Convert bytes from a file to an integer c++ - c++

Question

I am trying to parse a .dat file reading it byte by byte with this code.(the name of the file is in arv[1])
std::ifstream is (arv[1], std::ifstream::binary);
if (is) {
is.seekg (0, is.end);
int length = is.tellg();
is.seekg (0, is.beg);
char * buffer = new char [length];
is.read (buffer,length);
if (is)
std::cout << "all characters read successfully.";
else
std::cout << "error: only " << is.gcount() << " could be read";
is.close();
}
Now all file is in the buffer variable. The file contains numbers represented in 32 bits, how can I iterate over the buffer reading 4 bytes at a time and convert them to integer?

Answer 1

first of all , you have a memory leak, you dynamically allocate character array but never delete[] them.
use std::string instead:
std::string buffer(length,0);
is.read (&buffer[0],length);
now, assuming you had written the integer correctly, and have read it correctly into buffer, you can use this character array as pointer to integer:
int myInt = *(int*)&buffer[0];
(do you understand why?)
if you have more then one integer stored:
std::vector<int> integers;
for (int i=0;i<buffer.size();i+=sizeof(int)){
integers.push_back(*(int*)&buffer[i]);
}

Answer 2

Instead of:
char * buffer = new char [length];
is.read (buffer,length);
You can use:
int numIntegers = length/sizeof(int);
int* buffer = new int[numIntegers];
is.read(reinterpret_cast<char*>(buffer), numIntegers*sizeof(int));
Update, in response to OP's comment
I am not seeing any problems with the approach I suggested. Here's a sample program and the output I see using g++ 4.9.2.
#include <iostream>
#include <fstream>
#include <cstdlib>
void writeData(char const* filename, int n)
{
std::ofstream out(filename, std::ios::binary);
for ( int i = 0; i < n; ++i )
{
int num = std::rand();
out.write(reinterpret_cast<char*>(&num), sizeof(int));
}
}
void readData(char const* filename)
{
std::ifstream is(filename, std::ifstream::binary);
if (is)
{
is.seekg (0, is.end);
int length = is.tellg();
is.seekg (0, is.beg);
int numIntegers = length/sizeof(int);
int* buffer = new int [numIntegers];
std::cout << "Number of integers: " << numIntegers << std::endl;
is.read(reinterpret_cast<char*>(buffer), numIntegers*sizeof(int));
if (is)
std::cout << "all characters read successfully." << std::endl;
else
std::cout << "error: only " << is.gcount() << " could be read" << std::endl;
for (int i = 0; i < numIntegers; ++i )
{
std::cout << buffer[i] << std::endl;
}
}
}
int main()
{
writeData("test.bin", 10);
readData("test.bin");
}
Output
Number of integers: 10
all characters read successfully.
1481765933
1085377743
1270216262
1191391529
812669700
553475508
445349752
1344887256
730417256
1812158119