Copy data from fstream to stringstream with no buf

问题:

Is there anyway I can transfer data from an fstream (a file) to a stringstream (a stream in the memory)?

Currently, I'm using a buffer, but this requires double the memory, because you need to copy the data to a buffer, then copy the buffer to the stringstream, and until you delete the buffer, the data is duplicated in the memory.

std::fstream fWrite(fName,std::ios::binary | std::ios::in | std::ios::out);  
    fWrite.seekg(0,std::ios::end); //Seek to the end  
    int fLen = fWrite.tellg(); //Get length of file  
    fWrite.seekg(0,std::ios::beg); //Seek back to beginning  
    char* fileBuffer = new char[fLen];  
    fWrite.read(fileBuffer,fLen);  
    Write(fileBuffer,fLen); //This writes the buffer to the stringstream  
    delete fileBuffer;`

Does anyone know how I can write a whole file to a stringstream without using an inbetween buffer?

回答1:

// need to include <algorithm> and <iterator>, and of course <fstream> and <sstream>
ifstream fin("input.txt");
ostringstream sout;
copy(istreambuf_iterator<char>(fin),
     istreambuf_iterator<char>(),
     ostreambuf_iterator<char>(sout));

回答2:

 ifstream f(fName);
 stringstream s;
 if (f) {
     s << f.rdbuf();    
     f.close();
 }

回答3:

In the documentation for ostream, there are several overloads for operator<<. One of them takes a streambuf* and reads all of the streambuffer's contents.

Here is a sample use (compiled and tested):

#include <exception>
#include <iostream>
#include <fstream>
#include <sstream>

int main ( int, char ** )
try
{
        // Will hold file contents.
    std::stringstream contents;

        // Open the file for the shortest time possible.
    { std::ifstream file("/path/to/file", std::ios::binary);

            // Make sure we have something to read.
        if ( !file.is_open() ) {
            throw (std::exception("Could not open file."));
        }

            // Copy contents "as efficiently as possible".
        contents << file.rdbuf();
    }

        // Do something "useful" with the file contents.
    std::cout << contents.rdbuf();
}
catch ( const std::exception& error )
{
    std::cerr << error.what() << std::endl;
    return (EXIT_FAILURE);
}

回答4:

The only way using the C++ standard library is to use a ostrstream instead of stringstream.

You can construct a ostrstream object with your own char buffer, and it will take ownership of the buffer then (so no more copying is needed).

Note however, that the strstream header is deprecated (though its still part of C++03, and most likely, it will always be available on most standard library implementations), and you will get into big troubles if you forget to null-terminate the data supplied to the ostrstream.This also applies to the stream operators, e.g: ostrstreamobject << some_data << std::ends; (std::ends nullterminates the data).