Objects and Names

What are Objects

In C, there are two types of objects:

• Elemental: declared explicitly - int x, int* xp, float A[10], node_t node
  • Have identifiers as their names (x, xp, A, node)
  • May have either a basic or a derived type, type specification can surround name
• Compound: constructed implicitly - dereference xp, index to A, access a component of node
  • Has a tree structured name combining identifiers, constants, and accessors (*xp, A[6], node.children, node.children[0], node.children[*xp])
  • May have either a basic or derived type: int, float, (node_t*)[3], node_t*
  • Type is inferred by a post order traversal of the tree

Helper Methods

For a C object named x, define:

• TYPE(x) to be the C language type of x
• ADDR(x) to be the lowest numbered memory byte occupied by x at runtime (&x)
• SIZE(x) to be the number of memory bytes occupied by x at runtime (sizeof(x))
• BOX(x) = [ADDR(x), ADDR(x)+SIZE(x)-1], the range of memory bytes holding the machine representation of x
• VAL(x) to be the value of x, obtained by applying the type-specific interpretation to the machine representation of x