Olivier Langlois's blog

In the book The C++ Programming language from Bjarne Stroustrup, you can read in section 5.7 (Structures):

The size of an object of a structure type is not necessarily the sum of the sizes of its members. This is because many machines require objects of certain types to be allocated on architecture dependant boundaries or handle such objects much more efficiently if they are. For example, integers are often allocated on word boundaries. On such machines, objects are said to have to be aligned properly. This leads to "holes" in the structures...You can minimize wasted space by simply ordering members by size (largest member first).

As an example, on my platform if I write:

struct MyPOD1
{
  bool a;
  int  b;
  bool c
  int  d;
};

struct MyPOD2
{
  int  b;
  int  d;
  bool a;
  bool c;
};

sizeof(MyPOD1) is 16 and sizeof(MyPOD2) is 12. which means that 4 bytes are wasted by the MyPOD1 layout. Bjarne continues by adding that you should prefer readability over memory usage optimization but in my opinion taking the habit of always placing 1 byte variables such as bools and chars at the end of a class data members declaration can hardly obfuscate the code especially if the class encapsulation is tight.

Now that the demonstration that by just reordering data members around in a class declaration can save space has been done, let me explain to you the advantages of saving few bytes. Just imagine that you have a std::vector<MyPOD2> of 25K elements. You just saved 100KB of space to your program for free and since more elements will fit in the processor cache which should make the vector traversal faster.

I recommend taking this habit because it is easy to do and once you get used to do it, it will become an automatism and you will get the best performance from your data structures without having to stop and think about how you could optimize them before you need to.