mirror of
https://github.com/aantron/better-enums.git
synced 2025-12-06 16:56:42 +08:00
70 lines
2.3 KiB
Markdown
70 lines
2.3 KiB
Markdown
## Bit sets
|
|
|
|
If you want to use `std::bitset` or a similar library to have enums be keys into
|
|
a bit set, you need to know the number of bits at compile time. You can easily
|
|
automate this with Better Enums, even when constants are not declared in
|
|
increasing order.
|
|
|
|
---
|
|
|
|
We simply need to find the maximum value of any given enum type.
|
|
|
|
#include <bitset>
|
|
#include <iostream>
|
|
<em>#include <enum.h></em>
|
|
|
|
template <<em>typename Enum</em>>
|
|
constexpr <em>Enum max_loop</em>(Enum accumulator, size_t index)
|
|
{
|
|
return
|
|
<em>index >= Enum::_size() ? accumulator :
|
|
Enum::_values()[index] > accumulator ?
|
|
max_loop<Enum>(Enum::_values()[index], index + 1) :
|
|
max_loop<Enum>(accumulator, index + 1)</em>;
|
|
}
|
|
|
|
template <<em>typename Enum</em>>
|
|
constexpr <em>Enum max</em>()
|
|
{
|
|
return <em>max_loop<Enum>(Enum::_values()[0], 1)</em>;
|
|
}
|
|
|
|
And use that to declare a bit set template:
|
|
|
|
template <<em>typename Enum</em>>
|
|
using <em>EnumSet</em> = <em>std::bitset</em><<em>max<Enum>()._to_integral()</em> + <em>1</em>>;
|
|
|
|
Now, we can have bit sets that are wide enough to hold whatever range we
|
|
declared. We just declare enums, and the numeric values of their constants will
|
|
be bit indices. The rest is straightforward.
|
|
|
|
ENUM(<em>EFLAGS</em>, int,
|
|
<em>Carry</em>, <em>Parity</em> = 2, <em>Adjust</em> = 4, <em>Zero</em>, <em>Sign</em>, <em>Trap</em>, <em>Interrupt</em>, <em>Direction</em>,
|
|
<em>Overflow</em>, <em>NestedTask</em> = 14, <em>Resume</em> = 16, <em>V8086</em>, <em>AlignmentCheck</em>,
|
|
<em>CPUIDPresent</em> = 21)
|
|
|
|
int main()
|
|
{
|
|
<em>EnumSet</em><<em>EFLAGS</em>> eflags = <em>1 << EFLAGS::Carry</em> | <em>1 << EFLAGS::Zero</em>;
|
|
|
|
if (eflags.test(<em>EFLAGS::Carry</em>))
|
|
eflags.set(<em>EFLAGS::Trap</em>);
|
|
|
|
std::cout << <em>eflags</em> << std::endl;
|
|
|
|
return 0;
|
|
}
|
|
|
|
---
|
|
|
|
If we want bit sets of fixed known width instead, we can use the code above to
|
|
check that we haven't declared any bit indices out of range:
|
|
|
|
~~~comment
|
|
static_assert(max<EFLAGS>()._to_integral() < 32,
|
|
"some bit indices are out of range");
|
|
~~~
|
|
|
|
%% description = Finding the maximum value of a Better Enum for use in declaring
|
|
statically-sized bit set types.
|