c++ - Is there a way to std::move std::string into std::stringstream

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Learn more about Teams Probably because std::stringstrream does not store the data as a string internally but inside a stream buffer. So it needs to copy the data from the string into the string buffer. If it used move semantics it would need to take ownership of the string buffer thus need to know how it was allocated (which may not be dynamically allocated see short sting optimization) and know how to set string to a valid state. So allowing move semantics means tightly coupling the string and stringstream class. – Martin York May 14, 2016 at 17:25 @LokiAstari: " So allowing move semantics means tightly coupling the string and stringstream class. " Yes, God forbid that two classes from the same library be coupled together... – Nicol Bolas May 14, 2016 at 17:26 @NicolBolas: Coupling usually means that there are tradeoffs. Maybe the standard designers did not want to pay those tradeoffs. std::string is pretty crucial to most C++ applications. Do you want to limit optimizations on it just so this one corner case is more efficient? PS I have no intention of defending the IO library you can take that up with the committee (who are smarter than me). Just trying to provide a reasonable explanation for the limitation. – Martin York May 14, 2016 at 17:31 @LokiAstari: Your way limits optimizations for cases that you can't even say actually exist. And it's not an either/or kind of thing. The current behavior makes it impossible to allow an implementation to move the string data.


    str()

is a


    const

function; it's not allowed to modify the


    stringstream

's data at all. It would be very easily to create APIs that would permit the moving of string data without requiring it. After moving into the stream, the string would be "valid-but-unspecified". After moving from the stream, the stream would be "valid-but-unspecified." – Nicol Bolas May 14, 2016 at 17:36 @NicolBolas: How is this my fault? It looks like you have a good suggestion for the committee. You should write it up and propose it. – Martin York May 14, 2016 at 17:37

Since C++20 you can move a string into a stringstream : cppreference

Old answer for pre-C++20:

I do not see a std::stringstream constructor accepting rvalue reference of std::string

That's right. Even the str setter doesn't utilize move semantics, so moving a string into stringstream is not supported (not in the current standard, but hopefully in the next one).

Hopefully there are some good people creating proposals making c++ more straightforward :) – W.F. May 14, 2016 at 17:31

You'll be able to move a string into a string-stream in C++20.

Move semantics are supported by the constructor :

std::string myString{ "..." };
std::stringstream myStream{ std::move(myString) };
It can also be done after construction by calling str():
std::string myString{ "..." };
std::stringstream myStream;
myStream.str(std::move(myString));
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