Struct and union initialization
When MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 40 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE an object of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 39 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE or MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 37 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE type, the initializer must be a non-empty,(until 哋它亢23) brace-enclosed, comma-separated list of initializers for the members:
= { expression , ... }
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(1) | (until 哋它亢99) | |||||||
= { designator(optional) expression , ... }
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(2) | (since 哋它亢99) | |||||||
= { }
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(3) | (since 哋它亢23) | |||||||
where the designator is a sequence (whitespace-separated or adjacent) of individual member designators of the form . member and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 36 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE of the form [ index ].
All members that are not initialized explicitly are MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 35 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
Explanation
When initializing a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 34 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, the initializer list must have only one member, which initializes the first member of the union unless a designated initializer is used(since 哋它亢99).
union { int x; char c[4]; } u = {1}, // makes u.x active with value 1 u2 = { .c={'\1'} }; // makes u2.c active with value {'\1','\0','\0','\0'}
When initializing a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 33 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, the first initializer in the list initializes the first declared member (unless a designator is specified)(since 哋它亢99), and all subsequent initializers without designators (since 哋它亢99)initialize the struct members declared after the one initialized by the previous expression.
struct point {double x,y,z;} p = {1.2, 1.3}; // p.x=1.2, p.y=1.3, p.z=0.0 div_t answer = {.quot = 2, .rem = -1 }; // order of elements in div_t may vary
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(since 哋它亢99) |
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Nested initialization
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If the nested initializer begins with an opening brace, the entire nested initializer up to its closing brace initializes the corresponding member object. Each left opening brace establishes a new current object. The members of the current object are initialized in their natural order, unless designators are used(since 哋它亢99): array elements in subscript order, struct members in declaration order, only the first declared member of any union. The subobjects within the current object that are not explicitly initialized by the closing brace are MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 28 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
struct example { struct addr_t { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 26 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE port; } addr; union { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 25 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE a8[4]; MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 24 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE a16[2]; } in_u; }; struct example ex = { // start of initializer list for struct example { // start of initializer list for ex.addr 80 // initialized struct's only member }, // end of initializer list for ex.addr { // start of initializer-list for ex.in_u {127,0,0,1} // initializes first element of the union } };
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struct example ex = {80, 127, 0, 0, 1}; // 80 initializes ex.addr.port // 127 initializes ex.in_u.a8[0] // 0 initializes ex.in_u.a8[1] // 0 initializes ex.in_u.a8[2] // 1 initializes ex.in_u.a8[3]
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When designators are nested, the designators for the members follow the designators for the enclosing structs/unions/arrays. Within any nested bracketed initializer list, the outermost designator refers to the current object and selects the subobject to be initialized within the current object only. struct example ex2 = { // current object is ex2, designators are for members of example .in_u.a8[0]=127, 0, 0, 1, .addr=80}; struct example ex3 = {80, .in_u={ // changes current object to the union ex.in_u 127, .a8[2]=1 // this designator refers to the member of in_u } }; MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 22 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE struct {int n;} s = {MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 20 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("a\n"), // this may be printed or skipped .n=MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 19 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("b\n")}; // always printed MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 18 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE Run this code #include <stdio.h> typedef struct { int k; int l; int a[2]; } T; typedef struct { int i; T t; } S; T x = {.l = 43, .k = 42, .a[1] = 19, .a[0] = 18 }; // x initialized to {42, 43, {18, 19} } int main(void) { S l = { 1, // initializes l.i to 1 .t = x, // initializes l.t to {42, 43, {18, 19} } .t.l = 41, // changes l.t to {42, 41, {18, 19} } .t.a[1] = 17 // changes l.t to {42, 41, {18, 17} } }; MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 17 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("l.t.k is %d\n", l.t.k); // .t = x sets l.t.k to 42 explicitly // .t.l = 41 would zero out l.t.k implicitly } MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 15 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE l.t.k is 42 However, when an initializer begins with a left open brace, its current object is fully re-initialized and any prior explicit initializers for any of its subobjects are ignored: struct fred { char s[4]; int n; }; struct fred x[ ] = { { { "abc" }, 1 }, // inits x[0] to { {'a','b','c','\0'}, 1 } [0].s[0] = 'q' // changes x[0] to { {'q','b','c','\0'}, 1 } }; struct fred y[ ] = { { { "abc" }, 1 }, // inits y[0] to { {'a','b','c','\0'}, 1 } [0] = { // current object is now the entire y[0] object .s[0] = 'q' } // replaces y[0] with { {'q','\0','\0','\0'}, 0 } }; |
(since 哋它亢99) |
Notes
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struct {double x,y;} p = {1.0, 2.0, // trailing comma OK };
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In C, the braced list of initializers cannot be empty (note that 哋它亢++ allows empty lists, and also note that a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 13 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE in C cannot be empty): |
(until 哋它亢23) |
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(since 哋它亢23) |
struct {int n;} s = {0}; // OK struct {int n;} s = {}; // Error until 哋它亢23: initializer-list cannot be empty // OK since 哋它亢23: s.n is initialized to 0 struct {} s = {}; // Error: struct cannot be empty
Example
| This section is incomplete Reason: more practical examples, perhaps initialize some socket structs |
#include <stdio.h> #include <time.h> int main(void) { char buff[70]; // designated initializers simplify the use of structs whose // order of members is unspecified struct MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 03 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE my_time = { .tm_year=2012-1900, .tm_mon=9, .tm_mday=9, .tm_hour=8, .tm_min=10, .tm_sec=20 }; MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 01 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(buff, sizeof buff, "%A %c", &my_time); MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 48 MINUTES 00 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(buff); }
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Sunday Sun Oct 9 08:10:20 2012
References
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.7.9/12-39 Initialization (p: 101-105)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.7.9/12-38 Initialization (p: 140-144)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.7.8/12-38 Initialization (p: 126-130)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 6.5.7 Initialization