find a value in a vector of class objects

Question

I have written a class-based vector:

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 ;

    }
}

Now, I want to search for a value in a1. Let's say I want to find the index of element in which "CC" happens.

I found these similar solutions:

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?

As I implement all the comments in this section, I still get the errors! What did I miss? I guess the problem is in defining vector::iterator it;

error C2679: binary '=' : no operator found which takes a right-hand operand of type 'std::_Vector_iterator<_Myvec>' (or there is no acceptable conversion)

and,

error C2678: binary '!=' : no operator found which takes a left-hand operand of type 'std::_Vector_iterator<_Myvec>' (or there is no acceptable conversion)

When posting error messages you should really indicate the line the compiler points to, otherwise we have to guess. — cdhowie, Jul 13 '15 at 20:03

R Sahu · Answer 1 · 2015-07-13T20:20:50.827

1

To use a Predicate, you'll need to use std::find_if, not std::find.
```
it = std::find_if(a1.begin(), a1.end(), A::Finder("CC"));
```
Use const& in the argument type of Finder::operator().

Instead of
```
bool operator () (A & el) { return el.b == name; }
```
Use
```
bool operator () (A const& el) { return el.b == name; }
```
One of the requirements of UnaryPredicate is (from http://en.cppreference.com/w/cpp/concept/Predicate)

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

Many compilers take it to mean that the argument type must be either a value or a const&.

edited Jul 13 '15 at 20:20

answered Jul 13 '15 at 19:59

R Sahu

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Voted up. By not passing a reference, a copy is created each time. – Satish Chalasani Jul 13 '15 at 20:01

score 1 · Answer 2 · answered Jul 13 '15 at 20:03

1

You have to use standard algorithm std::find_if

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

Take into account that the inner clas should be defined like

struct Finder {

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

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

private:

    std::string name;

};

answered Jul 13 '15 at 20:03

Vlad from Moscow

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@Vlad from Moscow, @R Sahu : I modify my code (as above), however I still get the same errors! Do you have any idea what is wrong? – Afrand Jul 14 '15 at 03:56
@Vlad from Moscow, @R Sahu : I finally found the solution to my problem. Thanks for your great help. – Afrand Jul 14 '15 at 05:29
@Afrand It seems you have a wrong order of data member initialization Instead of A( std::string n, std::string l ) { b = l ;c = n; } there should be A( std::string n, std::string l ) { b = n ;c = l; } that is b should be initialized by n. And declare the parameters like const A & – Vlad from Moscow Jul 14 '15 at 07:24

score 1 · Accepted Answer · answered Jul 14 '15 at 11:37

I found two solution as for my problem:

You can implement std::find in vector class-based object in c++:

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;
}

You can implement std::find_if in vector class/structure based object in c++ (thanks to @Vlad from Moscow and @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;
 }

find a value in a vector of class objects

3 Answers3