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There are three kinds of templates: function templates, class templates and, since C++14, variable templates. Since C++11, templates may be either variadic or non-variadic; in earlier versions of C++ they are always non-variadic. C++ Templates are Turing complete. [2]
The use of templates as a metaprogramming technique requires two distinct operations: a template must be defined, and a defined template must be instantiated.The generic form of the generated source code is described in the template definition, and when the template is instantiated, the generic form in the template is used to generate a specific set of source code.
Templates in C++ may also be used for template metaprogramming, which is a way of pre-evaluating some of the code at compile-time rather than run-time. Using template specialization, C++ Templates are Turing complete.
In computability theory, a system of data-manipulation rules (such as a model of computation, a computer's instruction set, a programming language, or a cellular automaton) is said to be Turing-complete or computationally universal if it can be used to simulate any Turing machine [1] [2] (devised by English mathematician and computer scientist Alan Turing).
Because C++ templates are type-aware and Turing-complete, they can also be used to let the compiler resolve recursive conditionals and generate substantial programs through template metaprogramming. Contrary to some opinion, template code will not generate a bulk code after compilation with the proper compiler settings. [77]
It is used for specifying that a dependent name in a template definition or declaration is a type. [3] [4] In the original C++ compilers before the first ISO standard was completed, the typename keyword was not part of the C++ language and Bjarne Stroustrup used the class keyword for template arguments instead.
Are Templates Turing complete: in principle would it be possible to perform arbitrary computations during the compiling (rather than just by executing the program afterward)? Alternatively, is the phase of unravelling templates always guaranteed to halt (as it presumably is for macros)? Cesiumfrog 23:56, 31 January 2011 (UTC)
(Note that if value_type were an incomplete template instance, ADL would trigger its complete instantiation.) The Barton–Nackman trick originally relied not on ADL but on a C++ feature called "friend name injection", in which an in-class declaration of a friend function made the function name visible in the immediately surrounding namespace ...