STL -- binder

What is a binder ？ Why is it proposed ？ See the following code to understand the source and function of the binder .

#include <iostream>
#include <functional>
#include <algorithm>
#include <vector>
using namespace std;

int main()
{
    int arr[] = {12,34,42,52,64,62,90,102,46,256,78};
    int len = sizeof(arr) / sizeof(arr[0]);
    vector<int> vec(arr,arr+len);
    
    // The first query is greater than 100 The elements of   Greater than ：greater     Less than :less
    
    //vector<int>::iterator it = find_if(vec.begin(),vec.end(),(greater<int>(),100));

    //greater<int> great;
    //great(10,20);           The essence of an object is to call member methods  ==> great.operator(10,20) 
    //                        Pass in two parameters ( This is the binary function object ), Which is bigger  
    // but  find  The function requires the first two arguments to be passed into the iterator interval , The third is the unary function object 
    //greater<int> You need to pass in two function objects   How to solve ？
    // Rewrite -->no   Use binders ( Convert a binary function object to a unary function object )

    vector<int>::iterator it = find_if(vec.begin(),vec.end(),bind2nd(greater<int>(),100));

    if(it != vec.end())
    {
	cout << " The first one is greater than 100 The elements of :" << *it << endl;
    }

    return 0;
}

1. Binder Of Realization principle ：

Function object It is divided into Unary function object operator(lhs)  and Binary function objects operator()(lhs,rhs)

Binder bind2nd(greater<int>(),100)  Quite so 100 Bound to the second parameter rhs, namely operator()(lhs,100), The second parameter is given , It is equivalent to directly calling a unary function object , Just change the binary function object into the unary function object

Binary function objects The parameters of are Left operand   and Right operands

bind1st( Bind left operand ): Compare the size of all elements with the left operand   

bind2nd( Bind right operand ): Compare the size of all elements with the right operand   The above example is to combine all elements with 100 Compare , Find the first one 100 Big  

2. Binder The concrete realization of ：

#include <iostream>
#include <functional>
#include <algorithm>
#include <vector>
using namespace std;

template<typename Comp>// By looking at  bind2nd  The definition of , Find just one  Comp  Function object is OK 
class MyBinder
{
public:
	MyBinder(Comp& f,const typename Comp::second_argument_type& val)
	{
		fn = f;
		value = val;
	}

	bool operator()(const typename Comp::second_argument_type& first)// Pass in the first argument of the binary function object 
	{
		return fn(first,value);
	}

private:
	Comp fn;// Comp  Class generation  fn  object , The second member variable needs to be bound with a parameter 
        typename Comp::second_argument_type value;// The second parameter in the binary function object 
};

template<typename Comp,typename T>// Function object and value type are required 
MyBinder<Comp> MyBinderSecond(Comp &fn,const T& value)// Templates ：MyBinder   Class name ：MyBinder<Comp>  Add parameters to the template to form the class name 
{
	return MyBinder<Comp>(fn,value);
}

int main()
{
	// The first query is greater than 100 The elements of (greater)    Query the first less than 40 The elements of (less)
	int arr[] = {12,34,42,52,64,62,90,102,46,256,78};
	int len = sizeof(arr) / sizeof(arr[0]);
	vector<int> vec(arr,arr+len);
		
	// Use your own binder 
	vector<int>::iterator it1 = find_if(vec.begin(),vec.end(),MyBinderSecond(greater<int>(),100));
	vector<int>::iterator it2 = find_if(vec.begin(),vec.end(),MyBinderSecond(less<int>(),40));
	if(it1 != vec.end())
	{
		cout << " The first one is greater than 100 The elements of :" << *it1 << endl;
	}

	if(it2 != vec.end())
	{
		cout << " The first one is less than 40 The elements of :" << *it2 << endl;
	}

	return 0;
}

As shown in the figure below , The calling relationship of the binder

(1) Calling point ：MyBinderSecond(greater<int>(),100) . take   Show temporary objects greater<int>()  and   value 100  Pass in ,greater The generated object is transferred to the binder written by itself ,greater Pass to fn, value 100 Pass to value

greater  Prototypes of function objects ： No member variables , There is only one member method    
template<typename T>
class greater
{
public:
	bool operator()(first,second)
	{
		return first > second;
	}
};

(2) Take the two arguments passed externally to build a new function object

Initialize the member variables in the new function object through the binary function object fn , Data binding val Put it in the member variable value in

(3) Now generate a MyBinder The object of , It is equivalent to encapsulating binary function objects , Calling a member method , It can be used by passing only one parameter , Inside fn(first,value) also Call a binary function object , It is equivalent to encapsulating the binary function object

3. Binder The role of ： Put the binary function object convert to Unary function object