First, let's address some problems:
You can't do this, C/C++ grammar doesn't work this way at all, and not even horrible hacks can make it work:
UFlag<int> uf | 10; //I want also to be able to declare it like this... Saves one line.
Try:
UFlag<int> uf{10};
Second, your conditional idiom is slightly problematic even with plain integers, unless you particularly like getting lots of warnings:
if(f & uf) ;//do stuff. should be true with the example-argument above.
if(f & 55) ;//do stuff. should be true with the example-argument above.
Prefer:
if((f & uf) != uf)
Alright, on to the main event.
I don't like using complex wrapper types for flags. We have enums. Let's use them. On some calling conventions, you'll actually be penalized for using classes/structs where you could have used a primitive type (even small structs may always be passed via the stack and never registers), so it's doubly important that we stick to plain enums if they'll ever be used anywhere perf-sensitive in multi-platform code.
C++11 strong enums have further advantages (you can specify the size explicitly, and they're namespaced). However, since you can't use operators like | and & with C++11 strong enums, but we want them for flags, but we _only_ want them enabled for flags, we need a new type trait.
You could use a wrapper type and just adapt the type traits I have here to simply checking against your wrapper type if you prefer the wrapper or just dislike type traits.
Here's my "flagset.h" (minus some helper functions that I rarely use) that I made for my personal toys/experiments:
// Sean Middleditch <sean@middleditch.us> - 2014
// This is free and unencumbered software released into the public domain.
//
// Anyone is free to copy, modify, publish, use, compile, sell, or
// distribute this software, either in source code form or as a compiled
// binary, for any purpose, commercial or non-commercial, and by any
// means.
//
// In jurisdictions that recognize copyright laws, the author or authors
// of this software dedicate any and all copyright interest in the
// software to the public domain. We make this dedication for the benefit
// of the public at large and to the detriment of our heirs and
// successors. We intend this dedication to be an overt act of
// relinquishment in perpetuity of all present and future rights to this
// software under copyright law.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
// IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
// OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.
//
// For more information, please refer to <http://unlicense.org/>
#if !defined(_guard_STLX_FLAGSET_H)
#define(_guard_STLX_FLAGSET_H)
#pragma once
#include <type_traits>
namespace stlx
{
template <typename T> struct is_flagset;
template <typename T> constexpr T bitmask(T);
}
/// Specialize to indicate that an enum is a flag set (and should be combinable).
/// @tparam T the enumeration to mark.
template <typename T> struct stlx::is_flagset : std::false_type {};
/// Set a given bit 1, all other bits to 0.
/// @param bit the bit to set.
template <typename T>
constexpr T stlx::bitmask(T bit) { return T(1) << bit; }
// Operators for flagsets.
template <typename E, typename = std::enable_if_t<stlx::is_flagset<E>::value>>
constexpr E operator|(E lhs, E rhs) { return E(std::underlying_type_t<E>(lhs) | std::underlying_type_t<E>(rhs)); }
template <typename E, typename = std::enable_if_t<stlx::is_flagset<E>::value>>
constexpr E operator&(E lhs, E rhs) { return E(std::underlying_type_t<E>(lhs) & std::underlying_type_t<E>(rhs)); }
template <typename E, typename = std::enable_if_t<stlx::is_flagset<E>::value>>
constexpr E operator^(E lhs, E rhs) { return E(std::underlying_type_t<E>(lhs) ^ std::underlying_type_t<E>(rhs)); }
template <typename E, typename = std::enable_if_t<stlx::is_flagset<E>::value>>
constexpr E& operator|=(E& lhs, E rhs) { return lhs = E(std::underlying_type_t<E>(lhs) | std::underlying_type_t<E>(rhs)); }
template <typename E, typename = std::enable_if_t<stlx::is_flagset<E>::value>>
constexpr E& operator&=(E& lhs, E rhs) { return lhs = E(std::underlying_type_t<E>(lhs) & std::underlying_type_t<E>(rhs)); }
template <typename E, typename = std::enable_if_t<stlx::is_flagset<E>::value>>
constexpr E& operator^=(E& lhs, E rhs) { return lhs = E(std::underlying_type_t<E>(lhs) ^ std::underlying_type_t<E>(rhs)); }
template <typename E, typename = std::enable_if_t<stlx::is_flagset<E>::value>>
constexpr E operator~(E rhs) { return E(~std::underlying_type_t<E>(rhs)); }
#endif // defined(_guard_STLX_FLAGSET_H)
And some example usage modeled from a virtual file system wrapper I have:
enum class EFileMode : std::uint16_t
{
None = 0,
Read = stlx::bitmask(0U),
Write = stlx::bitmask(1U),
Create = stlx::bitmask(2U),
Exclusive = stlx::bitmask(3U),
Truncate = stlx::bitmask(4U),
MakeDir = stlx::bitmask(5U),
WriteNew = Write|Create|Truncate|MakeDir,
};
namespace stlx
{
template<> struct is_flagset<EFileMode> : std::true_type {};
}
File OpenFile(string_view path, EFileMode mode) {
// silly example
if ((mode & (EFileMode::Truncate | EFileMode::Create)) != EFileMode::None);
mode &= EFileMode::Write;
...
return file;
}
auto file = OpenFile(path, EFileMode::Write | EFileMode::Create | EFileMode::Truncate);
