c++设置容器的问题
当我尝试编译以下代码时:
When I try to compile the following code:
#include <iostream>
#include <set>
#include <vector>
using namespace std;
template <class T, class S>
class Property
{
public:
pair<T,S> p;
Property(T t, S s) { p = make_pair(t,s);}
};
int main()
{
set< Property<string, string> > properties;
Property<string, string> name("name", "Andy");
properties.insert(name);
}
我收到编译错误.但是,当我用向量替换 set 并因此使用 push_back 函数而不是插入函数时,一切正常.谁能解释一下我做错了什么?谢谢指教.
I get the compilation error. However, when I replace set by vector and hence use the the push_back function instead of insert function everything works fine. Could anyone explain me what am I doing wrong? Thanks in advice.
推荐答案
std::set
将其值存储在已排序的二叉树中,因此它需要知道如何比较它所保存的值.默认情况下,它使用 std::less
作为比较函数,对于非专业化的用户定义类型,它尝试调用 operator<
.因此,告诉集合如何比较您的对象的最简单方法是为您的类定义一个 operator<
:
std::set
stores its values in a sorted binary tree, so it needs to know how to compare the values it holds. By default it uses std::less
as a comparison function, which for un-specialized user defined types tries to call operator<
. So, the easiest way to tell the set how to compare your objects is to define an operator<
for your class:
template <class T, class S>
class Property
{
public:
pair<T,S> p;
Property(T t, S s) { p = make_pair(t,s);}
bool operator<(const Property<T,S>& rhs) const
{
return p < rhs.p;
}
};
然而,还有其他方法可以告诉 std::set
如何比较您的类型.一种是为您的类专门化 std::less
模板:
However, there are also other ways of telling std::set
how to compare your type. One is to specialize the std::less
template for your class:
namespace std {
template<typename T,typename S>
struct less<Property<T, S> >
{
bool operator()(const Property<T, S>& lhs, const Property<T,S>& rhs) const
{
return lhs.p < rhs.p;
}
};
}
另一种方法是用具有正确签名的函数或具有使用正确签名定义的 operator()
的类替换默认比较类型.这就是事情开始变得丑陋的地方.
Another is to replace the default comparison type with a function with the correct signature, or a class that has an operator()
defined with the correct signature. This is where things start to get ugly.
// Comparison function
template<typename T, typename S>
bool property_less_function(const Property<T,S>& lhs, const Property<T,S>& rhs)
{
return lhs.p < rhs.p;
}
// Comparison functor
template<typename T, typename S>
struct PropertyLess
{
bool operator()(const Property<T,S>& lhs, const Property<T,S>& rhs) const
{
return lhs.p < rhs.p;
}
};
int main()
{
// Set using comparison function.
// Have to pass the function pointer in the constructor so it knows
// which function to call. The syntax could be cleaned up with some
// typedefs.
std::set<Property<std::string, std::string>,
bool(*)(const Property<std::string, std::string>&,
const Property<std::string, std::string>&)>
set1(&property_less_function<std::string, std::string>);
// Set using comparison functor. Don't have to pass a value for the functor
// because it will be default constructed.
std::set<Property<std::string, std::string>, PropertyLess<std::string, std::string> > set2;
}
请记住,无论您使用什么小于函数,该函数都必须定义一个严格弱排序 适合您的类型.
Keep in mind that whatever less-than function you use, that function must define a strict weak ordering for your type.
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