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| -rw-r--r-- | articles/2015-01-11_a_look_at_compile_time_computation_in_cpp.md | 20 |
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diff --git a/articles/2015-01-11_a_look_at_compile_time_computation_in_cpp.md b/articles/2015-01-11_a_look_at_compile_time_computation_in_cpp.md index 345b97f..e6a12ae 100644 --- a/articles/2015-01-11_a_look_at_compile_time_computation_in_cpp.md +++ b/articles/2015-01-11_a_look_at_compile_time_computation_in_cpp.md @@ -87,10 +87,26 @@ Where the verbose Assembler output is acquired as follows (note that the same co ~~~ {: .language-sh} -## Types as values… +One area where the example above would not work and one would thus require the `constexpr` keyword, is when one wants to use the result of a function as e.g. a template parameter or any other value that is required by the standard to be defined at compile time. While this is certainly useful it - contrary to what one could think after first hearing about `constexpr` - doesn't quite enable one to explicitly write _compile time programs_ in the same way as _normal_ programs. -…and templates as functions. +## Types as values and templates as functions + +As I hinted at previously the only way to handle e.g. lists in the same way as in [ConstList] while enabling value-dependent result types is to think of types as values and templates as functions. This translates to each `CAR` value of a _Cons_ being stored in the form of a `std::integral_constant` specialization instantiation. + +This would limit all compile time operations performed in this manner to only dealing with values that can be passed as template parameters, to cite the standard: + +> A non-type _template-parameter_ shall have one of the following (optionally _cv-qualified_) types: +> - integral or enumeration type, [...] +> ([ISO C++ Standard draft, N3797], p. 329) + +Where integral types are defined as follows: + +> Types `bool`, `char`, `char16_t`, `char32_t`, `wchar_t`, and the signed and unsigned integer types are collectively called _integral_ types. A synonym for integral type is _integer type_. [...] +> ([ISO C++ Standard draft, N3797], p. 86) + +So as it turns out the restriction imposed by being forced to rely on template parameters is not as severe as I for one thought at first, especially since compositions of these types in e.g. structures are not hindered by this. For example usage of templates such as `std::tuple` and even compile time string operations via variadic template parameter packs of type `char` should be possible. [proof]: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.14.3670 [ConstList]: /page/const_list/ [test cases]: https://github.com/KnairdA/ConstList/blob/master/test.cc +[ISO C++ Standard draft, N3797]: http://www.open-std.org/jtc1/sc22/wg21/ |