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123 lines
3.1 KiB
C++
123 lines
3.1 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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namespace mozilla {
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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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class IsBaseOf
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{
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private:
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static char test(Base* b);
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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(static_cast<Derived*>(0))) == sizeof(char);
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};
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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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{
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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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/*
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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, class A, class 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<IsPodType<T>, 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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} /* namespace mozilla */
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#endif /* mozilla_TypeTraits_h_ */
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