How do you determine the size of an object in C++?

Question

For example, say I have a class Temp:

class Temp
{
    public:
        int function1(int foo) { return 1; }
        void function2(int bar) { foobar = bar; }         


        

Answer 1

There's a utility call pahole (for 'Poke-A-HOLE') that's nominally intended to study how object layouts get padded, but is great for visualizing object size and layout in general.

Answer 2

If you want detailed information about how objects are represented in memory at run-time, the ABI (Application Binary Interface) specification is the place to look. You'll need to look determine which ABI your compiler implements; for example, GCC versions 3.2 and above implement the Itanium C++ ABI.

Answer 3

If you want to examine the layout of a particular structure, the offsetof(s,member) macro may also be of use. It tells you how far from the base address of a structure a particular member lives:

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

// Print placement of foo's members
void printFoo() {
  printf("foo->a is %zu bytes into a foo\n", offsetof(struct foo, a));
  printf("foo->b is %zu bytes into a foo\n", offsetof(struct foo, b));
}

int main() {
  printFoo();
  return 0;
}

Would print on a typical 32-bit machine:

foo->a is 0 bytes into a foo
foo->b is 4 bytes into a foo

Whereas on a typical 64 bit machine, it would print

foo->a is 0 bytes into a foo
foo->b is 8 bytes into a foo

Answer 4

I've always wondered this sort of thing, so I decided to come up with a complete answer. It's about what you might expect, and it is predictable (yay)! Thus, with the information below, you ought to be able to predict the size of a class.

Using Visual Studio Community 2017 (Version 15.2), in Release mode with all optimizations disabled and RTTI (Run-time Type Information) off, I have determined the following:

---

Shortish answer:

First of all:

• In 32 (x86) bit, <size of pointer> == 4 bytes
• In 64 (x64) bit, <size of pointer> == 8 bytes
• When I say "virtual class inheritance", I mean e.g.: class ChildClass: virtual public ParentClass

Now, my findings are that:

• empty classes are 1 byte
• inheritance of an empty class is still 1 byte
• empty classes with functions are still 1 byte (?! see Note below for explanation)
• inheritance of an empty class with a function is still 1 byte
• adding a variable to an empty class is <size of variable> bytes
• inheriting a class with a variable and adding another variable is <size of variables> bytes
• inheriting a class and overriding its function adds a vtable (further explanation provided in Conclusions section) and is <size of pointer> bytes
• simply declaring a function virtual also adds a vtable, making it <size of pointer> bytes
• virtual class inheritance of an empty class (with or without a member function) also adds a vtable, and makes the class <size of pointer> bytes
• virtual class inheritance of a non-empty class also adds a vtable, but it gets somewhat complicated: it adds <size of pointer> bytes to the total, wrapping all of the member variables in as many <size of pointer> bytes increments as is necessary to cover <total size of member variables> - yeah, you read that right... (see my guess as to what's going on in Conclusions...)

Note that I even tried having the function() cout some text, creating an instance of the class, and calling the function; it doesn't change the size of the function class (it's not an optimization)! I was somewhat surprised, but it actually makes sense: member functions don't change, so they can be stored external to the class itself.

Conclusions:

• Empty classes are 1 byte, since that is the minimum required for it to have a presence in memory. Once data or vtable data is added, though, begin counting at 0 bytes.
• Adding a (non-virtual) member function does nothing to the size, because the member function is stored externally.
• Declaring a member function to be virtual (even if the class is not overridded!) or overriding a member function in a child class adds what is called a "vtable" or "virtual function table", which allows for Dynamic Dispatch (which is really super awesome to use though and I highly recommend using it). This vtable consumes <size of pointer> bytes, adding <size of pointer> bytes to said class. This vtable can only exist once per class (either it does or it doesn't), of course.
• Adding a member variable increases the size of the class by that member variable, regardless of whether said member variable is in the parent or child class (the parent class remains its own size though, of course).
• Virtual class inheritance is the only part that gets complicated... So... I think what's going on after a little experimentation is: the size of the class actually increments in <size of pointer> bytes at a time, even if it doesn't need to consume that much memory, I'm guessing because it's adding a vtable "helper block" for each <size of pointer> bytes of memory or something...

