2.38.txt

Exercise 2.38: Describe the differences in type deduction between decltype and
auto. Give an example of an expression where auto and decltype will deduce the
same type and an example where they will deduce differing types.

By Faisal Saadatmand

The type specifier auto deduces the type of an object from its initializer.
However, the type deduced by auto is not always exactly the same as the
initializer's type.  In the case of references the type deduced corresponds to
the type of the original object the reference is bound to, in which top-level
consts are ignored. To have a top-level const or a reference deduced, we must
add a const or & to auto respectively. In the latter case, top-level const will
not be ignored.

decltype deduces a type by analysing an expression without evaluating it. When
the expression is a variable, top-level consts and references are kept.  When
the expression is not a variable, "we get the type that the expression yields."
(p. 71) Some expressions yield objects (rvalues) while others references
(lvalues). Note: enclosing variables in parentheses will yield a reference.