fstream实现文件读取

 

写入文件，然后读取文件

#include <fstream>
#include <iostream>
#include <string>

void Approach1();
void Approach2();

void ProcessLine( std::string const & lineData );

int main()
{
    // Use first approach to write and read file
    std::cout << "Output from first approach:\n";
    Approach1();

    // Use second approach to write and read file
    std::cout << "\nOutput from second approach:\n";
    Approach2();

    return 0;
}

// Write and read file using first approach.
// Create a std::ofstream, write data to it and close stream.
// Open file using a std::ifstream, read and process lines
// from it.
void Approach1()
{
        // Open file for writing, truncating file data
        // if file exists.
        std::ofstream out("approach1.dat");

        // Write 4 lines to it
        out << "line1\nline2\nline3\nline4\n";

        // Explicitly close out file stream
        // Would also be closed on destruction of object out
        out.close();

        // Open file for reading
        std::ifstream in("approach1.dat");

        // Read and process data in file
        std::string lineData;
        while (in)
        {
                in >> lineData;

                if ( !in )
                {
                        if ( !in.eof() )
                        {
                                std::cerr << "Error! Input "
                                             "stream failure. "
                                             "Quitting.\n";
                        }
                }
                else
                {
                        ProcessLine( lineData );
                }
        }

        in.close();
}

// Write and read file using second approach
// Create a std:fstream, open for read write.
// Write data, rewind read position and read
// and process lines in file.
void Approach2()
{
        // Open file for reading and writing,
        // with truncation of existing file data
        std::fstream inOut( "approach2.dat"
                          , std::ios::in | std::ios::out
                                         | std::ios::trunc
                          );

        // Write 4 lines to it
        inOut << "line1\nline2\nline3\nline4\n";

        // Flush buffered written data to file:
        // This is not strictly necessary but shows you
        // how to do it (could also have used std::endl
        // instead of last \n above)
        inOut.flush();

        // Rewind read position to start of file
        inOut.seekg(0);

        // Read and process data in file
        std::string lineData;
        while ( !inOut.eof() )
        {
                inOut >> lineData;

                if ( !inOut )
                {
                        if ( !inOut.eof() )
                        {
                                std::cerr << "Error! Input "
                                             "stream failure. "
                                             "Quitting.\n";
                        }
                }
                else
                {
                        ProcessLine( lineData );
                }
        }

        inOut.close();
}

// Process a line of data.
// If the line starts with "line" then asume it
// has the correct format and is followed by a
// single digit. Output this digit to std::cout.
// otherwise output error to std:cerr.
void ProcessLine( std::string const & lineData )
{
        std::string prefix("line");
        if ( 0==lineData.find(prefix) )
        {
                std::cout << "Line value:"
                          << lineData.substr(prefix.length(),1)
                          << '\n';
        }
        else
        {
                std::cerr << "Error! badly formatted line.\n";
        }
}

 

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