FBB::TypeTrait is a traits class. It does not define any member functions or data members, but merely types. It can be used to determine the basic type and other characteristics of (const) plain, (const) pointer or (const) reference types. It is used, e.g., by the FBB::FnWrap* family of template classes and was designed after Alexandrescu's (2001) TypeTraits template class.
The FBB::IfElse class template allows the conditional (on a bool value) use of one of two types.
The FBB::LpromotesR class template is used to determine, compile-time, whether its second (right-hand side) template type can be promoted to its first (left-hand side) template type.
The FBB::Use class template expects two typenames LHS and RHS and defines typename Use<LHS, RHS>::type as LHS if RHS can be promoted to LHS or it defines typename Use<LHS, RHS>::type as RHS.
TYPE is the template type parameter whose characteristics must be determined.
TypeTrait<TYPE> defines the type Plain: the plain type (the type without const, pointer, reference type indication) of any const or non-const plain, pointer, lvalue- or rvalue-reference type.
TypeTrait<TYPE> defines the following bool values:
IfElse<bool, TrueType, FalseType> defines the type type equal to TrueType if its bool template non-type parameter is true, and equal to FalseType if its its bool template non-type parameter is false.
LpromotesR<LeftType, RightType> defines the enum constant yes as 1 if an RightType argument can be promoted to LeftType value or object. The enum value yes is defined as 0 if no such constructor is available or if such a constructor is defined using the explicit keyword.
The following example shows a fragment of the code used in the FBB::FnWrap1 class determining the basic type of its Type template parameter:
#include <bobcat/typetrait> ... template <typename Type, typename ReturnType = void> class FnWrap1 { ... public: typedef typename TypeTrait<Type>::Plain argument_type; ... };The above code could have used TypeTrait<Type>::isPointer to determine whether Type actually represents a pointer type.
Here is an example using IfElse:
int main() { IfElse<true, int, double>::type isInt; IfElse<false, int, double>::type isDouble; }
And an example using LpromotesR:
int main() { cout << "string promotes char const *: " << LpromotesR<string, char const *>::yes << '\n'; // 1 cout << "string promotes string " << LpromotesR<string, string>::yes << '\n'; // 1 cout << "char const * promotes string " << LpromotesR<char const *, string>::yes << '\n'; // 0 }