X3J11 Technical Report - Designated Initializers

Introduction

• Purpose
• Scope
• References

Language

• Designated Initializers
  Syntax
  Constraints
  Semantics
  Examples

Introduction

Purpose

Scope

References

All references to clauses of the ISO standard and to sections of the former ANSI standard will be presented in pairs. For example, subclause 6.4 or §3.4 references constant expressions.

Language

Designated Initializers

The syntax for initializers (subclause 6.5.7 or §3.5.7) is changed to the following, and the constraints and semantics are augmented by the following:[Footnote 1]

Syntax

	initializer:
		assignment-expression
		{ initializer-list }
		{ initializer-list , }

	initializer-list:
		designation(opt) initializer
		initializer-list , designation(opt) initializer

	designation:
		designator-list =

	designator-list:
		designator
		designator-list designator

	designator:
		[ constant-expression ]
		. identifier

Constraints

No initializer shall attempt to provide a value for an object not contained within the entity being initialized.[Footnote 2] 

If a designator has the form

	[ constant-expression ]

If a designator has the form

	. identifier

Semantics

Each brace-enclosed initializer list has an associated current object.  When no designations are present, subobjects of the current object are initialized in order according to the type of the current object: array elements in increasing subscript order, structure members in declaration order, and the first member of a union.[Footnote 3]  In contrast, a designation causes the following initializer to begin initialization of the subobject described by the designator.  Initialization continues forward, as normal, after the designated initializer.[Footnote 4]  (The designation can be thought of as a ``goto'' within the current object.)

Each designator list begins its description with the current object associated with the closest-surrounding brace pair.  Each item in the designator list (in order) specifies a particular member of its current object and changes the current object for the next designator (if any) to be that member.[Footnote 5]  The current object that results at the end of the designator list is the subobject to be initialized by the following initializer.

The initialization shall occur in initialization list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject; all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.

If an array of unknown size is initialized, its size is determined by the largest indexed element with an explicit initializer.[Footnote 6] 

Examples

	enum { Mem_One, Mem_Two, /*...*/ };
	const char *nm[] = {
		[Mem_Two] = "Mem Two",
		[Mem_One] = "Mem One",
		/*...*/
	};

	div_t answer = { .quot = 2, .rem = -1 };

	struct { int a[3], b; } w[] = { [0].a = {1}, [1].a[0] = 2 };

	int a[MAX] = {
		1, 3, 5, 7, 9, [MAX-5] = 8, 6, 4, 2, 0
	};

Finally, any member of a union can be initialized:

	union { /*...*/ } = { .any_member = 42 };