Subclause 220.127.116.11 of the C Standard [ISO/IEC 9899:2011] defines a compound literal as
A postfix expression that consists of a parenthesized type name followed by a brace-enclosed list of initializers. . . . The value of the compound literal is that of an unnamed object initiated by the initializer list.
The storage for this object is either static (if the compound literal occurs at file scope) or automatic (if the compound literal occurs at block scope), and the storage duration is associated with its immediate enclosing block. For example, in the function
following initialization, the
ip contains the address of an unnamed object of type
int, allocated on the stack. Once
func returns, any attempts to access this object will produce undefined behavior.
Note that only one object is created per compound literal—even if the compound literal appears in a loop and has dynamic initializers.
This recommendation is a specific instance of DCL30-C. Declare objects with appropriate storage durations.
Noncompliant Code Example
In this noncompliant code example, the programmer mistakenly assumes that the elements of the
ints array of the pointer to
int_struct are assigned the addresses of distinct
int_struct objects, one for each integer in the range
[0, MAX_INTS - 1]:
However, only one
int_struct object is created. At each iteration of the first loop, the
x member of this object is set equal to the current value of the loop counter
i. Therefore, just before the first loop terminates, the value of the
x member is
MAX_INTS - 1.
Because the storage duration of the compound literal is associated with the
for loop that contains it, dereferencing
ints in the second loop results in undefined behavior 9 (Annex J of the C Standard).
Even if the region of memory that contained the compound literal is not written to between loops, the print loop will display the value
MAX_INTS - 1 for
MAX_INTS lines. This is contrary to the intuitive expected result, which is that the integers
MAX_INTS - 1 would be printed in order.
This compliant solution uses an array of structures rather than an array of pointers. That way, an actual copy of each
int_struct (rather than a pointer to the object) is stored.
|[ISO/IEC 9899:2011]||Subclause 18.104.22.168, "Compound Literals"|