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

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< cpp‎ | utility‎ | variadic
m (fmt)
(Added LWG issue #2099 DR.)
 
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{{rrev|since=c++11|
 
{{rrev|since=c++11|
−
If the {{c|parm_n}} is a [[cpp/language/parameter pack#Pack expansion|pack expansion]] or an entity resulting from a [[cpp/language/lambda#Lambda capture|lambda capture]], the program is ill-formed, no diagnostic required.
+
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 declared with reference type or with a type not compatible with the type that results from [[cpp/language/variadic_arguments#Default_conversions|default argument promotions]], the behavior is undefined.
+
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 instance of the {{lc|va_list}} type}}
+
{{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}}
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===Defect reports===
 
===Defect reports===
 
{{dr list begin}}
 
{{dr list begin}}
−
{{dr list item|wg=cwg|dr=273|std=C++98|before={{tt|va_start}} may not work if unary {{tt|operator&}} is overloaded|after=required to work correctly even if {{tt|operator&}} is overloaded}}
+
{{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}}
 
{{dr list end}}
  

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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