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Added information about role of concepts in template meta programming with example, that illustrates how proper specialization is choosen. It's my first remark in codin, so i will be pleased for review of more advanced wiki user |
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==New possibilites in metaprogramming thanks to concepts==
C++20 standard brings C++ programmers new tools for meta template programming, which are concepts<ref>https://en.cppreference.com/w/cpp/language/constraints</ref>.
Thanks to them, now programmer can specify requirements for type, to make instantation of template possible.
Moreover standard says, that compiler should look for template with concpet, that has highest requirements.
Here is example of famous [[Fizz buzz]] problem fully resolved with Template Meta Programming
<syntaxhighlight lang="cpp">
#include <boost/type_index.hpp> // for pretty printing of types
#include <iostream>
#include <tuple>
/**
* Type representation of words to print
*/
struct Fizz {};
struct Buzz {};
struct FizzBuzz {};
template<size_t _N> struct number { constexpr static size_t N = _N; };
/**
* Concepts used to define condition for specializations
*/
template<typename Any> concept has_N = requires{ requires Any::N - Any::N == 0; };
template<typename A> concept fizz_c = has_N<A> && requires{ requires A::N % 3 == 0; };
template<typename A> concept buzz_c = has_N<A> && requires{ requires A::N % 5 == 0;};
template<typename A> concept fizzbuzz_c = fizz_c<A> && buzz_c<A>;
/**
* By specializing `res` structure, with concepts requirements, proper instantation is performed
*/
template<typename X> struct res;
template<fizzbuzz_c X> struct res<X> { using result = FizzBuzz; };
template<fizz_c X> struct res<X> { using result = Fizz; };
template<buzz_c X> struct res<X> { using result = Buzz; };
template<has_N X> struct res<X> { using result = X; };
/**
* Predeclaration of concentrator
*/
template <size_t cnt, typename... Args>
struct concatenator;
/**
* Recursive way of concatenating next types
*/
template <size_t cnt, typename ... Args>
struct concatenator<cnt, std::tuple<Args...>>
{ using type = typename concatenator<cnt - 1, std::tuple< typename res< number<cnt> >::result, Args... >>::type;};
/**
* Base case
*/
template <typename... Args> struct concatenator<0, std::tuple<Args...>> { using type = std::tuple<Args...>;};
/**
* Final result getter
*/
template<size_t Amount>
using fizz_buzz_full_template = typename concatenator<Amount - 1, std::tuple<typename res<number<Amount>>::result>>::type;
int main()
{
// printing result with boost, so it's clear
std::cout << boost::typeindex::type_id<fizz_buzz_full_template<100>>().pretty_name() << std::endl;
/*
Result:
std::tuple<number<1ul>, number<2ul>, Fizz, number<4ul>, Buzz, Fizz, number<7ul>, number<8ul>, Fizz, Buzz, number<11ul>, Fizz, number<13ul>, number<14ul>, FizzBuzz, number<16ul>, number<17ul>, Fizz, number<19ul>, Buzz, Fizz, number<22ul>, number<23ul>, Fizz, Buzz, number<26ul>, Fizz, number<28ul>, number<29ul>, FizzBuzz, number<31ul>, number<32ul>, Fizz, number<34ul>, Buzz, Fizz, number<37ul>, number<38ul>, Fizz, Buzz, number<41ul>, Fizz, number<43ul>, number<44ul>, FizzBuzz, number<46ul>, number<47ul>, Fizz, number<49ul>, Buzz, Fizz, number<52ul>, number<53ul>, Fizz, Buzz, number<56ul>, Fizz, number<58ul>, number<59ul>, FizzBuzz, number<61ul>, number<62ul>, Fizz, number<64ul>, Buzz, Fizz, number<67ul>, number<68ul>, Fizz, Buzz, number<71ul>, Fizz, number<73ul>, number<74ul>, FizzBuzz, number<76ul>, number<77ul>, Fizz, number<79ul>, Buzz, Fizz, number<82ul>, number<83ul>, Fizz, Buzz, number<86ul>, Fizz, number<88ul>, number<89ul>, FizzBuzz, number<91ul>, number<92ul>, Fizz, number<94ul>, Buzz, Fizz, number<97ul>, number<98ul>, Fizz, Buzz>
*/
}
</syntaxhighlight>
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