[英]Providing less than operator for one element of a pair
What would be the most elegant way too fix the following code: 什么是最优雅的方式也修复以下代码:
#include <vector>
#include <map>
#include <set>
using namespace std;
typedef map< int, int > row_t;
typedef vector< row_t > board_t;
typedef row_t::iterator area_t;
bool operator< ( area_t const& a, area_t const& b ) {
return( a->first < b->first );
};
int main( int argc, char* argv[] )
{
int row_num;
area_t it;
set< pair< int, area_t > > queue;
queue.insert( make_pair( row_num, it ) ); // does not compile
};
One way to fix it is moving the definition of less< to namespace std (I know, you are not supposed to do it. ) 修复它的一种方法是将less <to namespace命名为std(我知道, 你不应该这样做。 )
namespace std {
bool operator< ( area_t const& a, area_t const& b ) {
return( a->first < b->first );
};
};
Another obvious solution is defining less than< for pair< int, area_t > but I'd like to avoid that and be able to define the operator only for the one element of the pair where it is not defined. 另一个明显的解决方案是定义小于<for pair <int,area_t>,但我想避免这种情况,并且只能为未定义的对中的一个元素定义运算符。
When you are implementing a comparator that implements some specific and/or fairly exotic comparison approach, it is better to use a named function or a function object instead of hijacking the operator <
for that purpose. 当您实现一个实现某种特定和/或相当奇特的比较方法的比较器时,最好使用命名函数或函数对象而不是为此目的劫持
operator <
。 I'd say that the natural way to compare a std::pair
object would be to use lexicographical comparison. 我要说比较
std::pair
对象的自然方法是使用词典比较。 Since your comparison is not lexicographical, taking over operator <
might not be a good idea. 由于您的比较不是词典,因此接管
operator <
可能不是一个好主意。 Better implement a comparator class 更好地实现比较器类
typedef pair< int, area_t > Pair; // give it a more meaningful name
struct CompareFirstThroughSecond {
bool operator ()(const Pair& p1, const Pair& p2) const {
if (p1.first != p2.first) return p1.first < p2.first;
return p1.second->first < p2.second->first;
}
};
and use it with your container 并将其与容器一起使用
std::set< Pair, CompareFirstThroughSecond > queue;
(I hope I deciphered your intent from your original code correctly). (我希望我能正确地从原始代码中解读你的意图)。
You can also implement the above operator ()
method as a template method, thus making it usable with all std::pair
-based types with an iterator as a second
member. 您还可以将上面的
operator ()
方法实现为模板方法,从而使其可以与所有基于std::pair
的类型一起使用,并将迭代器作为second
成员。 It might not make mich sense though, since your comparison is "exotic" enough. 虽然你的比较充分“充满异国情调”,但它可能并不会让人感觉到这种感觉。
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