0 || i1 > E_C) {
/* error condition */
}
switch(static_cast<et1>(i1)) {
case E_A:
/* some action A */
case E_B:
/* some action B */
}
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