c++中的距离计算错误

Question

#include <iostream>
#include <cmath>
#include <vector>

using namespace std;

int square(int a){
    return a*a;
}
struct  Point{
    int x,y;

};
int distance (const  Point& a,const Point& b){
    int k=(int) sqrt((float)((a.x-b.x)*(a.x-b.x))+((a.y-b.y)*(a.y-b.y)));
    return k;

}
int main(){

    vector<Point>a(10);
    for (int i=0;i<10;i++){
        cin>>a[i].x>>a[i].y;
    }

    int s=0;
    int s1;
    int k=0; 
    for (int i=1;i<10;i++){

        s+=square(distance(a[0],a[i]));
    }
    for (int i=1;i<10;i++){
        s1=0;
        for (int j=0;j<10;j++){
            s1+=square(distance(a[i],a[j]));

            if (s1<s) {  s=s1; k=i;}

        }
    }
    cout<<k<<"Points are:";
    cout<<a[k].x;
    cout<<a[k].y;


    return 0;
}

我有以下代码，但下面是错误列表

1>------ Build started: Project: distance, Configuration: Debug Win32 ------
1>  distance.cpp
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(373): error C2039: 'iterator_category' : is not a member of 'Point'
1>          d:\c++_algorithms\distance\distance\distance.cpp(9) : see declaration of 'Point'
1>          d:\c++_algorithms\distance\distance\distance.cpp(30) : see reference to class template instantiation 'std::iterator_traits<_Iter>' being compiled
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(373): error C2146: syntax error : missing ';' before identifier 'iterator_category'
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(373): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(373): error C2602: 'std::iterator_traits<_Iter>::iterator_category' is not a member of a base class of 'std::iterator_traits<_Iter>'
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>          c:\program files\microsoft visual studio 10.0\vc\include\xutility(373) : see declaration of 'std::iterator_traits<_Iter>::iterator_category'
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(373): error C2868: 'std::iterator_traits<_Iter>::iterator_category' : illegal syntax for using-declaration; expected qualified-name
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(374): error C2039: 'value_type' : is not a member of 'Point'
1>          d:\c++_algorithms\distance\distance\distance.cpp(9) : see declaration of 'Point'
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(374): error C2146: syntax error : missing ';' before identifier 'value_type'
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(374): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(374): error C2602: 'std::iterator_traits<_Iter>::value_type' is not a member of a base class of 'std::iterator_traits<_Iter>'
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>          c:\program files\microsoft visual studio 10.0\vc\include\xutility(374) : see declaration of 'std::iterator_traits<_Iter>::value_type'
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(374): error C2868: 'std::iterator_traits<_Iter>::value_type' : illegal syntax for using-declaration; expected qualified-name
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(375): error C2039: 'difference_type' : is not a member of 'Point'
1>          d:\c++_algorithms\distance\distance\distance.cpp(9) : see declaration of 'Point'
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(375): error C2146: syntax error : missing ';' before identifier 'difference_type'
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(375): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(375): error C2602: 'std::iterator_traits<_Iter>::difference_type' is not a member of a base class of 'std::iterator_traits<_Iter>'
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>          c:\program files\microsoft visual studio 10.0\vc\include\xutility(375) : see declaration of 'std::iterator_traits<_Iter>::difference_type'
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(375): error C2868: 'std::iterator_traits<_Iter>::difference_type' : illegal syntax for using-declaration; expected qualified-name
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(377): error C2039: 'pointer' : is not a member of 'Point'
1>          d:\c++_algorithms\distance\distance\distance.cpp(9) : see declaration of 'Point'
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(377): error C2146: syntax error : missing ';' before identifier 'pointer'
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(377): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(377): error C2602: 'std::iterator_traits<_Iter>::pointer' is not a member of a base class of 'std::iterator_traits<_Iter>'
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>          c:\program files\microsoft visual studio 10.0\vc\include\xutility(377) : see declaration of 'std::iterator_traits<_Iter>::pointer'
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(377): error C2868: 'std::iterator_traits<_Iter>::pointer' : illegal syntax for using-declaration; expected qualified-name
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(378): error C2039: 'reference' : is not a member of 'Point'
1>          d:\c++_algorithms\distance\distance\distance.cpp(9) : see declaration of 'Point'
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(378): error C2146: syntax error : missing ';' before identifier 'reference'
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(378): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(378): error C2602: 'std::iterator_traits<_Iter>::reference' is not a member of a base class of 'std::iterator_traits<_Iter>'
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>          c:\program files\microsoft visual studio 10.0\vc\include\xutility(378) : see declaration of 'std::iterator_traits<_Iter>::reference'
1>          with
1>          [
1>              _Iter=Point
1>          ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xutility(378): error C2868: 'std::iterator_traits<_Iter>::reference' : illegal syntax for using-declaration; expected qualified-name
1>          with
1>          [
1>              _Iter=Point
1>          ]
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

这在一定程度上是由于您使用using namespace std;造成的。

在标准库std::distance中有一个函数，用于计算两个迭代器到容器中的距离。在重载解析过程中选择该函数，而不是选择distance函数。

如果你不使用using namespace std;，你就不会有这个问题。使用using namespace std;会将std名称空间中的所有名称放入全局名称空间中。这很糟糕，因为名称空间std中有许多常见的名称，并且在名称查找过程中可能会导致复杂的问题，正如您在这里所发现的那样。

通常，最好不要使用using namespace，尤其是在文件范围内。从长远来看，对您想要使用的每个名称进行限定要容易得多，也更清晰。

Answer 2

@James是对的，摆脱using namespace std会让这个问题消失。但这并不是真正的问题所在。

问题来自于在C++中解析名称的方式。如果没有找到合适的类成员，当解析自由函数的名称时，C++使用一种非常积极的方法来解析名称查找。这称为Argument-dependent name lookup (或ADL)，有时也称为Koenig lookup。同时，这也是C++如此强大，但也如此致命和令人困惑的原因之一。

这是基本的问题，简化了。

您可以调用返回vector::reference或vector::const_reference的vector::operator[]。您接受此返回值，并调用distance() unqualified。

因为在和可应用的类中找不到distance，所以ADL会起作用，它会得到太多可能匹配的名称。

然后，ADL遍历候选列表并选择“最佳匹配”。在这种情况下，operator[]返回的reference更接近std::distance期望的类型，而不是distance期望的类型，尽管两者都兼容，因此选择了std::distance。

解决此问题的几种方法：

1. ‘t using namespace std.取而代之的是，使用作用域解析操作符::distance()
2. Change using want.
3. Quantify来获取指针，而不是const &。这实际上关闭了ADL。用法：

示例：

int distance(const Point* a,const Point* b)
s+=square(distance(&a[0],&a[1]));

其他资源：

"A Modest Proposal: Fixing ADL (revision 2)" by Herb Sutter