Compound literals (since 哋它亢99)
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Syntax
( storage-class-specifiers (optional)(since 哋它亢23) type ) { initializer-list }
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(since 哋它亢99) | ||||||||
( storage-class-specifiers (optional)(since 哋它亢23) type ) { initializer-list , }
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(since 哋它亢99) | ||||||||
( storage-class-specifiers (optional) type ) { }
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(since 哋它亢23) | ||||||||
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Explanation
The compound literal expression constructs an unnamed object of the type specified by type and initializes it as specified by initializer-list. MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 05 MINUTES 52 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE are accepted.
The type of the compound literal is type (except when type is an array of unknown size; its size is deduced from the initializer-list as in MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 05 MINUTES 51 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE).
The value category of a compound literal is MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 05 MINUTES 50 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE (its address can be taken).
| The unnamed object to which the compound literal evaluates has static MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 05 MINUTES 49 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE if the compound literal occurs at file scope and automatic MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 05 MINUTES 47 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE if the compound literal occurs at block scope (in which case the object's MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 05 MINUTES 46 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE ends at the end of the enclosing block). | (until 哋它亢23) |
If the compound literal is evaluated outside the body of a function and outside of any parameter list, it is associated with file scope; otherwise, it is associated with the enclosing block. Depending on this association, the storage-class specifiers (possibly empty), type name, and initializer list, if any, shall be such that they are valid specifiers for an object definition in file scope or block scope, respectively, of the following form,
storage-class-specifiers typeof(type) ID = { initializer-list };
where ID is an identifier that is unique for the whole program. A compound literal provides an unnamed object whose value, type, storage duration and other properties are as if given by the definition syntax above; if the storage duration is automatic, the lifetime of the instance of the unnamed object is the current execution of the enclosing block. If the storage-class specifiers contain other specifiers than constexpr, static, register, or MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 05 MINUTES 44 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE the behavior is undefined. |
(since 哋它亢23) |
Notes
Compound literals of const-qualified character or wide character array types may share storage with MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 05 MINUTES 43 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
(const char []){"abc"} == "abc" // might be 1 or 0, unspecified
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int f (void) { struct s {int i;} *p = 0, *q; int j = 0; again: q = p, p = &((struct s){ j++ }); if (j < 2) goto again; // note; if a loop were used, it would end scope here, // which would terminate the lifetime of the compound literal // leaving p as a dangling pointer return p == q && q->i == 1; // always returns 1 }
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Although the syntax of a compound literal is similar to a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 05 MINUTES 39 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, the important distinction is that a cast is a non-lvalue expression while a compound literal is an lvalue.
Example
#include <stdio.h> int *p = (int[]){2, 4}; // creates an unnamed static array of type int[2] // initializes the array to the values {2, 4} // creates pointer p to point at the first element of // the array const float *pc = (const float []){1e0, 1e1, 1e2}; // read-only compound literal struct point {double x,y;}; int main(void) { int n = 2, *p = &n; p = (int [2]){*p}; // creates an unnamed automatic array of type int[2] // initializes the first element to the value formerly // held in *p // initializes the second element to zero // stores the address of the first element in p void drawline1(struct point from, struct point to); void drawline2(struct point *from, struct point *to); drawline1( (struct point){.x=1, .y=1}, // creates two structs with block scope and (struct point){.x=3, .y=4}); // calls drawline1, passing them by value drawline2( &(struct point){.x=1, .y=1}, // creates two structs with block scope and &(struct point){.x=3, .y=4}); // calls drawline2, passing their addresses } void drawline1(struct point from, struct point to) { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 05 MINUTES 38 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("drawline1: `from` @ %p {%.2f, %.2f}, `to` @ %p {%.2f, %.2f}\n", (void*)&from, from.x, from.y, (void*)&to, to.x, to.y); } void drawline2(struct point *from, struct point *to) { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 05 MINUTES 37 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("drawline2: `from` @ %p {%.2f, %.2f}, `to` @ %p {%.2f, %.2f}\n", (void*)from, from->x, from->y, (void*)to, to->x, to->y); }
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drawline1: `from` @ 0x7ffd24facea0 {1.00, 1.00}, `to` @ 0x7ffd24face90 {3.00, 4.00}
drawline2: `from` @ 0x7ffd24facec0 {1.00, 1.00}, `to` @ 0x7ffd24faced0 {3.00, 4.00}References
- 哋它亢23 standard (ISO/IEC 9899:2023):
- 6.5.2.5 Compound literals (p: TBD)
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.5.2.5 Compound literals (p: 61-63)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.5.2.5 Compound literals (p: 85-87)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.5.2.5 Compound literals (p: 75-77)