这里的(void)first2 ++有什么意义? [英] What's the point of (void) first2++ here?
问题描述
在这个页面的en.cppreference中有可能实施词典比较的例子。这是基本的:
template< class InputIt1,class InputIt2>
bool lexicographical_compare(InputIt1 first1,InputIt1 last1,
InputIt2 first2,InputIt2 last2)
{
for(;(first1!= last1)&(first2!= last2 ); first1 ++,(void)first2 ++){
if(* first1< * first2)return true;
if(* first2< * first1)return false;
}
return(first1 == last1)&& (first2!= last2);
}
IIRC行像(void)some_argument;
通常用于抑制关于未使用参数的编译器警告。但是在这个函数中,所有参数(包括first2)都使用 - 所以在for语句中写入(void)first2 ++
是什么意思?如果InputIt1重载 operator,
?
它确保内置的逗号运算符被使用 - 以防万一有用户定义的重载出现意外事件。 void
类型的值永远不能传递给一个函数,所以在这里不能选择这样的重载。
想象一下,如果这个函数存在,会发生什么:
void operator,(InputIt1 const& InputIt2 const&){launch_missiles (); }
编辑:这实际上是在相关的 cppreference讨论页面。
On this page of en.cppreference there are examples of possible implementation of lexicographical comparison. Here is the basic one:
template<class InputIt1, class InputIt2>
bool lexicographical_compare(InputIt1 first1, InputIt1 last1,
InputIt2 first2, InputIt2 last2)
{
for ( ; (first1 != last1) && (first2 != last2); first1++, (void) first2++ ) {
if (*first1 < *first2) return true;
if (*first2 < *first1) return false;
}
return (first1 == last1) && (first2 != last2);
}
IIRC lines like (void)some_argument;
are commonly used to supress compiler warnings about unused parameters. But in this function, all parameters, including first2 are used - so what's the point of writing (void) first2++
in the for statement?
Is it some syntax workaround in case InputIt1 overloads operator,
?
It ensures that the built-in comma operator is used - just in case there is a user-defined overload that does something unexpected. A value of void
type can never be passed to a function, so no such overload could be selected here.
Imagine otherwise what would happen if this function existed:
void operator , (InputIt1 const &, InputIt2 const &) { launch_missiles(); }
EDIT: This is actually mentioned on the related cppreference discussion page.
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