mirror of
https://github.com/mozilla/gecko-dev.git
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264 lines
7.4 KiB
C++
264 lines
7.4 KiB
C++
/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* Template-based metaprogramming and type-testing facilities. */
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#ifndef mozilla_TypeTraits_h_
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#define mozilla_TypeTraits_h_
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/*
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* These traits are approximate copies of the traits and semantics from C++11's
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* <type_traits> header. Don't add traits not in that header! When all
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* platforms provide that header, we can convert all users and remove this one.
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*/
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#include <wchar.h>
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namespace mozilla {
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/**
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* Helper class used as a base for various type traits, exposed publicly
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* because <type_traits> exposes it as well.
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*/
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template<typename T, T Value>
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struct IntegralConstant
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{
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static const T value = Value;
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typedef T ValueType;
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typedef IntegralConstant<T, Value> Type;
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};
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/** Convenient aliases. */
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typedef IntegralConstant<bool, true> TrueType;
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typedef IntegralConstant<bool, false> FalseType;
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namespace detail {
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// The trickery used to implement IsBaseOf here makes it possible to use it for
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// the cases of private and multiple inheritance. This code was inspired by the
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// sample code here:
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//
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// http://stackoverflow.com/questions/2910979/how-is-base-of-works
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template<class Base, class Derived>
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struct BaseOfHelper
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{
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public:
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operator Base*() const;
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operator Derived*();
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};
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template<class Base, class Derived>
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struct BaseOfTester
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{
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private:
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template<class T>
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static char test(Derived*, T);
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static int test(Base*, int);
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public:
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static const bool value =
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sizeof(test(BaseOfHelper<Base, Derived>(), int())) == sizeof(char);
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};
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template<class Base, class Derived>
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struct BaseOfTester<Base, const Derived>
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{
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private:
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template<class T>
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static char test(Derived*, T);
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static int test(Base*, int);
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public:
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static const bool value =
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sizeof(test(BaseOfHelper<Base, Derived>(), int())) == sizeof(char);
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};
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template<class Base, class Derived>
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struct BaseOfTester<Base&, Derived&> : FalseType {};
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template<class Type>
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struct BaseOfTester<Type, Type> : TrueType {};
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template<class Type>
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struct BaseOfTester<Type, const Type> : TrueType {};
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} /* namespace detail */
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template<bool Condition, typename A, typename B>
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struct Conditional;
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/*
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* IsBaseOf allows to know whether a given class is derived from another.
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*
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* Consider the following class definitions:
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*
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* class A {};
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* class B : public A {};
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* class C {};
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*
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* mozilla::IsBaseOf<A, B>::value is true;
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* mozilla::IsBaseOf<A, C>::value is false;
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*/
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template<class Base, class Derived>
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struct IsBaseOf
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: Conditional<detail::BaseOfTester<Base, Derived>::value, TrueType, FalseType>::Type
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{};
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namespace detail {
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template<typename From, typename To>
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struct ConvertibleTester
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{
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private:
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static From create();
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template<typename From1, typename To1>
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static char test(To to);
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template<typename From1, typename To1>
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static int test(...);
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public:
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static const bool value =
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sizeof(test<From, To>(create())) == sizeof(char);
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};
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} // namespace detail
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/**
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* IsConvertible determines whether a value of type From will implicitly convert
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* to a value of type To. For example:
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*
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* struct A {};
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* struct B : public A {};
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* struct C {};
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*
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* mozilla::IsConvertible<A, A>::value is true;
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* mozilla::IsConvertible<A*, A*>::value is true;
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* mozilla::IsConvertible<B, A>::value is true;
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* mozilla::IsConvertible<B*, A*>::value is true;
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* mozilla::IsConvertible<C, A>::value is false;
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* mozilla::IsConvertible<A, C>::value is false;
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* mozilla::IsConvertible<A*, C*>::value is false;
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* mozilla::IsConvertible<C*, A*>::value is false.
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*
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* For obscure reasons, you can't use IsConvertible when the types being tested
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* are related through private inheritance, and you'll get a compile error if
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* you try. Just don't do it!
