如何在没有c ++副本的情况下从ostringstream获取char指针

Question

I have a ostringstream variable which contains some data. I want to get set a char * pointer to the data inside the ostringstream.

If I do the following:

std::ostringstream ofs;
.....
const char *stam = (ofs.str()).c_str();

There is a copy of the content of the string in ofs. I want to get a pointer to that content without a copy.

Is there a way to do so?

Answer 1

This actually answers the question... took a while but I wanted to do it for the same reasons (efficiency vs portability is fine for my situation):

class mybuf : public std::stringbuf {
public:
    // expose the terribly named end/begin pointers
    char *eback() {
        return std::streambuf::eback();
    }
    char *pptr() {
        return std::streambuf::pptr();
    }
};


class myos : public std::ostringstream {
    mybuf d_buf;
public:
    myos() {
        // replace buffer
        std::basic_ostream<char>::rdbuf(&d_buf);
    }
    char *ptr();
};

char *myos::ptr() {
    // assert contiguous
    assert (  tellp() == (d_buf.pptr()-d_buf.eback()) );
    return d_buf.eback();
}

int main() {
    myos os;
    os << "hello";
    std::cout << "size: " << os.tellp()  << std::endl;
    std::string dat(os.ptr(),os.tellp());
    std::cout << "data: " << dat  << std::endl;
}

This points to, yet again, the deeper, underlying problem with the standard library - a confusion between contracts and "safety". When writing a messaging service, I need a library with efficient contracts... not safety. Other times, when writing a UI, I want strong safety - and cares less about efficiency.

Answer 2

Although you can't get a pointer to the character buffer in the ostringstream, you can get access to its characters without copying them if you switch to using stringstream. A stringstream allows input and output (reading from and writing to the stream), whereas ostringstream allows only output (writing to the stream). Example:

std::stringstream ss;
ss << "This is a test.";

// Read stringstream from index 0.  Use different values to look at any character index.
ss.seekg(0);

char ch;
while (ss.get(ch)) {                  // loop getting single characters
    std::cout << ch;
}

ss.clear(); // Clear eof bit in case you want to read more from ss

This site has a pretty good overview of stringstreams and what you can do with them.