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// Copyright 2018 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include <initializer_list>
#include <tuple>
#include <type_traits>
#include <utility>
#include "base/optional.h"
// A bag of Traits (structs / enums / etc...) can be an elegant alternative to
// the builder pattern and multiple default arguments for configuring things.
// Traits are terser than the builder pattern and can be evaluated at compile
// time, however they require the use of variadic templates which complicates
// matters. This file contains helpers that make Traits easier to use.
// WARNING: Trait bags are currently too heavy for non-constexpr usage in prod
// code due to template bloat, although adding NOINLINE to template constructors
// configured via trait bags can help.
// E.g.
// struct EnableFeatureX {};
// struct UnusedTrait {};
// enum Color { RED, BLUE };
// struct ValidTraits {
// ValidTraits(EnableFeatureX);
// ValidTraits(Color);
// };
// ...
// DoSomethingAwesome(); // Use defaults (Color::BLUE &
// // feature X not enabled)
// DoSomethingAwesome(EnableFeatureX(), // Turn feature X on
// Color::RED); // And make it red.
// DoSomethingAwesome(UnusedTrait(), // Compile time error.
// Color::RED);
// DoSomethingAwesome might be defined as:
// template <class... ArgTypes,
// class CheckArgumentsAreValid = std::enable_if_t<
// trait_helpers::AreValidTraits<ValidTraits,
// ArgTypes...>::value>>
// constexpr void DoSomethingAwesome(ArgTypes... args)
// : enable_feature_x(trait_helpers::HasTrait<EnableFeatureX>(args...)),
// color(trait_helpers::GetEnum<Color, EnumTraitA::BLUE>(args...)) {}
namespace base {
namespace trait_helpers {
// Checks if any of the elements in |ilist| is true.
// Similar to std::any_of for the case of constexpr initializer_list.
inline constexpr bool any_of(std::initializer_list<bool> ilist) {
for (auto c : ilist) {
if (c)
return true;
return false;
// Checks if all of the elements in |ilist| are true.
// Similar to std::any_of for the case of constexpr initializer_list.
inline constexpr bool all_of(std::initializer_list<bool> ilist) {
for (auto c : ilist) {
if (!c)
return false;
return true;
// Counts the elements in |ilist| that are equal to |value|.
// Similar to std::count for the case of constexpr initializer_list.
template <class T>
inline constexpr size_t count(std::initializer_list<T> ilist, T value) {
size_t c = 0;
for (const auto& v : ilist) {
c += (v == value);
return c;
// Represents a trait that has been removed by a predicate.
struct EmptyTrait {};
// Checks if |Type| occurs in the parameter pack.
template <typename Type, typename... Pack>
struct TypeInPack {
static constexpr auto value = any_of({std::is_same<Type, Pack>::value...});
// Predicate used to remove any traits from the given list of types by
// converting them to EmptyTrait. E.g.
// template <typename... Args>
// void MyFunc(Args... args) {
// DoSomethingWithTraits(
// base::trait_helpers::Exclude<UnwantedTrait1,
// UnwantedTrait2>::Filter(args)...);
// }
// NB It's possible to actually remove the unwanted trait from the pack, but
// that requires constructing a filtered tuple and applying it to the function,
// which isn't worth the complexity over ignoring EmptyTrait.
template <typename... TraitsToExclude>
struct Exclude {
template <
typename T,
std::enable_if_t<TypeInPack<T, TraitsToExclude...>::value>* = nullptr>
static constexpr EmptyTrait Filter(T t) {
return EmptyTrait();
template <
typename T,
std::enable_if_t<!TypeInPack<T, TraitsToExclude...>::value>* = nullptr>
static constexpr T Filter(T t) {
return t;
// CallFirstTag is an argument tag that helps to avoid ambiguous overloaded
// functions. When the following call is made:
// func(CallFirstTag(), arg...);
// the compiler will give precedence to an overload candidate that directly
// takes CallFirstTag. Another overload that takes CallSecondTag will be
// considered iff the preferred overload candidates were all invalids and
// therefore discarded.
struct CallSecondTag {};
struct CallFirstTag : CallSecondTag {};
// A trait filter class |TraitFilterType| implements the protocol to get a value
// of type |ArgType| from an argument list and convert it to a value of type
// |TraitType|. If the argument list contains an argument of type |ArgType|, the
// filter class will be instantiated with that argument. If the argument list
// contains no argument of type |ArgType|, the filter class will be instantiated
// using the default constructor if available; a compile error is issued
// otherwise. The filter class must have the conversion operator TraitType()
// which returns a value of type TraitType.
