Call a member function using for_each - c++

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..?

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");
}

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;
}