Data structure padding

Question

What is data structure padding in c++ and how do i check the number of bytes padded bytes?

class a { public: int x; int y; int z; };

Answer 1

padding is done for performance reasons - see this article Data Structure Alignment for more info.

To see whether the compiler pads your data structure you could write a simple program:

#include <iostream>

class a {
public:
    int x;
    int y;
    int z;
};

int main()
{
    std::cout << sizeof(a) << std::endl; // print the total size in bytes required per class instance

    a anInstance;
    std::cout << &anInstance.x << std::endl; // print the address of the x member
    std::cout << &anInstance.y << std::endl; // as above but for y
    std::cout << &anInstance.z << std::endl; // etc
}

I added the public declaration to avoid compiler errors - It will not affect the size or padding.

Edit: Running this on my macbook air gives the following output: 12 0x7fff5fbff650 0x7fff5fbff654 0x7fff5fbff658

This shows that on my machine the total size is 12 bytes, and each member is 4 bytes apart. The ints are 4 bytes each (which can be confirmed with sizeof(int)). There is no padding.

Try this with different members in your class, for example:

class b {
    public:
        char      w;
        char      x[6];
        int       y;
        long long z;
};

Answer 2

Processors require that certain types of data have particular alignments. For example, a processor might require that an int be on a 4-byte boundary. So, for example, an int could start at memory location 0x4000 but it could not start at 0x4001. So if you defined a class:

class a
{
public:
    char c;
    int i;
};

the compiler would have to insert padding between c and i so that i could start on a 4-byte boundary.

Answer 3

struct A
{
    char c;
    int i;
};

int main(int argc, char *argv[])
{
    A a;

    cout << "sizeof struct = " << sizeof(A) << endl;

    cout << "sizeof items  = " << sizeof(a.c) + sizeof(a.i) << endl;

    return 0;
}

Answer 4

Lol just create 2 identical structs, make one of them packed e.g.

struct foo {
  int  a;
  char b;
  int  c;
} 

struct bar {
  int  a;
  char b;
  int  c;
} __attribute__((__packed__));

sizeof(foo) - sizeof(bar) will give you the amount of padding. Or you could also calculate manually like Duck suggested.