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*/
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template<typename From, typename To>
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struct IsConvertible
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: Conditional<detail::ConvertibleTester<From, To>::value, TrueType, FalseType>::Type
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{};
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/**
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* Conditional selects a class between two, depending on a given boolean value.
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*
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* mozilla::Conditional<true, A, B>::Type is A;
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* mozilla::Conditional<false, A, B>::Type is B;
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*/
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template<bool Condition, typename A, typename B>
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struct Conditional
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{
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typedef A Type;
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};
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template<class A, class B>
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struct Conditional<false, A, B>
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{
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typedef B Type;
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};
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/**
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* EnableIf is a struct containing a typedef of T if and only if B is true.
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*
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* mozilla::EnableIf<true, int>::Type is int;
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* mozilla::EnableIf<false, int>::Type is a compile-time error.
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*
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* Use this template to implement SFINAE-style (Substitution Failure Is not An
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* Error) requirements. For example, you might use it to impose a restriction
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* on a template parameter:
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*
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* template<typename T>
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* class PodVector // vector optimized to store POD (memcpy-able) types
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* {
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* EnableIf<IsPod<T>::value, T>::Type* vector;
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* size_t length;
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* ...
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* };
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*/
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template<bool B, typename T = void>
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struct EnableIf
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{};
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template<typename T>
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struct EnableIf<true, T>
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{
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typedef T Type;
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};
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/**
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* IsSame tests whether two types are the same type.
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*
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* mozilla::IsSame<int, int>::value is true;
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* mozilla::IsSame<int*, int*>::value is true;
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* mozilla::IsSame<int, unsigned int>::value is false;
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* mozilla::IsSame<void, void>::value is true;
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* mozilla::IsSame<const int, int>::value is false;
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* mozilla::IsSame<struct S, struct S>::value is true.
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*/
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template<typename T, typename U>
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struct IsSame : FalseType {};
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template<typename T>
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struct IsSame<T, T> : TrueType {};
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/**
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* Traits class for identifying POD types. Until C++11 there's no automatic
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* way to detect PODs, so for the moment this is done manually. Users may
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* define specializations of this class that inherit from mozilla::TrueType and
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* mozilla::FalseType (or equivalently mozilla::IntegralConstant<bool, true or
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* false>, or conveniently from mozilla::IsPod for composite types) as needed to
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* ensure correct IsPod behavior.
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*/
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template<typename T>
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struct IsPod : public FalseType {};
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template<> struct IsPod<char> : TrueType {};
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template<> struct IsPod<signed char> : TrueType {};
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template<> struct IsPod<unsigned char> : TrueType {};
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template<> struct IsPod<short> : TrueType {};
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template<> struct IsPod<unsigned short> : TrueType {};
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template<> struct IsPod<int> : TrueType {};
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template<> struct IsPod<unsigned int> : TrueType {};
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template<> struct IsPod<long> : TrueType {};
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template<> struct IsPod<unsigned long> : TrueType {};
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template<> struct IsPod<long long> : TrueType {};
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template<> struct IsPod<unsigned long long> : TrueType {};
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template<> struct IsPod<bool> : TrueType {};
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template<> struct IsPod<float> : TrueType {};
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template<> struct IsPod<double> : TrueType {};
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template<> struct IsPod<wchar_t> : TrueType {};
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template<typename T> struct IsPod<T*> : TrueType {};
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/**
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* IsPointer determines whether a type is a pointer type (but not a pointer-to-
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* member type).
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*
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* mozilla::IsPointer<struct S*>::value is true;
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* mozilla::IsPointer<int**>::value is true;
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* mozilla::IsPointer<void (*)(void)>::value is true;
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* mozilla::IsPointer<int>::value is false;
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* mozilla::IsPointer<struct S>::value is false.
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*/
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template<typename T>
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struct IsPointer : FalseType {};
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template<typename T>
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struct IsPointer<T*> : TrueType {};
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} /* namespace mozilla */
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#endif /* mozilla_TypeTraits_h_ */
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