struct to/from std::tuple conversion

Question

Assuming I have struct and std::tuple with same type layout:

struct MyStruct { int i; bool b; double d; }
using MyTuple = std::tuple<         


        

Answer 1

Is there any standartized way to cast one to another?

There is no way to "cast" the one to the other.

The easiest may be to use a std::tie to pack the tuple out into the struct;

struct MyStruct { int i; bool b; double d; };
using MyTuple = std::tuple;

auto t = std::make_tuple(42, true, 5.1);
MyStruct s;

std::tie(s.i, s.b, s.d) = t;

Demo.

You can further wrap this up in higher level macros or "generator" (make style) functions, e.g;

std::tuple from_struct(MyStruct const& src)
{
  return std::make_tuple(src.i, src.b, src.d);
}

MyStruct to_struct(std::tuple const& src)
{
  MyStruct s;
  std::tie(s.i, s.b, s.d) = src;
  return s;
}

I know that trivial memory copying can do the trick, but it is alignment and implementation dependent?

You mention the "trivial memory copy" would work - only for copying the individual members. So basically, a memcpy of the entire structure to the tuple and vice-versa is not going to always behave as you expect (if ever); the memory layout of a tuple is not standardised. If it does work, it is highly dependent on the implementation.