Question

In C++, what is the difference in making member variables values or pointers? For example, what is the difference between pThread and thread in the following code:

class A {
public:
    boost::thread *pThread;
    boost::thread thread;
}

Answer 1

The difference is the memory layout of the object.

Member items are stored like a struct – sequentially and continuously stored in the memory (possibly with 2-4-8 bytes alignment but otherwise sequentially one after the other). Pointers are stored in the memory address as a 4/8 byte pointer while the actual object can be stored anywhere in the memory.

Consider the situation where you have a struct

struct grades {
    unsigned char math;
    unsigned char english;
    unsigned char history;
    unsigned char physics;
    unsigned char chemistry;
    unsigned char biology;
    unsigned char computers;
};

The size of the struct is 7 bytes.

Now if you have a class Student_member:

class Student_member {
    unsigned long id;
    struct grades _grades;
};

and a class Student_pointer:

class Student_pointer {
    unsigned long id;
    struct grades *_grades;
};

Then the size in memory of the class Student_member would be 4+7=11 bytes (long + the struct) which would be allocated as a continuous block, while the size in memory of class Student_pointer would be 4+4=8 byte (long + pointer) and another 7 bytes that are allocated somewhere else in memory storing the actual grades.