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Document an emulation limitation of scoped enum (#28)

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Piotr Jawniak
2019-04-23 02:00:26 +02:00
committed by Peter Dimov
parent fb417474ae
commit c7f0fa8900
1 changed files with 22 additions and 3 deletions
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25
doc/scoped_enum.qbk
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@@ -67,9 +67,9 @@ The enumeration can be forward declared:
BOOST_SCOPED_ENUM_FORWARD_DECLARE(future_errc); BOOST_SCOPED_ENUM_FORWARD_DECLARE(future_errc);
There are however some limitations. First, the emulated scoped enum is not a C++ enum, so `is_enum< future_errc >` will be `false_type`. There are however some limitations. The emulated scoped enum is not a C++ enum, so `is_enum< future_errc >` will be `false_type`.
Second, the emulated scoped enum can not be used in switch nor in template arguments. For these cases the user needs to use some helpers. Instead of The emulated scoped enum can not be used in switch nor in template arguments. For these cases the user needs to use some helpers. Instead of
switch (ev) switch (ev)
{ {
@@ -99,10 +99,29 @@ use
{ {
}; };
Lastly, explicit conversion to the underlying type should be performed with `boost::underlying_cast` instead of `static_cast`: Explicit conversion to the underlying type should be performed with `boost::underlying_cast` instead of `static_cast`:
unsigned int val = boost::underlying_cast< unsigned int >(ev); unsigned int val = boost::underlying_cast< unsigned int >(ev);
In C++03, scoped enums behave differently in case of calling an overloaded function when one overload takes a scoped enum as a paramter, and the other takes a parameter of an integral type. Consider the following code:
enum enum_regular { REGULAR_A, REGULAR_B };
BOOST_SCOPED_ENUM_DECLARE_BEGIN(enum_scoped)
{
a, b
}
BOOST_SCOPED_ENUM_DECLARE_END(enum_scoped)
void regular_or_int(enum_regular); // (1)
void regular_or_int(int); // (2)
void scoped_or_int(enum_scoped); // (3)
void scoped_or_int(int); // (4)
regular_or_int(REGULAR_A); // calls (1) in C++03 and C++11
scoped_or_int(enum_scoped::a); // calls (3) in C++11 but (4) in C++03!
scoped_or_int(enum_scoped(enum_scoped::a)); // calls (3) in C++03 and C++11
Here is usage example: Here is usage example:
BOOST_SCOPED_ENUM_UT_DECLARE_BEGIN(algae, char) BOOST_SCOPED_ENUM_UT_DECLARE_BEGIN(algae, char)