C++ 上界/下界的比较函数
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compare function for upper_bound / lower_bound
提问by Martin Beckett
I want to find the first item in a sorted vector that has a field less than some value x.
I need to supply a compare function that compares 'x' with the internal value in MyClass but I can't work out the function declaration.
Can't I simply overload '<' but how do I do this when the args are '&MyClass' and 'float' ?
我想在一个字段小于某个值 x 的排序向量中找到第一项。
我需要提供一个比较函数，将“x”与 MyClass 中的内部值进行比较，但我无法计算出函数声明。
我不能简单地重载 '<' 但是当参数是 '&MyClass' 和 'float' 时我该怎么做？
float x;
std::vector< MyClass >::iterator last = std::upper_bound(myClass.begin(),myClass.end(),x);
回答by Mark Ransom
What function did you pass to the sort algorithm? You should be able to use the same one for upper_bound and lower_bound.
你传递给排序算法的函数是什么？您应该能够对 upper_bound 和 lower_bound 使用相同的一个。
The easiest way to make the comparison work is to create a dummy object with the key field set to your search value. Then the comparison will always be between like objects.
进行比较的最简单方法是创建一个虚拟对象，并将键字段设置为您的搜索值。那么比较将始终在相似的对象之间进行。
Edit:If for some reason you can't obtain a dummy object with the proper comparison value, then you can create a comparison functor. The functor can provide three overloads for operator() :
编辑：如果由于某种原因您无法获得具有正确比较值的虚拟对象，那么您可以创建一个比较函子。函子可以为 operator() 提供三个重载：
struct MyClassLessThan
{
bool operator() (const MyClass & left, const MyClass & right)
{
return left.key < right.key;
}
bool operator() (const MyClass & left, float right)
{
return left.key < right;
}
bool operator() (float left, const MyClass & right)
{
return left < right.key;
}
};
As you can see, that's the long way to go about it.
正如你所看到的，这是很长的路要走。
回答by Ghostrider
You can further improve Mark's solution by creating a static instance of MyClassLessThan in MyClass
您可以通过在 MyClass 中创建 MyClassLessThan 的静态实例来进一步改进 Mark 的解决方案
class CMyClass
{
static struct _CompareFloatField
{
bool operator() (const MyClass & left, float right) //...
// ...
} CompareFloatField;
};
This way you can call lower_bound in the following way:
这样你就可以通过以下方式调用lower_bound：
std::lower_bound(coll.begin(), coll.end(), target, CMyClass::CompareFloatField);
This makes it a bit more readable
这使它更具可读性
回答by Borbus
I think what you need is std::bind2nd(std::less<MyClass>(), x). But, of course, the operator< must be defined for MyClass.
我认为你需要的是std::bind2nd(std::less<MyClass>(), x). 但是，当然，必须为 MyClass 定义 operator<。
Edit: oh and I think you will need a constructor for MyClass that accepts only a float so that it can be implicitly converted. However, there might be a better way to do this.
编辑：哦，我认为您需要一个 MyClass 的构造函数，它只接受一个浮点数，以便可以隐式转换。但是，可能有更好的方法来做到这一点。
