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Bug 953296 - Implement mozilla::IsClass and mozilla::IsEmpty. r=jcranmer

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This commit is contained in:
Jeff Walden 2013-12-30 01:07:39 -06:00
parent 12220c1619
commit 5ee5d4c63b
2 changed files with 126 additions and 0 deletions
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85
mfbt/TypeTraits.h
View File

@ -169,6 +169,33 @@ struct IsEnum
: detail::IsEnumHelper<typename RemoveCV<T>::Type>
{};
namespace detail {
// __is_class is a supported extension across all of our supported compilers:
// http://llvm.org/releases/3.0/docs/ClangReleaseNotes.html
// http://gcc.gnu.org/onlinedocs/gcc-4.4.7/gcc/Type-Traits.html#Type-Traits
// http://msdn.microsoft.com/en-us/library/ms177194%28v=vs.100%29.aspx
template<typename T>
struct IsClassHelper
: IntegralConstant<bool, __is_class(T)>
{};
} // namespace detail
/**
* IsClass determines whether a type is a class type (but not a union).
*
* struct S {};
* union U {};
* mozilla::IsClass<int>::value is false;
* mozilla::IsClass<const S>::value is true;
* mozilla::IsClass<U>::value is false;
*/
template<typename T>
struct IsClass
: detail::IsClassHelper<typename RemoveCV<T>::Type>
{};
/* 20.9.4.2 Composite type traits [meta.unary.comp] */
/**
@ -243,6 +270,64 @@ template<> struct IsPod<char16_t> : TrueType {};
#endif
template<typename T> struct IsPod<T*> : TrueType {};
namespace detail {
// __is_empty is a supported extension across all of our supported compilers:
// http://llvm.org/releases/3.0/docs/ClangReleaseNotes.html
// http://gcc.gnu.org/onlinedocs/gcc-4.4.7/gcc/Type-Traits.html#Type-Traits
// http://msdn.microsoft.com/en-us/library/ms177194%28v=vs.100%29.aspx
template<typename T>
struct IsEmptyHelper
: IntegralConstant<bool, IsClass<T>::value && __is_empty(T)>
{};
} // namespace detail
/**
* IsEmpty determines whether a type is a class (but not a union) that is empty.
*
* A class is empty iff it and all its base classes have no non-static data
* members (except bit-fields of length 0) and no virtual member functions, and
* no base class is empty or a virtual base class.
*
* Intuitively, empty classes don't have any data that has to be stored in
* instances of those classes. (The size of the class must still be non-zero,
* because distinct array elements of any type must have different addresses.
* However, if the Empty Base Optimization is implemented by the compiler [most
* compilers implement it, and in certain cases C++11 requires it], the size of
* a class inheriting from an empty |Base| class need not be inflated by
* |sizeof(Base)|.) And intuitively, non-empty classes have data members and/or
* vtable pointers that must be stored in each instance for proper behavior.
*
* static_assert(!mozilla::IsEmpty<int>::value, "not a class => not empty");
* union U1 { int x; };
* static_assert(!mozilla::IsEmpty<U1>::value, "not a class => not empty");
* struct E1 {};
* struct E2 { int : 0 };
* struct E3 : E1 {};
* struct E4 : E2 {};
* static_assert(mozilla::IsEmpty<E1>::value &&
* mozilla::IsEmpty<E2>::value &&
* mozilla::IsEmpty<E3>::value &&
* mozilla::IsEmpty<E4>::value,
* "all empty");
* union U2 { E1 e1; };
* static_assert(!mozilla::IsEmpty<U2>::value, "not a class => not empty");
* struct NE1 { int x; };
* struct NE2 : virtual E1 {};
* struct NE3 : E2 { virtual ~NE3() {} };
* struct NE4 { virtual void f() {} };
* static_assert(!mozilla::IsEmpty<NE1>::value &&
* !mozilla::IsEmpty<NE2>::value &&
* !mozilla::IsEmpty<NE3>::value &&
* !mozilla::IsEmpty<NE4>::value,
* "all empty");
*/
template<typename T>
struct IsEmpty : detail::IsEmptyHelper<typename RemoveCV<T>::Type>
{};
namespace detail {
template<typename T, bool = IsFloatingPoint<T>::value>

41
mfbt/tests/TestTypeTraits.cpp
View File

@ -7,13 +7,54 @@
#include "mozilla/TypeTraits.h"
using mozilla::IsBaseOf;
using mozilla::IsClass;
using mozilla::IsConvertible;
using mozilla::IsEmpty;
using mozilla::IsSame;
using mozilla::IsSigned;
using mozilla::IsUnsigned;
using mozilla::MakeSigned;
using mozilla::MakeUnsigned;
struct S1 {};
union U1 { int x; };
static_assert(!IsClass<int>::value, "int isn't a class");
static_assert(IsClass<const S1>::value, "S is a class");
static_assert(!IsClass<U1>::value, "U isn't a class");
static_assert(!mozilla::IsEmpty<int>::value, "not a class => not empty");
static_assert(!mozilla::IsEmpty<bool[5]>::value, "not a class => not empty");
static_assert(!mozilla::IsEmpty<U1>::value, "not a class => not empty");
struct E1 {};
struct E2 { int : 0; };
struct E3 : E1 {};
struct E4 : E2 {};
static_assert(IsEmpty<const volatile S1>::value, "S should be empty");
static_assert(mozilla::IsEmpty<E1>::value &&
mozilla::IsEmpty<E2>::value &&
mozilla::IsEmpty<E3>::value &&
mozilla::IsEmpty<E4>::value,
"all empty");
union U2 { E1 e1; };
static_assert(!mozilla::IsEmpty<U2>::value, "not a class => not empty");
struct NE1 { int x; };
struct NE2 : virtual E1 {};
struct NE3 : E2 { virtual ~NE3() {} };
struct NE4 { virtual void f() {} };
static_assert(!mozilla::IsEmpty<NE1>::value &&
!mozilla::IsEmpty<NE2>::value &&
!mozilla::IsEmpty<NE3>::value &&
!mozilla::IsEmpty<NE4>::value,
"all empty");
static_assert(!IsSigned<bool>::value, "bool shouldn't be signed");
static_assert(IsUnsigned<bool>::value, "bool should be unsigned");