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I am trying to write a stringstream into a file but it not working.


int main() {
    std::stringstream stream;
    stream << "Hello world";
    cout << stream.rdbuf()<<endl;//prints fine

    std::ofstream p{ "hi.txt" };
    p << stream.rdbuf();//nothing is writtten
    p.close();
    std::ifstream ip{ "hi.txt" };
    std::string s;
    ip >> s;
    cout << s;//nothing printed
    p.close();
    return 0;
}

This answer here follows the same process. But it's not working in my case.

lorem1213
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    After you do `cout << stream.rdbuf() << endl;`, `stream.rdbuf()` has been emptied out. So `p << stream.rdbuf();` won't write anything, and `ip >> s;` will read an empty file. – Nathan Pierson Jul 16 '22 at 06:17
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    @NathanPierson it is not emptied, just read through. You can seek the stringstream's read pointer back to the beginning so you can then read the same data again. – Remy Lebeau Jul 16 '22 at 06:27

2 Answers2

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Internally, the stringstream maintain a read pointer/offset into its data buffer. When you read something from the stream, it reads from the current offset and then increments it.

So, cout << stream.rdbuf() reads the entire stream's data, leaving the read pointer/offset at the end of the stream. Thus, there is nothing left for p << stream.rdbuf() to read afterwards. In order to do that, you have to seek the read pointer/offset back to the beginning of the steam, eg:

cout << stream.rdbuf() << endl;
stream.clear(); // resets the error state, particularly eofbit
stream.seekg(0);
p << stream.rdbuf();
Remy Lebeau
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1

The C++ file streams use input and output pointers to keep track of it's data buffer. When you write something it is written from the position where the current write pointer is or we can say the write pointer offset is added and then the data is simply written, after which the write pointer is incremented by that much value

The stream.rdbuf() function reads all the data in the current file and leaves the read pointer at the end of the file so when we run the same function again there is nothing to read

This can Simply be solved by using stream.seekg(0)

 cout << stream.rdbuf() << endl;
 stream.seekg(0);  //This puts the pointer at the beginning of the file
 p << stream.rdbuf();  //Should work'
polkamonster
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