类型转换在存储受限的枚举中失败
Type cast failed in switch for enum with restricted storage
enum class confirm {yes};
struct item
{
confirm s:4; // (1) limiting storage size required
};
int main()
{
item itm;
itm.s = confirm::yes; // (2) OK
switch (itm.s)
{
case confirm::yes: // (3) Failure, need static data cast here?
break;
}
}
产生错误：
In function ‘int main()’:
error: could not convert ‘yes’ from ‘confirm’ to ‘int’
case confirm::yes:
^
在使用 g++ 编译时，但通过 clang++ 编译得很好。 为什么 标记为 (2) 的赋值可能但 case 标记为 (3) 的子句不行？
关于 too small storage 的警告属于
这看起来像一个 gcc 错误，我们可以看到它有效 in the latest gcc version:
来自 C++11 标准草案部分 6.4.2 [stmt.switch]:
The condition shall be of integral type, enumeration type, or of a
class type for which a single non-explicit conversion function to
integral or enumeration type exists (12.3). [...] Integral promotions
are performed. Any statement within the switch statement can be
labeled with one or more case labels as follows:
case constant-expression :
where the constant-expression shall be a converted constant expression (5.19) of the promoted type of the switch condition.
转换后的常量表达式在 5.19 节中介绍：
[...]A converted constant expression of type T is a literal constant expression, implicitly converted to type T,
where the implicit conversion (if any) is permitted in a literal constant expression and the implicit conversion
sequence contains only user-defined conversions, lvalue-to-rvalue conversions (4.1), integral promotions (4.5),
and integral conversions (4.7) other than narrowing conversions (8.5.4). [ Note: such expressions may be used
as case expressions (6.4.2), as enumerator initializers if the underlying type is fixed (7.2), and as integral or
enumeration non-type template arguments (14.3). —end note ] [...]
也许这与defect report 1767: Scoped enumeration in a switch statement有关。所以也许它强制提升为 int，然后在这种情况下比较会失败。
enum class confirm {yes};
struct item
{
confirm s:4; // (1) limiting storage size required
};
int main()
{
item itm;
itm.s = confirm::yes; // (2) OK
switch (itm.s)
{
case confirm::yes: // (3) Failure, need static data cast here?
break;
}
}
产生错误：
In function ‘int main()’:
error: could not convert ‘yes’ from ‘confirm’ to ‘int’
case confirm::yes:
^
在使用 g++ 编译时，但通过 clang++ 编译得很好。 为什么 标记为 (2) 的赋值可能但 case 标记为 (3) 的子句不行？
关于 too small storage 的警告属于
这看起来像一个 gcc 错误，我们可以看到它有效 in the latest gcc version:
来自 C++11 标准草案部分 6.4.2 [stmt.switch]:
The condition shall be of integral type, enumeration type, or of a class type for which a single non-explicit conversion function to integral or enumeration type exists (12.3). [...] Integral promotions are performed. Any statement within the switch statement can be labeled with one or more case labels as follows:
case constant-expression :where the constant-expression shall be a converted constant expression (5.19) of the promoted type of the switch condition.
转换后的常量表达式在 5.19 节中介绍：
[...]A converted constant expression of type T is a literal constant expression, implicitly converted to type T, where the implicit conversion (if any) is permitted in a literal constant expression and the implicit conversion sequence contains only user-defined conversions, lvalue-to-rvalue conversions (4.1), integral promotions (4.5), and integral conversions (4.7) other than narrowing conversions (8.5.4). [ Note: such expressions may be used as case expressions (6.4.2), as enumerator initializers if the underlying type is fixed (7.2), and as integral or enumeration non-type template arguments (14.3). —end note ] [...]
也许这与defect report 1767: Scoped enumeration in a switch statement有关。所以也许它强制提升为 int，然后在这种情况下比较会失败。