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The keyword tt(decltype) is a tool for determining the type of an
expression. To use it an expression to which tt(decltype) is applied must be
available. But what if a function template defines a tt(typename Class)
template parameter and the function template should use the return type of the
function tt(Class::fun())? Since two classes may define members tt(fun) having
different return types, the return type to use is not immediately available.
These kinds of problems are solved by using the function template
hi(declval)tt(std::declval), defined in the tthi(utility) header
file. This function template defines one template type parameter, and returns
an rvalue reference to an object of the template type parameter's class,
without actually creating a temporary object. But since an rvalue reference is
available, its tt(fun) function can be called, and the return type of em(that)
function can then be produced by tt(decltype). There are no specific
requirements for the constructors of the class type that's passed to
tt(declval). Specifically: it doesn't have to have a default or public
constructor (but access rights em(are) used). Consider this
function template:
verbinsert(-s4 //1 examples/declval.cc)
The function tt(value's) return type is defined as the as yet unknown
tt(Type::fun's) return type.
By defining two structs, both having tt(fun) member functions tt(value's)
actual return type can now be returned. This is used in tt(main) where
respectively an tt(int) and a tt(double) is returned, resulting in the output
tt(12 12.5):
verbinsert(-s4 //2 examples/declval.cc)
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