Question

我写了一个基于类的矢量：

class A {

private:

    string b;

    string c;

public:

    A(string n, string l) { b = l ;c = n; }

    struct Finder {

         Finder(std::string const& n) : name(n) { }  

         bool operator () ( const A & el) const  { return el.b == name; }

    private:

         std::string name;

    };

};


int main()

{

    vector<A> a1;

    a1.push_back(A("AA","aa"));

    a1.push_back(A("BB","bb"));

    a1.push_back(A("CC","cc"));

    a1.push_back(A("DD","dd"));


    vector<string>::iterator it;

    it = find_if(a1.begin(), a1.end(), A::Finder("CC"));

    if (it != a1.end()) {

        auto pos = it - a1.begin();

        cout << "CC is found at " << pos ;

    }
}

现在，我想在a1中搜索一个值。假设我想找到“CC”发生的元素索引。

我发现了这些类似的解决方案：

Search a vector of objects by object attribute

std::find Object by Member

How can I find an object in a vector based on class properties?

How to find an object with specific field values in a std::set?

当我实现本节中的所有注释时，我仍然会收到错误！我错过了什么？我想问题是定义vector :: iterator it;

错误C2679：binary'='：找不到运算符，它采用'std :: _ Vector_iterator＆lt; _Myvec＆gt;'类型的右手操作数（或者没有可接受的转换）

和

错误C2678：binary'！='：找不到运算符，该运算符采用类型为'std :: _ Vector_iterator＆lt; _Myvec＆gt;'的左手操作数（或者没有可接受的转换）

Answer 1

要使用Predicate，您需要使用std::find_if，而不是std::find。
```
it = std::find_if(a1.begin(), a1.end(), A::Finder("CC"));
```
在参数类型const&中使用Finder::operator()。

而不是
```
bool operator () (A & el) { return el.b == name; }
```
使用
```
bool operator () (A const& el) { return el.b == name; }
```
UnaryPredicate的要求之一是（来自http://en.cppreference.com/w/cpp/concept/Predicate）

函数对象pred不应通过解除引用的迭代器应用任何非常量函数。

许多编译器认为参数类型必须是值或const&。

Answer 2

您必须使用标准算法std::find_if

it = find_if(a1.begin(), a1.end(), A::Finder("CC"));

考虑到内部clas应该被定义为

struct Finder {

    Finder(std::string const& n) : name(n) { }  

    bool operator () ( const A & el) const  { return el.b == name; }

private:

    std::string name;

};

Answer 3

我找到了两个解决方案：

您可以在c ++中实现基于矢量类的对象中的std :: find：

class A {
    private:
       string b;
       string c;
    public:
    A(string i) : b(i) {}
    A(string n, string l) { b = n ;c = l; }
    string Getb(){ return b; }
    string Getc(){ return c; }
    bool operator==(const A & obj2) const
    {
       return (this->b.compare(obj2.b) == 0); 
    }
};


int main()
{
   vector<A> a1;
   a1.push_back(A("AA","aa"));
   a1.push_back(A("BB","bb"));
   a1.push_back(A("CC","cc"));
   a1.push_back(A("DD","dd"));

   auto it = find(a1.begin(), a1.end(), A("CC"));
   if (it != a1.end()) {
      auto idx = distance(a1.begin(), it);
      cout << "b= " << it->Getb() << " c= " << it->Getc() << endl;
      cout << "Index= " << idx << endl;
    } else
      cout << "CC is not found" << endl;
return 0;
}

你可以在c ++中的vector class / structure对象中实现std :: find_if（感谢来自莫斯科的@Vlad和@R Sahu）：

class A {
    private:
        string b;
        string c;
    public:
        A(string n, string l) { b = n ;c = l; }
        string Getb(){ return b; }
        string Getc(){ return c; }
        struct Finder {
            Finder(string const & n) : name(n) { }  
            bool operator () (const A & el) const { 
                 return el.Pos == name; 
             }
            string name;
        };
 };


int main()
 {
     vector<A> a1;
     a1.push_back(A("AA","aa"));
     a1.push_back(A("BB","bb"));
     a1.push_back(A("CC","cc"));
     a1.push_back(A("DD","dd"));

     vector<A>::iterator it;
     it = find_if(a1.begin(), a1.end(), A::Finder ("CC"));
     if (it != a1.end()) {
         auto idx = distance(a1.begin(), it);
         cout << "b= " << it->Getb() << " c= " << it->Getc() << endl;
         cout << "Index= " << idx << endl;
     } else
         cout << "CC is not found" << endl;

  return 0;
 }

在类对象的向量中查找值

3 个答案: