在 class 个对象的向量中找到一个值

Question

我写了一个基于class的向量：

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" 发生的元素的索引。

我找到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?

当我执行本节中的所有注释时，我仍然遇到错误！我错过了什么？我想问题在于定义 vector::iterator 它；

error C2679: 二进制“=”: 没有找到接受 'std::_Vector_iterator<_Myvec>' 类型右手操作数的运算符（或者没有可接受的转换）

并且

error C2678: 二进制“!=”: 未找到采用 'std::_Vector_iterator<_Myvec>' 类型左操作数的运算符（或没有可接受的转换）

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Finder::operator()的参数类型中使用const&。

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）

The function object pred shall not apply any non-constant function through the dereferenced iterator.

许多编译器认为参数类型必须是值或 const&.

Answer 2

你必须使用标准算法std::find_if

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

考虑到内部类应该这样定义

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

针对我的问题，我找到了两个解决方案：

您可以在 c++ 中基于向量 class 的对象中实现 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;
}

您可以在 c++ 中基于向量 class/structure 的对象中实现 std::find_if（感谢莫斯科的 @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 class 个对象的向量中找到一个值

find a value in a vector of class objects

c++

class

vector

object

find