C++ 中的距离计算错误 [已关闭]

distance calculation error in c++ [closed]

提问人： 提问时间：11/19/2010 最后编辑：sbi 更新时间：11/19/2010 访问量：5546

问：

10年前关闭。
#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 ==========
C++ 编译器错误 using-directives 重载解析 名称查找

评论

6赞 Paul R 11/19/2010
迄今为止有 271 个问题，但仍然无法（或不愿意）格式化代码以提高可读性？
0赞 sbi 11/19/2010
通过 VS 的自动格式化程序运行代码。

答：

26赞 James McNellis 11/19/2010 #1

这在一定程度上是由于您使用了 。using namespace std;

标准库中有一个函数 ，用于计算两个迭代器进入容器之间的距离。在过载解决期间选择该函数，而不是您的函数。std::distancedistance

如果不使用 ，则不会有此问题。使用 会将命名空间中的所有名称带入全局命名空间。这很糟糕，因为命名空间中有很多通用名称，并且可能会在名称查找过程中导致复杂的问题，正如您在此处发现的那样。using namespace std;using namespace std;stdstd

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

评论

0赞 winwaed 11/19/2010
然后，Op 将需要使用 std：：vector 进行向量定义。（为了完整性而添加，以尝试抢占下一个编译器错误！
10赞 Moo-Juice 11/19/2010
+1，这是一个很好的示例帖子，它显示了“使用命名空间 xyz;”来节省键入的固有问题。
3赞 Fred Larson 11/19/2010
@winwaed：另一种选择是using std::vector;
1赞 Rob Kennedy 11/19/2010
为什么重载分辨率会选择模板而不是此处提供的模板？std::distancedistance
2赞 sbi 11/19/2010
好吧，现在我知道为什么詹姆斯问我关于我以前的咆哮了。在这里。请注意，tsubasa 在 std：:d istance（） 方面也遇到了麻烦。巧合？using namespace std;
5赞 John Dibling 11/19/2010 #2

@James是对的，因为摆脱将使这个问题消失。但这并不是真正的问题所在。using namespace std

问题来自在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。这称为与参数相关的名称查找（或 ADL），有时称为 Koenig 查找。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。

这是基本问题，简化。

调用 which 返回 a 或 .获取此返回值并调用 unqualified。vector::operator[]vector::referencevector::const_referencedistance()

因为在适用的类中找不到，所以 ADL 会启动，它会消失并获得太多潜在的匹配名称。distance

然后，ADL 会浏览候选人列表并选择“最佳匹配”。在本例中，返回的 that 与预期的类型比 ur 更接近，尽管两者都兼容，因此被选中。referenceoperator[]std::distancedistancestd::distance

解决此问题的几种方法：

  1. 不要。取而代之的是，只有你想要的碎片。using namespace stdusing
  2. 使用范围解析运算符显式量化您引用的内容：distance::distance()
  3. 更改为 take 指针而不是 。这有效地关闭了 ADL。distanceconst &

例：

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

评论

1赞 UncleBens 11/19/2010
仍然不明白模板如何比非模板更匹配。顺便说一句，向量无关紧要，如果是普通数组，误差也是一样的？a

上一个：detail 命名空间中的 using 指令有问题吗？

下一个：为什么“使用系统”不被认为是不好的做法？