How is Memory Organized for C Objects?

Memory Layout of C Objects

Dynamic casts and reinterpreting operations often involve manipulating object memory pointers. Let's delve into how C organizes objects in memory to understand these operations better.

According to the C standard, the memory layout of nonstatic data members within a class or struct is primarily determined by their order of declaration. Members with the same access specifier are ordered in their declared sequence.

Aside from member variables, objects also allocate space for:

• Member function pointers (virtual function management)
• Base class subobjects
• Padding and alignment requirements

The actual memory layout is implementation-dependent, but many compilers adhere to the Itanium ABI specification. This ABI provides a detailed description of the object memory layout, including the order of virtual function tables, base classes, and member variables.

Memory Layout Demo

Using clang, we can visualize the memory layout of a complex class:

<code class="cpp">class Class {
    // ...
};</code>

<code class="bash">clang -cc1 -fdump-record-layouts layout.cpp</code>

Output:

   0 | class Class
   0 |   class SBase1 (primary base)
   0 |     (SBase1 vtable pointer)
   8 |     int k
  16 |   class SBase2 (base)
  16 |     (SBase2 vtable pointer)
  24 |     int k
  28 |   class SBase3 (base)
  28 |     int k
  32 |   int i
  36 |   char c
  40 |   float f
  48 |   double d
  56 |   short s
  64 |   class VBase (virtual base)
  64 |     (VBase vtable pointer)
  72 |     int j
     | [sizeof=80, dsize=76, align=8
     |  nvsize=58, nvalign=8]

This output shows the specific memory offsets for each member variable, as well as the locations of the virtual function tables and base classes.

By understanding the memory layout of objects, you can gain insight into how dynamic casts and reinterpreting operations work and make informed decisions when manipulating object pointers.

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