Difference between revisions of "cpp/utility/variadic/va start"
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(Added LWG issue #2099 DR.) |
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| − | {{ctitle | + | {{ctitle|va_start}} |
{{cpp/utility/variadic/navbar}} | {{cpp/utility/variadic/navbar}} | ||
| − | {{ | + | {{ddcl|header=cstdarg| |
| − | + | void va_start( std::va_list ap, parm_n ); | |
| − | + | ||
| − | void va_start( va_list ap, parm_n ); | + | |
}} | }} | ||
| − | |||
| − | The {{tt|va_start}} macro enables access to the variable arguments following the named argument {{ | + | The {{tt|va_start}} macro enables access to the variable arguments following the named argument {{c|parm_n}}. |
| − | {{tt|va_start}} should be invoked with an instance to a valid {{lc|va_list}} object {{ | + | {{tt|va_start}} should be invoked with an instance to a valid {{lc|va_list}} object {{c|ap}} before any calls to {{lc|va_arg}}. |
| − | If {{ | + | {{rrev|since=c++11| |
| + | If the {{c|parm_n}} is a {{lsd|cpp/language/parameter pack#Pack expansion}} or an entity resulting from a {{lsd|cpp/language/lambda#Lambda capture}}, the program is ill-formed, no diagnostic required. | ||
| + | }} | ||
| + | |||
| + | If {{c|parm_n}} is of reference type, or of a type not compatible with the type that results from [[cpp/language/variadic arguments#Default conversions|default argument promotions]], the behavior is undefined. | ||
===Parameters=== | ===Parameters=== | ||
{{par begin}} | {{par begin}} | ||
| − | {{par | ap | an | + | {{par|ap|an object of the {{lc|va_list}} type}} |
| − | {{par | parm_n | the named parameter preceding the first variable parameter}} | + | {{par|parm_n|the named parameter preceding the first variable parameter}} |
{{par end}} | {{par end}} | ||
===Expanded value=== | ===Expanded value=== | ||
(none) | (none) | ||
| + | |||
| + | ===Notes=== | ||
| + | {{tt|va_start}} is required to support {{c|parm_n}} with overloaded {{tt|operator&}}. | ||
===Example=== | ===Example=== | ||
{{example | {{example | ||
| − | + | |code= | |
| − | + | ||
| − | + | ||
#include <cstdarg> | #include <cstdarg> | ||
| + | #include <iostream> | ||
| − | int add_nums(int count | + | int add_nums(int count...) |
{ | { | ||
int result = 0; | int result = 0; | ||
| − | va_list args; | + | std::va_list args; |
va_start(args, count); | va_start(args, count); | ||
| − | for (int i = 0; i < count; ++i) | + | for (int i = 0; i < count; ++i) |
result += va_arg(args, int); | result += va_arg(args, int); | ||
| − | |||
va_end(args); | va_end(args); | ||
return result; | return result; | ||
} | } | ||
| − | int main() | + | int main() |
{ | { | ||
std::cout << add_nums(4, 25, 25, 50, 50) << '\n'; | std::cout << add_nums(4, 25, 25, 50, 50) << '\n'; | ||
} | } | ||
| − | + | |output= | |
150 | 150 | ||
}} | }} | ||
| − | === | + | ===Defect reports=== |
| + | {{dr list begin}} | ||
| + | {{dr list item|wg=cwg|dr=273|std=C++98|before=it was unclear whether {{tt|va_start}} is required to<br>support {{c|parm_n}}s with overloaded {{tt|operator&}}|after=required}} | ||
| + | {{dr list item|wg=lwg|dr=2099|std=C++98|before=the behavior was undefined if {{c|parm_n}} is<br>declared with a function, array, or reference type|after=the behavior is undefined if<br>{{c|parm_n}} is of reference type}} | ||
| + | {{dr list end}} | ||
| + | ===See also=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/utility/variadic/dsc va_arg}} | + | {{dsc inc|cpp/utility/variadic/dsc va_arg}} |
| − | {{dsc inc | cpp/utility/variadic/dsc va_end}} | + | {{dsc inc|cpp/utility/variadic/dsc va_end}} |
| + | {{dsc see c|c/variadic/va_start}} | ||
{{dsc end}} | {{dsc end}} | ||
| − | + | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} | |
| − | + | ||
| − | + | ||
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Latest revision as of 22:10, 25 February 2024
| Defined in header <cstdarg>
|
||
| void va_start( std::va_list ap, parm_n ); |
||
The va_start macro enables access to the variable arguments following the named argument parm_n.
va_start should be invoked with an instance to a valid va_list object ap before any calls to va_arg.
|
If the parm_n is a pack expansion or an entity resulting from a lambda capture, the program is ill-formed, no diagnostic required. |
(since C++11) |
If parm_n is of reference type, or of a type not compatible with the type that results from default argument promotions, the behavior is undefined.
Contents |
[edit] Parameters
| ap | - | an object of the va_list type |
| parm_n | - | the named parameter preceding the first variable parameter |
[edit] Expanded value
(none)
[edit] Notes
va_start is required to support parm_n with overloaded operator&.
[edit] Example
Run this code
Output:
150
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| CWG 273 | C++98 | it was unclear whether va_start is required tosupport parm_ns with overloaded operator&
|
required |
| LWG 2099 | C++98 | the behavior was undefined if parm_n is declared with a function, array, or reference type |
the behavior is undefined if parm_n is of reference type |
[edit] See also
| accesses the next variadic function argument (function macro) | |
| ends traversal of the variadic function arguments (function macro) | |
| C documentation for va_start
| |
