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Expression templates are a C++ template metaprogramming technique that builds structures representing a computation at compile time, where expressions are evaluated only as needed to produce efficient code for the entire computation. [1]
In C++11, this technique is known as generalized constant expressions (constexpr). [2] C++14 relaxes the constraints on constexpr – allowing local declarations and use of conditionals and loops (the general restriction that all data required for the execution be available at compile-time remains).
C++11 introduced template aliases, which act like parameterized typedefs. The following code shows the definition of a template alias StrMap. This allows, for example, StrMap<int> to be used as shorthand for std::unordered_map<int,std::string>.
In C++, associative containers are a group of class templates in the standard library of the C++ programming language that implement ordered associative arrays. [1] Being templates , they can be used to store arbitrary elements, such as integers or custom classes.
However, C++11 constexpr functions could only contain a single expression that is returned (as well as static_asserts and a small number of other declarations). C++14 relaxes these restrictions. Constexpr-declared functions may now contain the following: [3] Any declarations except: static or thread_local variables.
For example, in C, int const x = 1; declares an object x of int const type – the const is part of the type, as if it were parsed "(int const) x" – while in Ada, X: constant INTEGER:= 1_ declares a constant (a kind of object) X of INTEGER type: the constant is part of the object, but not part of the type. This has two subtle results.
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The curiously recurring template pattern (CRTP) is an idiom, originally in C++, in which a class X derives from a class template instantiation using X itself as a template argument. [1] More generally it is known as F-bound polymorphism , and it is a form of F -bounded quantification .