sizeof variadic template (sum of sizeof of all elements)

问题

Considering the following function :

template<typename... List> 
inline unsigned int myFunction(const List&... list)
{
    return /* SOMETHING */; 
}

What is the most simple thing to put instead of /* SOMETHING */ in order to return the sum of sizeof all arguments ?

For example myFunction(int, char, double) = 4+1+8 = 13

回答1:

unsigned myFunction() {return 0;}

template <typename Head, typename... Tail>
unsigned myFunction(const Head & head, const Tail &... tail) {
    return sizeof head + myFunction(tail...);
}

回答2:

In C++17, use a fold expression:

template<typename... List> 
inline constexpr unsigned int myFunction(const List&... list)
{
    return (0 + ... + sizeof(List));
}

回答3:

Based off of this comment and the following comments on the question, you could use this (note: completely untested)

std::initializer_list<std::size_t> sizeList = {sizeof(List)...}; //sizeList should be std::initializer_list, according to the comments I linked to
return std::accumulate(sizeList.begin(), sizeList.end(), 0);

回答4:

Two years late but an alternative solution guaranteed to be computed by the compiler (if you don't mind the different syntax):

template < typename ... Types >
struct SizeOf;

template < typename TFirst >
struct SizeOf < TFirst >
{
    static const auto Value = (sizeof(TFirst));
};

template < typename TFirst, typename ... TRemaining >
struct SizeOf < TFirst, TRemaining ... >
{
    static const auto Value = (sizeof(TFirst) + SizeOf<TRemaining...>::Value);
};

Used as const int size = SizeOf<int, char, double>::Value; // 4 + 1 + 8 = 13

回答5:

Here's a template way:

#include <iostream>

template<typename T, typename ...Ts>
class PPackSizeOf
{
  public:
  static const unsigned int size = sizeof(T) + PPackSizeOf<Ts...>::size;
};


template<typename T>
class PPackSizeOf<T>
{
  public:
  static const unsigned int size = sizeof(T);
};

template<typename ...Ts>
class FixedSizeBlock
{
   private:
      char block[PPackSizeOf<Ts...>::size];
   public:

};

int main( )
{
  FixedSizeBlock<char,long> b;
  std::cout << sizeof(b) << std::endl; 
  return 0;
}

回答6:

I've just found that :

template<typename... List> 
inline unsigned int myFunction(const List&... list)
{
    return sizeof(std::make_tuple(list...)); 
}

But :

1) Do I have the guarantee that the result will always be the same on all compilers ?

2) Do the make_tuple will make and overhead at compile-time ?

来源：https://stackoverflow.com/questions/12666761/sizeof-variadic-template-sum-of-sizeof-of-all-elements

标签

c++

c++11

tuples

sizeof

variadic-templates