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In the C++ programming language, decltype is an operator for querying the type of an expression. It was introduced in C++11. Its primary intended use is in generic programming, where it is often difficult, or even impossible, to express types that depend on template parameters.

As generic programming techniques became increasingly popular throughout the 1990s, the need for a type-deduction mechanism was recognized. Many compiler vendors implemented their own versions of the operator, typiBjarne Stroustrup proposed that a standardized version of the operator be added to the C++ language, and suggested the name "decltype", to reflect that the operator would yield the "declared type" of an expression.

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