通过自定义比较与不同类型的值来查找std :: set的元素 [英] Find elements of std::set by custom comparison with value of different type
本文介绍了通过自定义比较与不同类型的值来查找std :: set的元素的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
考虑以下带有自定义比较器的 std :: set
玩具示例:
Consider the following toy example of an std::set
with a custom comparator:
#include <set>
struct A {
A() : a(cnt++) {}
const int a;
static int cnt;
};
int A::cnt = 0;
struct comp {
bool operator()(const A& left, const A& right)
{
return left.a < right.a;
}
};
int main()
{
std::set<A, comp> sa;
for (int i = 0; i < 10; ++i) sa.insert(A());
return 0;
}
请注意, A
我想寻找一个 A
给定值< sa
中的code> A :: a ,无需构造 A
,即我正在搜索类似
I would like to look for an A
with a given value of A::a
in sa
, without constructing a temporary object of type A
, i.e. I am searching for something like
sa.find(4)
带有自定义比较器,该比较器允许直接比较整数与对象 A
。
with a custom comparator that allows for direct comparison of integers with objects of type A
. Is that possible?
推荐答案
使用C ++ 14,您可以利用透明比较器:
With C++14 you can utilize "transparent" comparator:
#include <iostream>
#include <set>
#include <type_traits>
class A
{
public: explicit A() : a{cnt++} {}
private: explicit A(int) = delete;
public: const int a;
private: static int cnt;
};
int A::cnt{};
class Comparator
{
// this member is required to let container be aware that
// comparator is capable of dealing with types other than key
public: using is_transparent = std::true_type;
public: bool operator()(const int & left, const A& right) const
{
return left < right.a;
}
public: bool operator()(const A & left, const int& right) const
{
return left.a < right;
}
public: bool operator()(const A& left, const A& right) const
{
return left.a < right.a;
}
};
int main()
{
std::set<A, Comparator> sa{};
for (int i{}; i < 10; ++i)
{
sa.emplace();
}
std::cout << sa.find(3)->a << std::endl;
return 0;
}
在 :: boost :: intrusive :: set
:
#include <boost/intrusive/set.hpp>
#include <iostream>
namespace bi = ::boost::intrusive;
// hook contains set node data, supports various options, can be a member
class A: public bi::set_base_hook
<
bi::link_mode<bi::link_mode_type::safe_link>
>
{
public: explicit A() : a{cnt++} {}
private: explicit A(int) = delete;
public: const int a;
private: static int cnt;
};
int A::cnt{};
class Comparator
{
public: bool operator()(const int & left, const A& right) const
{
return left < right.a;
}
public: bool operator()(const A & left, const int& right) const
{
return left.a < right;
}
public: bool operator()(const A& left, const A& right) const
{
return left.a < right.a;
}
};
int main()
{
bi::set<A, bi::compare<Comparator>> sa{Comparator{}};
for (int i{0}; i < 10; ++i)
{
sa.insert(*new A{}); // typically user manages object creation
}
// comparators may vary
std::cout << sa.find(3, Comparator{})->a << std::endl;
return 0;
}
这篇关于通过自定义比较与不同类型的值来查找std :: set的元素的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!
查看全文