Correct usage of alignas in std::ostringstream wrapper

Please take a look at this code:

class error_stream: boost::noncopyable
{
public:
    template
    std::ostream& operator(arg);
    }
    std::string str() { return strm().str(); }
    bool empty() const { return !holder_.valid(); }
private:
    std::ostringstream& strm() { return holder_; }
    struct alignas(std::ostringstream) holder
    {
        holder(): ptr_{} {}
        ~holder()
        {
            if(valid())
                ptr_->~basic_ostringstream();
        }
        operator std::ostringstream&()
        {
            if(!valid())
                ptr_ = new (memory_) std::ostringstream{};
            return *ptr_;
        }
        bool valid() const { return ptr_; }
    private:
        char memory_[sizeof(std::ostringstream)];
        std::ostringstream* ptr_;
    } holder_;
};

Here I want to avoid creating std::ostringstream if there was no output made into it:

error_stream strm;
//strm << "...";
if(!strm.empty())
{
    //...get strm.str();
}

I use placement new/delete when a client outputs something into the stream. Do I need to use alingas for block of memory that will hold instance of std::ostringstream?

UPDATE: To give more context - this is part of THROW wrapper:

...
#define THROW_EX_WITH_LOCATION(EXCEPTION, LOCATION)                \
    for(error_stream strm;;                                        \
        strm.empty()?                                              \
            throw_exception(LOCATION):                  \
            throw_exception(LOCATION, strm.str()))      \
        strm
...

I don't want to create any stream if ther is no error message but solution should work with minumum overhead over version with just std::ostringstream.

Consider the following cases:

THROW();

and

THROW() << "some error";

I agree this looks like nitpicking.

This seems overcomplicated to me. Lazy-loading something doesn't require placement-new trickery and mucking around with alignments. Why not simply store a pointer to the stream which is only initialised when the user performs some kind of operation? This is the general way to do lazy-loading:

struct error_stream
{
private:
    std::unique_ptr stream;

    // This should use make_unique, which you can find 
    // or write yourself without too much hassle.
    void construct()
    {
        stream = std::unique_ptr(
            new std::ostringstream);
    }

public:

    template 
    std::ostream& operator(arg);
        return *stream;
    }

    bool empty() const
    {
        return stream ? false : true;
    }

    std::string str() 
    {
        if(empty()) {
             construct();
        }
        return stream->str();
    }
};

This seems to have the semantics you want (including being non-copyable due to using a std::unique_ptr) with much less hassle. You don't have to worry about placement new or any alignment requirements, which is going to make the code a -lot- harder to maintain.

Edit: If you really what to avoid the heap, I suppose you can use placement new. You allocate the memory for the stream when you create an error_stream, so I'm curious as to what problem you're trying to solve exactly. That being said, what you want to align is the storage for the ostringstream.

char memory_[sizeof(std::stringstream)];

Hence I'd use:

struct holder
{
....
private:
    alignas(std::ostringstream) char memory_[sizeof(std::stringstream)];
    ...
};

StackExchange Code Review Q#42248, answer score: 7