Missing data when reading/writing to stream

Question

Here is complete example - compiles and runs, writes contents of map to the file and reads it right after:

#include <map>
#include <fstream>
#include <iostream>
using namespace std;

int main(int argc, char* argv[])
{
    std::string fname("test.bin");

    std::map<unsigned,unsigned> testMap;
    testMap[0]=103;
    testMap[1]=2;
    testMap[5]=26;
    testMap[22]=4;

    std::ofstream output(fname.c_str(),std::ios_base::binary|std::ios_base::trunc);
    for(std::map<unsigned,unsigned>::iterator iter = testMap.begin();iter != testMap.end();++iter)
    {
        unsigned temp = iter->first;
        output.write((const char*)&temp,sizeof(temp));
        unsigned temp1 = iter->second;
        output.write((const char*)&temp1,sizeof(temp1));
        std::cerr << temp <<" "<<temp1<<" "<<std::endl;
    }
    std::cerr << "wrote bytes.........."<<output.tellp()<<", map size "<<testMap.size()<<std::endl;
    output.flush();
    output.close();

    std::ifstream input(fname.c_str());
    // retrieve length of file:
    input.seekg (0, input.end);
    unsigned streamSize = input.tellg();
    input.seekg (0, input.beg);

    char* buff = new char[streamSize];
    input.read(buff,streamSize);
    cerr << "sizeof of input......"<<streamSize << endl;
    cerr << "read bytes..........."<<input.gcount() << endl;
    ::getchar();
    return 0;
}

It gives the following output:

0 103 
1 2 
5 26 
22 4 
wrote bytes..........32, map size 4
sizeof of input......32
read bytes...........20

The question is why bytes read does not match bytes written, and how to read/write whole map.

P.S. Online compiler gives me expected output of 32 read bytes, I'm getting wrong output while compiling with Visual Studio 2010 proffesional.

Answer 1

Make sure you're opening the file as a binary file.