In my API I have a function that returns std::istringstream
.
The std::istringstream
class is non-copyable but supports moving so on a conforming compiler there is no problem returning a local std::istringstream
.
However, on gcc 4.9, there is no support for moving std::istringstream
.
Is there some workaround that I can use that std::istringstream
without changing the API from the user's perspective?
The workaround suggested here, of using a unique_ptr<std::istringstream>
will change the semantics of the API.
Answering my own question for completeness and future reference.
The goal was to find a workaround for the gcc (< 5) bug where std::istringstream
does not provide a move ctor that will work in cases where I want to return the un-copyable and (bugly-) unmovable stream.
As mentioned in the comments, I can in fact change my function signature (at least on gcc < 5) to return a proxy object that allows copying or moving without changing the API for code used on newer/other compilers.
The idea, suggested and implemented by a colleague, is to create a proxy object around std::istringstream
which provides a similar API, but also provides a copy-ctor which manually creates and initializes a new internal std::istringstream
from the copied-from stream. This proxy is used only on the offending compilers.
The code in its natural habitat is here.
Here's the relevant part:
#if !defined(__GNUC__) || (__GNUC__ >= 5)
using string_stream = std::istringstream;
#else
// Until GCC 5, istringstream did not have a move constructor.
// stringstream_proxy is used instead, as a workaround.
class stringstream_proxy
{
public:
stringstream_proxy() = default;
// Construct with a value.
stringstream_proxy(std::string const& value) :
stream_(value)
{}
// Copy constructor.
stringstream_proxy(const stringstream_proxy& other) :
stream_(other.stream_.str())
{
stream_.setstate(other.stream_.rdstate());
}
void setstate(std::ios_base::iostate state) { stream_.setstate(state); }
// Stream out the value of the parameter.
// If the conversion was not possible, the stream will enter the fail state,
// and operator bool will return false.
template<typename T>
stringstream_proxy& operator >> (T& thing)
{
stream_ >> thing;
return *this;
}
// Get the string value.
std::string str() const { return stream_.str(); }
std::stringbuf* rdbuf() const { return stream_.rdbuf(); }
// Check the state of the stream.
// False when the most recent stream operation failed
operator bool() const { return !!stream_; }
~stringstream_proxy() = default;
private:
std::istringstream stream_;
};
using string_stream = stringstream_proxy;
#endif