c++ creating 4 integer from the 16 byte char array

Question

Here is my question

I've read a binary sequence from a binary file into a char array. My binary file is much bigger than my buffer so, I've read it iteratively.

I've read 16 Bytes from the file and store it into char * myPointer pointer. Now from myPointer I need to create 4 integer and I dont know how could I do that. Is there anyone to help me ?

Note: I dont have to store it into char array if you have better solution

Answer 1

If endianess of stored data is same as endianess of your system, then you can use following code to store read data in int :

int ints[4];
char bytes[4*sizeof(int)];
memcpy( ints, bytes, sizeof( ints ) );

If you have different endianess in input binary file and on your computer, you might want to swap the bits:

template< class T>
void changeEndianess( T & input)
{
    char temp[ sizeof( T ) ];
    for( int j=sizeof( T )-1; j>=0; --j )
        temp[sizeof(T)-j-1] = *((char *)&input+j);
    memcpy( &input, temp, sizeof( T ) );
}

// and somewhere in your program:
for( size_t i=0; i<sizeof( ints ); ++i )
{
    changeEndianess<int>( ints[i] );
}

To make it use short change all int in above code to short , but it's up to you to decide when it is needed.

Answer 2

Reading from a binary file:

#include <fstream>
#include <assert.h>

void main()
{
    std::ifstream file("file.dat", std::ios_base::binary);
    assert(file.is_open());

    int i = 0;
    file.read((void*)&i, sizeof(i));
    short s = 0;
    file.read((void*)&s, sizeof(s));

    ... // And so on
}

Obviously, you need to know exactly how to interpret the data, ie the file structure (which data type is written where).