Writing data to memory in C++ - c++

Question

I want to write a a mix of int, char, real in void *data.
I am using a file pointer to run through the data block.
Now my question is that since the data type is void, I have to typecast it to int while writing integer and char for writing string.
While typecasting I used the following sample code:
*((int *)data+0) = 14; //writing int
*((int *)data+4) = 5; //writing int, left a space of 4 bytes for int
*((char *)data+8) = 'a'; //writing char
*((char *)data+9) = 'f'; //writing char
But then while reading the values back it didnt give the correct value.
cout<<*((int *)data+0);
cout<<*((int *)data+3);
cout<<*((char *)data+8);
Is the way my code is written correct? I am doubtful about it as data is void.

Answer 1

*((int *)data+4) = 5; // writing 4th int
cout<<*((int *)data+3); // but reading third one
And just in case, ((int *)data+4) points to 4th integer (that is, 16th byte given int size = 4), not to 4th byte. That is, you code overwrites bytes 0-3, then 16-19, then 8th, then 9th. What you probably meant is: *(int *)( (char*)data + X )

Answer 2

Edited to correct mistake pointed out by MSalters
Apart from the typo that others have mentioned (data+3 instead of data+4), you also need to change e.g.
*((int *)data+4)
to
*((int *)data+1)
because adding 4 to an int * doesn't add 4 to the address, it adds 4 * sizeof (int).
If you need to write to an offset that is not a multiple of sizeof(int) (say, 7), you need:
*(int *)((char *)data+7)
For this reason, it might be better to make data a char * to start with, so you can just say
*(int *)(data+7)

Answer 3

Use a class or a struct.
Here, pointer arithmetics is misleading you. When you add 4 to an int * you are adding actually four times sizeof int.

Answer 4

If your data has a constant layout, why don't you just use a struct such as
struct MemoryLayout {
int _first;
int _second;
char _c1;
char _c2;
};
?

Answer 5

You're writing the second one with:
*((int *)data+4) = 5; // offset = 4
and reading it back with:
cout<<*((int *)data+3); // offset = 3
In addition, the (int*) cast is binding to data, not data+4 so that your 4 is scaled up by the size of an int.
If you really want to do this (and a struct is not a possibility due to variances in the data formats), you should cast data to a char*, then add the number of bytes to get the char offset, then cast that to your desired type.
That would be something like:
*((int*)((char*)(data + 0)) = 14; //writing int
*((int*)((char*)(data + 4)) = 5; //writing int
*((char*)data + 8) = 'a'; //writing char
*((char*)data + 9) = 'f'; //writing char

Answer 6

int Data;
//char Data;
//float Data;
FILE *File = fopen("File.txt","wb");
fwrite((char *)&Data,sizeof(Data),1,File);
fclose(File);
File = fopen("File.txt","rb");
fread((char *)&Data,sizeof(Data),1,File);
fclose(File);
...
int DataInt1 = 200;
char DataChar1 = 'N';
FILE *File = fopen("File.txt","wb");
fwrite((char *)&DataInt1,sizeof(DataInt1),1,File);
fwrite((char *)&DataChar1,sizeof(DataChar1),1,File);
fclose(File);
int DataInt2 = 0;
char DataChar2 = 0;
File = fopen("File.txt","rb");
fread((char *)&DataInt2,sizeof(DataInt2),1,File);
fread((char *)&DataChar2,sizeof(DataChar2),1,File);
fclose(File);
printf("%d %d!\n",DataInt2,DataChar2);