---

Long answer:

I determined all of this using this code:

#include <iostream>

using namespace std;

class TestA
{

};

class TestB: public TestA
{

};

class TestC: virtual public TestA
{

};

class TestD
{
    public:
        int i;
};

class TestE: public TestD
{
    public:
        int j;
};

class TestF: virtual public TestD
{
    public:
        int j;
};

class TestG
{
    public:
        void function()
        {

        }
};

class TestH: public TestG
{
    public:
        void function()
        {

        }
};

class TestI: virtual public TestG
{
    public:
        void function()
        {

        }
};

class TestJ
{
    public:
        virtual void function()
        {

        }
};

class TestK: public TestJ
{
    public:
        void function() override
        {

        }
};

class TestL: virtual public TestJ
{
    public:
        void function() override
        {

        }
};

void main()
{
    cout << "int:\t\t" << sizeof(int) << "\n";
    cout << "TestA:\t\t" << sizeof(TestA) << "\t(empty class)\n";
    cout << "TestB:\t\t" << sizeof(TestB) << "\t(inheriting empty class)\n";
    cout << "TestC:\t\t" << sizeof(TestC) << "\t(virtual inheriting empty class)\n";
    cout << "TestD:\t\t" << sizeof(TestD) << "\t(int class)\n";
    cout << "TestE:\t\t" << sizeof(TestE) << "\t(inheriting int + int class)\n";
    cout << "TestF:\t\t" << sizeof(TestF) << "\t(virtual inheriting int + int class)\n";
    cout << "TestG:\t\t" << sizeof(TestG) << "\t(function class)\n";
    cout << "TestH:\t\t" << sizeof(TestH) << "\t(inheriting function class)\n";
    cout << "TestI:\t\t" << sizeof(TestI) << "\t(virtual inheriting function class)\n";
    cout << "TestJ:\t\t" << sizeof(TestJ) << "\t(virtual function class)\n";
    cout << "TestK:\t\t" << sizeof(TestK) << "\t(inheriting overriding function class)\n";
    cout << "TestL:\t\t" << sizeof(TestL) << "\t(virtual inheriting overriding function class)\n";

    cout << "\n";
    system("pause");
}

---

Output:

32 (x86) bits:

int:            4
TestA:          1       (empty class)
TestB:          1       (inheriting empty class)
TestC:          4       (virtual inheriting empty class)
TestD:          4       (int class)
TestE:          8       (inheriting int + int class)
TestF:          12      (virtual inheriting int + int class)
TestG:          1       (function class)
TestH:          1       (inheriting function class)
TestI:          4       (virtual inheriting function class)
TestJ:          4       (virtual function class)
TestK:          4       (inheriting overriding function class)
TestL:          8       (virtual inheriting overriding function class)

64 (x64) bits:

int:            4
TestA:          1       (empty class)
TestB:          1       (inheriting empty class)
TestC:          8       (virtual inheriting empty class)
TestD:          4       (int class)
TestE:          8       (inheriting int + int class)
TestF:          24      (virtual inheriting int + int class)
TestG:          1       (function class)
TestH:          1       (inheriting function class)
TestI:          8       (virtual inheriting function class)
TestJ:          8       (virtual function class)
TestK:          8       (inheriting overriding function class)
TestL:          16      (virtual inheriting overriding function class)

---

If you want information on multiple inheritance, go figure it out your darn self! -.-

Answer 5

Member functions dont account for the size of the objects of a particular class. The size of the object depends only on the member variables. In case of classes that contain virtual functions, the VPTR gets added to the object layout. So the size of the objects is basically size of the member variables + the size of the VPTRs. Sometimes this may not be true as Compilers try to locate member variables at the DWORD boundary.