JTC1/SC22/WG14
N716
C9X Addition WG14/N716 X3J11/97-079
Compound Literals
David Prosser ([email protected])
David Keaton ([email protected])
20 June, 1997
1. Introduction
1.1 Purpose
This document specifies the form and interpretation of a pure
extension to the language portion of the C standard to provide important
5 additional flexibility to literals in expressions.
1.2 Scope
This document, although extending the C standard, still falls within
the scope of that standard, and thus follows all rules and guidelines of
that standard except where explicitly noted herein.
10 1.3 References
1. ISO/IEC 9899:1990, Programming Languages -- C.
2. WG14/N494 X3J11/95-095, Prosser & Keaton. C9X Addition, Designated
Initializers, 8 December, 1995.
All references to the ISO C standard will be presented as subclause
15 numbers. For example, S6.4 references constant expressions.
1.4 Rationale
Compound literals provide a mechanism for specifying constants of
aggregate or union type. This eliminates the requirement for temporary
variables when an aggregate or union value will only be needed once.
20 Compound literals integrate easily into the C grammar and do not
impose any additional run-time overhead on a user's program. They also
combine well with designated initializers (see [2]) to form an even more
convenient aggregate or union constant notation. Their initial C
implementation appeared in a compiler by Ken Thompson at AT&T Bell
25 Laboratories.
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2. Language
2.1 Compound Literals
The syntax for postfix-expression in S6.3.2 is augmented by the
following:
5 Syntax
postfix-expression:
( type-name ) { initializer-list }
( type-name ) { initializer-list , }
A new subclause is added, S6.3.2.5, as follows:
10 6.3.2.5 Compound literals
Constraints
The type name shall specify an object type or an array of unknown
size.
No initializer shall attempt to provide a value for an object not
15 contained within the entire unnamed object specified by the compound
literal.1
If the compound literal occurs outside the body of a function, the
initializer list shall consist of constant expressions.
Semantics
20 A postfix expression that consists of a parenthesized type name
followed by a brace-enclosed list of initializers is a compound literal.
It provides an unnamed object whose value is given by the initializer
list.2
If the type name specifies an array of unknown size, the size is
25 determined by the initializer list as specified in S6.5.7, and the type
of the compound literal is that of the completed array type. Otherwise
(when the type name specifies an object type), the type of the compound
__________
1. This is the ``There shall be no more initializers...'' constraint
modified to take into account designated initializers[2].
2. Note that this differs from a cast expression. For example, a cast
specifies a conversion to scalar types or void only, and the result of a
cast expression is not an lvalue.
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literal is that specified by the type name. In either case, the result
is an lvalue.
The value of the compound literal is that of an unnamed object
initialized by the initializer list. The object has static storage
5 duration if and only if the compound literal occurs outside the body of a
function; otherwise, it has automatic storage duration associated with
the enclosing block.
Except that the initializers need not be constant expressions (when
the unnamed object has automatic storage duration), all the semantic
10 rules and constraints for initializer lists in S6.5.7 are applicable to
compound literals.3 The order in which any side effects occur among the
initialization list expressions is unspecified.4
String literals, and compound literals with const-qualified types,
need not designate distinct objects.5
15 Examples
1. The file scope definition
int *p = (int []){2, 4};
initializes p to point to the first element of an array of two
ints, the first having the value two and the second, four. The
20 expressions in this compound literal must be constant. The unnamed
object has static storage duration.
__________
3. For example, subobjects without explicit initializers are initialized to
zero.
4. In particular, the evaluation order need not be the same as the order of
subobject initialization. The extensions to initializers described in [2]
prescribe an ordering for the implicit assignments to the subobjects that
comprise the unnamed object.
5. This allows implementations to share storage for string literals and
constant compound literals with the same or overlapping representations.
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2. In contrast, in
void f(void)
{
int *p;
5 /*...*/
p = (int [2]){*p};
/*...*/
p is assigned the address of an unnamed automatic storage duration
object that is an array of two ints, the first having the value
10 previously pointed to by p and the second, zero.
3. Initializers with designations can be readily combined with
compound literals. On-the-fly structure objects can be passed to
functions without depending on member order:
drawline((struct point){.x=1, .y=1},
15 (struct point){.x=3, .y=4});
Or, if drawline instead expected pointers to struct point:
drawline(&(struct point){.x=1, .y=1},
&(struct point){.x=3, .y=4});
4. A read-only compound literal can be specified through constructions
20 like:
(const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}
5. The following three expressions have different meanings:
"/tmp/fileXXXXXX"
(char []){"/tmp/fileXXXXXX"}
25 (const char[]){"/tmp/fileXXXXXX"}
The first always has static storage duration and has type array of
char, but need not be modifiable; the last two have automatic
storage duration when they occur within the body of a function, and
the first of these two is modifiable.
30 6. Like string literals, const-qualified compound literals can be be
placed into read-only memory and can even be shared. For example,
(const char[]){"abc"} == "abc"
might yield 1 if the literals' storage is shared.
7. Since compound literals are unnamed, a single compound literal
35 cannot specify a circularly linked object. For example, there is
no way to write a self-referential compound literal that could be
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used as the function argument in place of the named object
endless_zeros below:
struct int_list { int car; struct int_list *cdr; };
struct int_list endless_zeros = {0, &endless_zeros};
5 eval(endless_zeros);
8. Outside the body of a function, a compound literal is an
initialization of a static object; however, inside a function
body, it is an assignment to an automatic object. Therefore, the
following two loops produce the same sequence of values for the
10 objects associated with their respective compound literals.
for (i = 0; i < 10; i++) {
f((struct foo){.a = i, .b = 42});
}
for (i = 0; i < 10; i++)
15 f((struct foo){.a = i, .b = 42});
2.2 Design Discussion
There has been some discussion that perhaps compound literals could
be made to have C++ style expression scope, a concept that does not
currently exist in C. However, the principle of least surprise dictates
20 that the following should work.
void f(void)
{
int *p = (int []){1, 1, 2, 3, 5, 8, 13};
for (; *p < 10; p++) {
25 /*...*/
}
/*...*/
Therefore, the scope of a compound literal inside a function body should
encompass the enclosing block.
30
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