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Generic allocator class without variadic templates?


Generic allocator class without variadic templates?

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Generic allocator class without variadic templates?
Tag : cpp , By : dlouzan
Date : January 02 2021, 06:48 AM


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How to use variadic templates to make a generic Lua function wrapper?


Tag : cpp , By : Saul
Date : March 29 2020, 07:55 AM
it helps some times The trick is to exploit template argument deduction by partially specializing a class template that contains the wrapper function:
// Lua API dummies ...

struct lua_State {};

template<class T> void luaU_push(lua_State*,T);
template<class T> T* luaW_check(lua_State*,int);
template<class T> T luaU_check(lua_State*,int);


// metaprogramming for creating indices ...

template<int...Ints>
struct int_pack {};

template<int Begin, int Count, int...Tail>
struct make_int_range_type {
    typedef typename make_int_range_type<Begin,Count-1,Begin+Count-1,Tail...>::type type;
};

template<int Begin, int...Tail>
struct make_int_range_type<Begin,0,Tail...> {
    typedef int_pack<Tail...> type;
};

template<int Begin, int Count>
inline typename make_int_range_type<Begin,Count>::type
make_int_range()
{ return typename make_int_range_type<Begin,Count>::type(); }


// the actual wrapper ...

template<class MemFunPtrType, MemFunPtrType PMF>
struct lua_mem_func_wrapper;

template<class Clazz, class ReturnType, class...Args, ReturnType(Clazz::*PMF)(Args...)>
struct lua_mem_func_wrapper<ReturnType(Clazz::*)(Args...),PMF> {
    static int doit(lua_State* L) {
        return doit_impl(L,make_int_range<2,sizeof...(Args)>());
    }
private:
    template<int...Indices>
    static int doit_impl(lua_State* L, int_pack<Indices...>) {
        luaU_push<ReturnType>(L,
            (luaW_check<Clazz>(L, 1)->*PMF)(
                luaU_check<Args>(L, Indices)...
            )
        );
        return 1;
    }
};

#define GET_MEM_FUN_WRAPPER(...) &lua_mem_func_wrapper<decltype(__VA_ARGS__),__VA_ARGS__>::doit


// testing ...

struct foo {
    int baz(int, float);
};

void test() {
    auto* ptr = GET_MEM_FUN_WRAPPER(&foo::baz);
}

Variadic templates of Variadic templates class in c++


Tag : cpp , By : user177910
Date : March 29 2020, 07:55 AM
like below fixes the issue You simply cannot write that, syntactically. What would Ts hold in that situation? It would be different for each type in Tuples so it wouldn't really be usable.
What you can do instead is take advantage of Columbo's bool_pack trick:
template <bool...> struct bool_pack;

template <bool... v>
using all_true = std::is_same<bool_pack<true, v...>, bool_pack<v..., true>>;
template <typename > struct is_a_tuple : std::false_type { };
template <typename... T> struct is_a_tuple<tuple<T...>> : std::true_type { };

template <typename... Tuples>
struct VariadicTuples {
    static_assert(all_true<is_a_tuple<Tuples>::value...>::value, "!");
};

specialization of variadic templates with class templates


Tag : cpp , By : Eniac
Date : March 29 2020, 07:55 AM
this will help If I understand correctly what you want... yes, you can create a templated struct that can count "class templates", so you can write something like
 count<std::vector, std::map, std::set, std::pair>::value
 count<std::vector, int &, float, std::set>::value
 template <typename... _Pp>
    struct count;
template <template<typename...> class ...>
struct countC;

template <>
struct countC<>
 { static const int value = 0; };

// ignore uninteresting templates
template<template<typename...> class F, template<typename...> class ... R>
struct countC<F, R...>
 { static const int value = countC<R...>::value; };

template <template<typename...> class ... R>
struct countC<std::vector, R...>
 { static const int value = 1 + countC<R...>::value; };
#include <map>
#include <set>
#include <vector>
#include <utility>
#include <iostream>

// countC for templates

template <template<typename...> class ...>
struct countC;

template <>
struct countC<>
 { static const int value = 0; };

// ignore uninteresting templates
template<template<typename...> class F, template<typename...> class ... R>
struct countC<F, R...>
 { static const int value = countC<R...>::value; };

template <template<typename...> class ... R>
struct countC<std::vector, R...>
 { static const int value = 1 + countC<R...>::value; };

template <template<typename...> class ... R>
struct countC<std::map, R...>
 { static const int value = 1 + countC<R...>::value; };

template <template<typename...> class ... R>
struct countC<std::pair, R...>
 { static const int value = 1 + countC<R...>::value; };


// countV for for values of a fixed type

template <typename T, T ... v>
struct countV;

template <typename T>
struct countV<T>
 { static const int value = 0; };

// ignore uninteresting values
template <typename T, T f, T ... r>
struct countV<T, f, r...>
 { static const int value = countV<T, r...>::value; };

// count only int odd values
template <int f, int ... r>
struct countV<int, f, r...>
 { static const int value = (f % 2) + countV<int, r...>::value; };


// countT for typenames

template <typename...>
struct countT;

template <>
struct countT<>
 { static const int value = 0; };

// ignore uninteresting types
template <typename F, typename ... R>
struct countT<F, R...>
 { static const int value = countT<R...>::value; };

template <typename F, typename ... R>
struct countT<F*, R...>
 { static const int value = 1 + countT<R...>::value; };

template<typename F, typename ... R>
struct countT<F&, R...>
 { static const int value = 1 + countT<R...>::value; };

template<typename ... R>
struct countT<int, R...>
 { static const int value = 1 + countT<R...>::value; };


int main()
 {
   std::cout << "countC vector + map + set + pair                   = " 
      << countC<std::vector, std::map, std::set, std::pair>::value
      << std::endl;

   std::cout << "countT int + float + bool* + double& + bool + int& = " 
      << countT<int, float, bool*, double&, bool, int&>::value
      << std::endl;

   std::cout << "countV int, 1 + 4 + 4 + 5 + 7 + 10 + 11 + 16 + 15  = " 
      << countV<int, 1, 4, 4, 5, 7, 10, 11, 16, 15>::value
      << std::endl;

   std::cout << "countV long, 1 + 4 + 4 + 5 + 7 + 10 + 11 + 16 + 15 = " 
      << countV<long, 1, 4, 4, 5, 7, 10, 11, 16, 15>::value
      << std::endl;

   return 0;
 }

Deduction guides and variadic class templates with variadic template constructors - mismatched argument pack lengths


Tag : cpp , By : Eric
Date : March 29 2020, 07:55 AM
I hope this helps . To simplify your example further, it appears that GCC does not implement variadic template arguments in deduction guides:
https://wandbox.org/permlink/4YsacnW9wYcoceDH

C++ Generic command parser using variadic templates


Tag : cpp , By : user140973
Date : March 29 2020, 07:55 AM
To fix the issue you can do I'm trying to write some sort of command handler, which can tokenize an istringstream, automatically convert the tokens into variables of specific types and call a callback function with the converted variables as arguments. Here is simplified version of my code:
struct Caller {
    template<class...Args>
    Caller(Args&&... args) { Callback(std::forward<Args>(args)...); }
};

template<typename ...Args>
void ParseAndExecute(std::istringstream& input_stream) {
  Caller{GetNextArgument<Args>(input_stream)...};
}
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