Why Copying Stringstream Is Not Allowed

Incomplete type is not allowed: stringstream

#include <sstream> and use the fully qualified name i.e. std::stringstream ss;

how copy from one stringstream object to another in C++?

Indeed, streams are non-copyable (though they are movable).

Depending on your usage, the following works quite well:

#include <iostream>
#include <sstream>

int main()
{
    std::stringstream ss1;
    ss1 << "some " << 123 << " stuff" << std::flush;

    std::stringstream ss2;
    ss2 << ss1.rdbuf(); // copy everything inside ss1's buffer to ss2's buffer

    std::cout << ss1.str() << std::endl;
    std::cout << ss2.str() << std::endl;
}

Output:

some 123 stuff

some 123 stuff

Does stringstream copy the string on which it is constructed?

Yes. From http://en.cppreference.com/w/cpp/io/basic_stringstream/basic_stringstream

2) Uses a copy of str as initial contents of the underlying string device. The underlying basic_stringbuf object is constructed as basic_stringbuf<Char,Traits,Allocator>(str, mode).

Problem with ostringstream and copy constructor

Copy constructor and copy-assignment of any stream class in C++ has been made private. That means, you cannot make copy of std::ostringstream object:

std::ostringstream ss;

std::ostringstream ss1(ss); //not allowed - copy-constructor is private
ss1=ss; //not allowed - copy-assignment is private

stringstream::str copy lifetime

This is not a problem of stringstream. It is the problem of c_str function - it returns pointer to a char* representation of std::string, but it lives only during lifetime of an original string. When you call char *str = ss.str().c_str() the following actually happens:

string tmp = ss.str();
char *str = tmp.c_str();
tmp.~string (); // after that line the pointer `str` is no longer valid

c_str is a dangerous function provided only for compatibility and speed purposes and you should avoid using it.

C++: Why re-input stringstream doesn't work? the content remains the same

The last read of sso >> r1 >> r2 >> r3; puts the stream into fail state because no more characters were available while it was still trying to read more characters for r3.

In fail state the stream cannot be written to or read from. To fix your problem move sso.clear(); up to before sso << "D".

You can see different behaviour if you output an extra space after C because then the stream does not enter fail state (it successfully reads r3 while there is still the space in the stream).

C++ incomplete type is not allowed when using stringstream

You've failed to #include <sstream>.

This is allowed to compile (any standard header can include an arbitrary number of other standard headers), but you should normally include the headers for the "stuff" you're using (and stringstream is declared in <sstream>). Once you include the header, VC++ compiles it just fine.

[Thanks Remy] Since you're using std::string, you should also #include <string>.

non-copying istringstream

It's fairly trivial to write a basic std::streambuf class that reads from a given memory area. You can then construct an istream from this and read from that.

initializing a C++ std::istringstream from an in memory buffer?

Note that the lifetime of the buffer pointed to be c_str() is very limited, though, and there's no guarantee that a call to c_str() want cause some copying although I don't know of any implementations where it does.

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