How do I make my sizeof sum struct work with an empty parameter pack
You simply must specialize also for <>
Example:
template < typename... T> struct TotalSizeOf;
template < typename U, typename... T> struct TotalSizeOf<U, T...>
: std::integral_constant<size_t, sizeof(U) + TotalSizeOf<T...>::value> {};
template <> struct TotalSizeOf<> :
std::integral_constant<size_t, 0 > { };
int main()
{
std::cout << TotalSizeOf< int, char>::value << std::endl;
std::cout << TotalSizeOf< char>::value << std::endl;
std::cout << TotalSizeOf< >::value << std::endl;
}
With C++17 you can get this without elaborate template metaprogramming, using fold expressions:
#include <iostream>
#include <type_traits>
template<class... T>
struct TotalSizeOf: std::integral_constant<std::size_t, (0 + ... + sizeof(T))> {};
int main()
{
std::cout << TotalSizeOf< int, char>::value << std::endl;
std::cout << TotalSizeOf< char>::value << std::endl;
std::cout << TotalSizeOf< >::value << std::endl;
}
This should also be more efficient when compiling (of course at runtime, these are the same).
PS: Just read, that you only have C++14, but will let this stand here, since I think it is nice to see, that we are less forced to do awkward TMP in newer C++ versions.
Addendum: Less elegant than C++17, but C++14 and pretty much tmp-free
#include <iostream>
#include <type_traits>
#include <initializer_list>
constexpr size_t sum(std::initializer_list<size_t> arr) {
// Accumulate is sadly not constexpr in C++14
auto ret = 0ul;
for(auto i: arr) {
ret += i;
}
return ret;
}
template<class... T>
struct TotalSizeOf: std::integral_constant<std::size_t, sum({sizeof(T)...})> {};
int main()
{
std::cout << TotalSizeOf< int, char>::value << std::endl;
std::cout << TotalSizeOf< char>::value << std::endl;
std::cout << TotalSizeOf< >::value << std::endl;
}