[RESOLVED] mem_fun & templates

[JohnW@Wessex]

This is the first time I've used std::mem_fun and I'm at a loss to figure out what I have to do to get this to compile.

#include <algorithm>
#include <vector>
#include <functional>

using namespace std;

template <typename T>
class Test
{
public:

    void Function1(T &t)
    {
    }

    void Function2()
    {
        for_each(data.begin(), data.end(), mem_fun(&Test<T>::Function1)); // Error here.
    }

private:
    vector<T> data;
};

int main()
{
    Test<int> test;

    test.Function2();
}

[JohnW@Wessex]

The fact it is a template seems to be irrelevant.

#include <algorithm>
#include <vector>
#include <functional>

using namespace std;

class Test
{
public:

    void Function1(int t)
    {
    }

    void Function2()
    {
        for_each(data.begin(), data.end(), mem_fun(&Test::Function1));
    }

private:
    vector<int> data;
};

int main()
{
    Test test;

    test.Function2();
}

[JohnW@Wessex]

Found the answer!

#include <algorithm>
#include <vector>
#include <functional>

using namespace std;

class Test
{
public:

    void Function1(int t)
    {
    }

    void Function2()
    {
        for_each(data.begin(), data.end(), bind1st(mem_fun(&Test::Function1), this));
    }

private:
    vector<int> data;
};

int main()
{
    Test test;

    test.Function2();
}

[Peter_APIIT]

Can you teach me how bind1st and bind2nd works because i read a lot of doc but never understand ?

Thanks.

[JohnW@Wessex]

bind1st & bind2nd allow you to use a binary function (two parameters) where a unary function (one parameter) is expected.

Take std::greater for example.
This is a binary function, but it's often used where a unary function is expected, for example std::count_if.

If you wanted to count how many items in a container were greater than 10 then you would use the following.

#include <functional>
#include <vector>
#include <algorithm>
#include <cstdlib>

using namespace std;

int main()
{
    vector<int> data;

    for (int i = 0; i < 100; ++i)
    {
        data.push_back(rand());
    }

    vector<int>::size_type count = count_if(data.begin(), data.end(), bind2nd(greater<int>(), 10));
}

bind2ndt makes the second argument to greater always equal to 10.
The first argument is the value of the element in the vector.

It's functionally equivalent to

#include <functional>
#include <vector>
#include <algorithm>
#include <cstdlib>

using namespace std;

int main()
{
    vector<int> data;

    for (int i = 0; i < 100; ++i)
    {
        data.push_back(rand());
    }

    greater<int> test;

    vector<int>::size_type count = 0;
    for (vector<int>::size_type i = 0; i < data.size(); ++i)
    {
        if (test(data[i], 10))
        {
            ++count;
        }
    }
}