c++ - 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，但这里是错误列表

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

这部分是由于您使用using namespace std;.

标准库中有一个函数，std::distance用于计算两个迭代器到容器中的距离。在重载决议期间选择了该函数而不是您的distance函数。

如果你不使用using namespace std;，你不会有这个问题。您的使用using namespace std;将命名空间中的所有std名称带入全局命名空间。这很糟糕，因为命名空间中有很多常用名称，std并且在名称查找期间可能会导致复杂的问题，正如您在此处发现的那样。

一般来说，不喜欢使用using namespace，尤其是在文件范围内。从长远来看，对您要使用的每个名称进行限定会更加容易和清晰。

Answer 2

@James 是正确的，摆脱using namespace std将使这个问题消失。但这并不是真正的问题所在。

问题来自 C++ 中解析名称的方式。如果没有找到合适的类成员，C++ 在解析自由函数的名称时使用非常激进的方法来解析名称查找。这称为依赖于参数的名称查找（或 ADL），有时也称为 Koenig 查找。同时，它也是让 C++ 如此强大但又如此致命和令人困惑的原因之一。

这是简化的基本问题。

您调用vector::operator[]which 返回 avector::reference或 a vector::const_reference。您获取此返回值并调用distance()不合格。

因为distance在适用的类中找不到，所以 ADL 开始发挥作用，这会导致太多潜在的匹配名称。

然后，ADL 遍历候选人列表并选择“最佳匹配”。在这种情况下，reference返回operator[]的 与期望的类型std::distance相比，返回的 与 your的类型更接近distance，尽管两者都是兼容的，因此std::distance被选中。

解决这个问题的几种方法：

1. 不要using namespace std。相反，using只是你想要的。
2. 使用范围解析运算符明确量化distance您所指的内容：::distance()
3. 更改distance为采用指针而不是const &. 这有效地关闭了 ADL。：

例子：

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

其他资源：

“一个适度的建议：修复 ADL（修订版 2）”，作者：Herb Sutter