C++ - decltype vs declval

Comparing decltype vs declval in C++

Feature	std::decltype	std::declval
Purpose	Deduces the type of an expression.	Simulates an instance of a type.
Operation	Works on actual expressions.	Works purely on type information.
Evaluation	Does not evaluate the expression.	Does not create any object; pure utility.
Common Use	Type deduction of expressions.	Type deduction for inaccessible instances.
Header	Built into the language.