# Correct usage of alignas in std::ostringstream wrapper

Please take a look at this code:

class error_stream: boost::noncopyable
{
public:
template<typename T>
std::ostream& operator<<(T&& arg)
{
return strm() << std::forward<T>(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<EXCEPTION>(LOCATION):                  \
throw_exception<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<std::ostringstream> stream;

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

public:

template <typename T>
std::ostream& operator<<(T&& arg)
{
if(!stream) {
construct();
}
(*stream) << std::forward<T>(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)];
...
};

• Yes your solution is cleaner. I'm trying to avoid going to free store if possible. – AlexT Feb 20 '14 at 5:21
• @AlexT See my edit. – Yuushi Feb 20 '14 at 5:36