Traits

'folly/Traits.h'

Implements traits complementary to those provided in <type_traits>

• Implements IsRelocatable trait.
• Implements IsOneOf trait
• Macros to state the assumptions easily

Motivation

---

<type_traits> is the Standard type-traits library defining a variety of traits such as is_integral or is_floating_point. This helps to gain more information about a given type.

folly/Traits.h implements traits complementing those present in the Standard.

IsRelocatable

---

In C++, the default way to move an object is by calling the copy constructor and destroying the old copy instead of directly copying the memory contents by using memcpy(). The conservative approach of moving an object assumes that the copied object is not relocatable. The two following code sequences should be semantically equivalent for a relocatable type:

{
  void conservativeMove(T * from, T * to) {
    new(to) T(from);
    (*from).~T();
  }
}

{
  void optimizedMove(T * from, T * to) {
    memcpy(to, from, sizeof(T));
  }
}

Very few C++ types are non-relocatable. The type defined below maintains a pointer inside an embedded buffer and hence would be non-relocatable. Moving the object by simply copying its memory contents would leave the internal pointer pointing to the old buffer.

class NonRelocatableType {
private:
  char buffer[1024];
  char * pointerToBuffer;
  ...
public:
  NonRelocatableType() : pointerToBuffer(buffer) {}
  ...
};

We can optimize the task of moving a relocatable type T using memcpy. IsRelocatable::value describes the ability of moving around memory a value of type T by using memcpy.

Usage

---

• Declaring types

template <class T1, class T2>
class MyParameterizedType;

class MySimpleType;

• Declaring a type as relocatable

Appending the lines below after definition of My*Type (MyParameterizedType or MySimpleType) will declare it as relocatable

/* Definition of My*Type goes here */
// global namespace (not inside any namespace)
namespace folly {
  // defining specialization of IsRelocatable for MySimpleType
  template <>
  struct IsRelocatable<MySimpleType> : std::true_type {};
  // defining specialization of IsRelocatable for MyParameterizedType
  template <class T1, class T2>
  struct IsRelocatable<MyParameterizedType<T1, T2>>
      : ::std::true_type {};
}

• To make it easy to state assumptions for a regular type or a family of parameterized type, various macros can be used as shown below.

• Stating that a type is Relocatable using a macro

// global namespace
namespace folly {
  // For a Regular Type
  FOLLY_ASSUME_RELOCATABLE(MySimpleType);
  // For a Parameterized Type
  FOLLY_ASSUME_RELOCATABLE(MyParameterizedType<T1, T2>);
}

fbvector only works with relocatable objects. If assumptions are not stated explicitly, fbvector<MySimpleType> or fbvector<MyParameterizedType> will fail to compile due to assertion below:

static_assert(IsRelocatable<My*Type>::value, "");

FOLLY_ASSUME_FBVECTOR_COMPATIBLE*(type) macros can be used to state that type is relocatable and has nothrow constructor.

• Stating that a type is fbvector-compatible using macros i.e. relocatable and has nothrow default constructor

// at global level, i.e no namespace
// macro for regular type
FOLLY_ASSUME_FBVECTOR_COMPATIBLE(MySimpleType)
// macro for types having 2 template parameters (MyParameterizedType)
FOLLY_ASSUME_FBVECTOR_COMPATIBLE_2(MyParameterizedType)

Similarly,

• FOLLY_ASSUME_FBVECTOR_COMPATIBLE_1(MyTypeHavingOneParameter) macro is for family of parameterized types having 1 parameter

• FOLLY_ASSUME_FBVECTOR_COMPATIBLE_3(MyTypeHavingThreeParameters) macro is for family of parameterized types having 3 parameters

• FOLLY_ASSUME_FBVECTOR_COMPATIBLE_4(MyTypeHavingFourParameters) macro is for family of parameterized types having 4 parameters

Few common types, namely std::basic_string, std::vector, std::list, std::map, std::deque, std::set, std::unique_ptr, std::shared_ptr, std::function, which are compatible with fbvector are already instantiated and declared compatible with fbvector. fbvector can be directly used with any of these C++ types.

std::pair can be safely assumed to be compatible with fbvector if both of its components are.

IsOneOf

---

std::is_same<T1, T2>::value can be used to test if types of T1 and T2 are same. folly::IsOneOf<T, T1, Ts...>::value can be used to test if type of T1 matches the type of one of the other template parameter, T1, T2, ...Tn. Recursion is used to implement this type trait.