Question: In trying to learn how to use std::aligned\_union I am having trouble finding any examples. My attempt is running into problems that I do not know how to solve.

```
struct include
{
    std::string file;
};
struct use
{
    use(const std::string &from, const std::string &to) : from{ from }, to{ to }
    {
    }
    std::string from;
    std::string to;
};
std::aligned_union<sizeof(use), include, use>::type item;
*reinterpret_cast<use*>(&item_) = use{ from, to };

```

When I attempt to run the program in VC++2013 debug mode I get a runtime error in `memcpy(unsigned char * dst, unsigned char * src, unsigned long count)`. I assume that this is how VC++ implements the assignment from the temporary.

How would I change this so that I do not have this issue?

Answer: 
Expanding on [Kerrek's answer](https://stackoverflow.com/a/21568809/923854): I suggest using `unique_ptr` with custom deleter to handle the destruction for you automatically. You can wrap everything up nicely in a factory ([Live at Rextester](http://rextester.com/PSXH66839)):

```
struct placement_deleter {
  template <typename T>
  void operator () (T* ptr) const {
    ptr->~T();
  }
};

template <typename T, typename...Args>
std::unique_ptr<T, placement_deleter>
make_in_place(void* place, Args&&...args) {
  return std::unique_ptr<T, placement_deleter>{
    ::new (place) T(std::forward<Args>(args)...)
  };
}

int main() {
  std::aligned_union<0, int, std::string>::type storage;
  {
    auto i = make_in_place<int>(&storage, 42);
    std::cout << *i << '\n';
  }
  {
    auto s = make_in_place<std::string>(&storage, "this is");
    *s += " a test";
    std::cout << *s << '\n';
  }
}

```
I'm also expanding on [Kerrek's answer](https://stackoverflow.com/a/21568809/923854). A possible (C++14) implementation of `aligned_union` is:

```
template <std::size_t Len, class... Types>
struct aligned_union
{
  static constexpr std::size_t alignment_value = std::max({alignof(Types)...});

  struct type
  {
    alignas(alignment_value) char _s[std::max({Len, sizeof(Types)...})];
  };
};

```

So it's clear that:

* you have to construct your own object (*placement new*)
* `type` is a POD type of a size and alignment suitable for use as uninitialized storage for an object of any of the types listed in `Types` (but actually none of those types)
* you can use 0 as value for `Len` parameter (I think it's for generic programming situations, e.g. see <https://stackoverflow.com/a/27069379/3235496>)

For further details:

* [N2140](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2140.pdf)
* [Eggs.Variant - Part I](http://talesofcpp.fusionfenix.com/post-17/eggs.variant---part-i)