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Difference between revisions of "cpp/utility/variadic/va start"

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< cpp‎ | utility‎ | variadic
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(Added LWG issue #2099 DR.)
 
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{{ctitle | n={{trim|va_start}} }}
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{{ctitle|va_start}}
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{{cpp/utility/variadic/sidebar}}
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{{cpp/utility/variadic/navbar}}
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{{ddcl | void va_start(va_list ap, parm_n);}}
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{{ddcl|header=cstdarg|
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void va_start( std::va_list ap, parm_n );
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}}
  
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The {{tt|va_start}} macro enables access to the variable arguments following the named argument {{tt|parm_n}}.
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The {{tt|va_start}} macro enables access to the variable arguments following the named argument {{c|parm_n}}.
  
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{{tt|va_start}} should be invoked with an instance to a valid {{c|va_list}} object {{tt|ap}} before any calls to {{c|va_arg}}.
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{{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}}.
 +
 
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{{rrev|since=c++11|
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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.
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}}
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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===
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{{param list begin}}
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{{par begin}}
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{{param list item | ap | an instance of the {{c|va_list}} type}}
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{{par|ap|an object of the {{lc|va_list}} type}}
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{{param list item | parm_n | the named parameter preceding the first variable parameter}}
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{{par|parm_n|the named parameter preceding the first variable parameter}}
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{{param list end}}
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{{par end}}
  
 
===Expanded value===
 
===Expanded value===
 
(none)
 
(none)
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 +
===Notes===
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{{tt|va_start}} is required to support {{c|parm_n}} with overloaded {{tt|operator&}}.
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 +
===Example===
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{{example
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|code=
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#include <cstdarg>
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#include <iostream>
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int add_nums(int count...)
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{
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    int result = 0;
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    std::va_list args;
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    va_start(args, count);
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    for (int i = 0; i < count; ++i)
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        result += va_arg(args, int);
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    va_end(args);
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    return result;
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}
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 +
int main()
 +
{
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    std::cout << add_nums(4, 25, 25, 50, 50) << '\n';
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}
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|output=
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150
 +
}}
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 +
===Defect reports===
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{{dr list begin}}
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{{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}}
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{{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}}
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{{dr list end}}
  
 
===See also===
 
===See also===
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{{dsc begin}}
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{{dsc inc|cpp/utility/variadic/dsc va_arg}}
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{{dsc inc|cpp/utility/variadic/dsc va_end}}
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{{dsc see c|c/variadic/va_start}}
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{{dsc end}}
  
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{{dcl list begin}}
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{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
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{{dcl list template | cpp/utility/variadic/dcl list va_arg}}
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−
{{dcl list template | cpp/utility/variadic/dcl list va_end}}
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−
{{dcl list end}}
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Latest revision as of 22:10, 25 February 2024

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Variadic functions
 
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
#include <cstdarg>
#include <iostream>
 
int add_nums(int count...)
{
    int result = 0;
    std::va_list args;
    va_start(args, count);
    for (int i = 0; i < count; ++i)
        result += va_arg(args, int);
    va_end(args);
    return result;
}
 
int main()
{
    std::cout << add_nums(4, 25, 25, 50, 50) << '\n';
}

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 to
support 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) [edit]
ends traversal of the variadic function arguments
(function macro) [edit]
C documentation for va_start
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