// |InvalidTrait| is used to return from GetTraitFromArg when the argument is
// not compatible with the desired trait.
struct InvalidTrait {};
// Returns an object of type |TraitFilterType| constructed from |arg| if
// compatible, or |InvalidTrait| otherwise.
template <class TraitFilterType,
class ArgType,
class CheckArgumentIsCompatible = std::enable_if_t<
std::is_constructible<TraitFilterType, ArgType>::value>>
constexpr TraitFilterType GetTraitFromArg(CallFirstTag, ArgType arg) {
return TraitFilterType(arg);
template <class TraitFilterType, class ArgType>
constexpr InvalidTrait GetTraitFromArg(CallSecondTag, ArgType arg) {
return InvalidTrait();
// Returns an object of type |TraitFilterType| constructed from a compatible
// argument in |args...|, or default constructed if none of the arguments are
// compatible. This is the implementation of GetTraitFromArgList() with a
// disambiguation tag.
template <class TraitFilterType,
class... ArgTypes,
class TestCompatibleArgument = std::enable_if_t<any_of(
{std::is_constructible<TraitFilterType, ArgTypes>::value...})>>
constexpr TraitFilterType GetTraitFromArgListImpl(CallFirstTag,
ArgTypes... args) {
return std::get<TraitFilterType>(std::make_tuple(
GetTraitFromArg<TraitFilterType>(CallFirstTag(), args)...));
template <class TraitFilterType, class... ArgTypes>
constexpr TraitFilterType GetTraitFromArgListImpl(CallSecondTag,
ArgTypes... args) {
"The traits bag is missing a required trait.");
return TraitFilterType();
// Constructs an object of type |TraitFilterType| from a compatible argument in
// |args...|, or using the default constructor, and returns its associated trait
// value using conversion to |TraitFilterType::ValueType|. If there are more
// than one compatible argument in |args|, generates a compile-time error.
template <class TraitFilterType, class... ArgTypes>
constexpr typename TraitFilterType::ValueType GetTraitFromArgList(
ArgTypes... args) {
count({std::is_constructible<TraitFilterType, ArgTypes>::value...},
true) <= 1,
"The traits bag contains multiple traits of the same type.");
return GetTraitFromArgListImpl<TraitFilterType>(CallFirstTag(), args...);
// Helper class to implemnent a |TraitFilterType|.
template <typename T, typename _ValueType = T>
struct BasicTraitFilter {
using ValueType = _ValueType;
constexpr BasicTraitFilter(ValueType v) : value(v) {}
constexpr operator ValueType() const { return value; }
ValueType value = {};
template <typename ArgType, ArgType DefaultValue>
struct EnumTraitFilter : public BasicTraitFilter<ArgType> {
constexpr EnumTraitFilter() : BasicTraitFilter<ArgType>(DefaultValue) {}
constexpr EnumTraitFilter(ArgType arg) : BasicTraitFilter<ArgType>(arg) {}
template <typename ArgType>
struct OptionalEnumTraitFilter
: public BasicTraitFilter<ArgType, Optional<ArgType>> {
constexpr OptionalEnumTraitFilter()
: BasicTraitFilter<ArgType, Optional<ArgType>>(nullopt) {}
constexpr OptionalEnumTraitFilter(ArgType arg)
: BasicTraitFilter<ArgType, Optional<ArgType>>(arg) {}
// Tests whether multiple given argtument types are all valid traits according
// to the provided ValidTraits. To use, define a ValidTraits
template <typename ArgType>
struct RequiredEnumTraitFilter : public BasicTraitFilter<ArgType> {
constexpr RequiredEnumTraitFilter(ArgType arg)
: BasicTraitFilter<ArgType>(arg) {}
// Note EmptyTrait is always regarded as valid to support filtering.
template <class ValidTraits, class T>
inline constexpr bool IsValidTrait() {
return std::is_constructible<ValidTraits, T>::value ||
std::is_same<T, EmptyTrait>::value;
// Tests whether a given trait type is valid or invalid by testing whether it is
// convertible to the provided ValidTraits type. To use, define a ValidTraits
// type like this:
// struct ValidTraits {
// ValidTraits(MyTrait);
// ...
// };
// You can 'inherit' valid traits like so:
// struct MoreValidTraits {
// MoreValidTraits(ValidTraits); // Pull in traits from ValidTraits.
// MoreValidTraits(MyOtherTrait);
// ...
// };
template <class ValidTraits, class... ArgTypes>
struct AreValidTraits
: std::integral_constant<bool,
{IsValidTrait<ValidTraits, ArgTypes>()...})> {
// Helper to make getting an enum from a trait more readable.
template <typename Enum, typename... Args>
static constexpr Enum GetEnum(Args... args) {
return GetTraitFromArgList<RequiredEnumTraitFilter<Enum>>(args...);
// Helper to make getting an enum from a trait with a default more readable.
template <typename Enum, Enum DefaultValue, typename... Args>
static constexpr Enum GetEnum(Args... args) {
return GetTraitFromArgList<EnumTraitFilter<Enum, DefaultValue>>(args...);
// Helper to make getting an optional enum from a trait with a default more
// readable.
template <typename Enum, typename... Args>
static constexpr Optional<Enum> GetOptionalEnum(Args... args) {
return GetTraitFromArgList<OptionalEnumTraitFilter<Enum>>(args...);
// Helper to make checking for the presence of a trait more readable.
template <typename Trait, typename... Args>
static constexpr bool HasTrait(Args... args) {
count({std::is_constructible<Trait, Args>::value...}, true) <= 1,
"The traits bag contains multiple traits of the same type.");
return TypeInPack<Trait, Args...>::value;
// If you need a template vararg constructor to delegate to a private
// constructor, you may need to add this to the private constructor to ensure
// it's not matched by accident.
struct NotATraitTag {};
} // namespace trait_helpers
} // namespace base
#endif // BASE_TRAITS_BAG_H_