Call a member function using for_each

Question

This was my original code

#include "stdafx.h"
#include <string>
#include <list>
#include <algorithm>
using namespace std;


class testing
{
public:
    int value;
    testing(int v)
    {
        value = v;
    }

    int getval()
    {
        return(value);
    }

};

void func(testing& ob)
{
    printf("The value is %d\n", ob.value);
}

int _tmain(int argc, _TCHAR* argv[])
{
    std::list<testing> testvar[3];

    testing t1(0);
    testing t2(1);
    testing t3(3);

    testvar[0].push_back(t1);
    testvar[0].push_back(t2);
    testvar[0].push_back(t3);

    std::for_each(testvar[0].begin(), testvar[0].end(), func);

    printf("Reached End");
    getchar();
    return 0;
}

I modified it to make func a member function and got weird compile errors, I searched online and someone had told use bind1st, bind2nd

#include "stdafx.h"
#include <string>
#include <list>
#include <algorithm>
using namespace std;

class testing
{
public:
int value;
testing(int v)
{
    value = v;
}

int getval()
{
    return(value);
}

};

class testing2
{
public:
    std::list<testing> testvar[3];

    void func(testing& ob)
    {
        printf("The value is %d\n", ob.value);
    }

    void driver()
    {
        std::for_each(testvar[0].begin(), testvar[0].end(), func);
    }

};



int _tmain(int argc, _TCHAR* argv[])
{
    testing2 testob;

    testob.driver();


    printf("Reached End");
    getchar();
    return 0;
}

So I modified the driver function to this

    void driver()
    {
std::for_each(testvar[0].begin(), testvar[0].end(),     std::bind1st(std::mem_fun(&testing2::func), this));
    }

I still get some weird compile erros, could someone please expain why we need to call a member function is such weird way..? and how does bind1st help..?

Actinia

ssce please people are less inclined to answer when they see all this code for what is most likely a simple question – Chadwickchae

I think the problem comes from void func(testing& ob) taking a reference vs. binder1st (the return type of bind1st) provides two overloads for operator(): one that takes a const second_argument& and one that takes a second_argument&. As the second argument of the return type of mem_fun(..) (which is mem_fun1_t) already is a reference, because func(testing&) takes a reference, these two overloads of operator() declare a function of the same signature. – Spout

Why did you make func a member function? – Several

If you can use C++11 or boost, you can (and should) use sehe's answer, otherwise you can try std::reference_wrapper (either from TR1 or write your own, it's simple) as a parameter for func. – Spout

Answer 1

Use std::bind

 std::for_each(testvar[0].begin(), testvar[0].end(), std::bind(&testing2::func, this, std::placeholders::_1));

Or use std::bind/lambdas

 std::for_each(testvar[0].begin(), testvar[0].end(), [this](testing& ob) { func(ob); });

Full:

#include <string>
#include <list>
#include <algorithm>
using namespace std;

struct testing {
    int value;
    testing(int v) { value = v; }
    int getval() { return(value); }
};

struct testing2 {
    std::list<testing> testvar[3];

    void func(testing& ob) {
        printf("The value is %d\n", ob.value);
    }

    void driver() {
        auto f = std::bind(&testing2::func, this, std::placeholders::_1);
        std::for_each(testvar[0].begin(), testvar[0].end(), f);
        std::for_each(testvar[0].begin(), testvar[0].end(), [this](testing& ob) { func(ob); });
    }
};

int main() {
    testing2 testob;
    testob.driver();
    printf("Reached End");
}

Torture

There is also std::mem_fn in C++11 – Executory

@Executory I don't think it applies here, as func is non-static member of a different class; a bind is required – Torture

Answer 2

You can also use std::mem_fn for cleaner syntax like this:

std::for_each(testvar[0].begin(), testvar[0].end(), std::mem_fn(&testing::func));

Complete code:

#include <stdio.h>
#include <bits/stdc++.h>

class testing
{
public:
    testing(int v) : value(v) {}

    int getval() { 
        return(value);
    }

    void func()
    {
        printf("The value is %d\n", value);
    }

private:
    int value;
};

int main() {
    std::list<testing> testvar[3];

    testing t1(0);  
    testing t2(1);
    testing t3(3);

    testvar[0].push_back(t1);
    testvar[0].push_back(t2);
    testvar[0].push_back(t3);

    std::for_each(testvar[0].begin(), testvar[0].end(), std::mem_fn(&testing::func));

    return 0;
}

Sharpnosed