find_if function build problems

Question

I'm trying to build the following block of code in a 1-file .cpp file:

#include <iostream>

#include <algorithm>
using namespace std;

class test
{

public:

    int a[10];
    int index;

    test();
    ~test();
    bool equals(int p);
    void search();
};

test::test()
{
    int temp[10] = {4, 9, 5, 6, 9, 10, 9, 255, 60, 0};

    memcpy(a, temp, sizeof(temp));

    index = -1;
}

bool test::equals(int p)
{
    return p == 9;
}

void test::search()
{
    int* p = std::find_if(a, a+10, &test::equals);
    while (p != a+10)
    {
     cout<< *p;
     index = p - a;
     p = std::find_if(p+1, a+10, &test::equals);
    }
}

int main(int argc, char *argv[])
{
    test object;

    object.search();

    return 0;
}

I am getting an error as shown below, and I'm not sure exactly what's happening when I use the find_if function in a member method of a class, and I'm getting this error whenever I am doing so.

1>c:\program files\microsoft visual studio 8\vc\include\algorithm(87) : error C2064: term does not evaluate to a function taking 1 arguments
1>        c:\program files\microsoft visual studio 8\vc\include\algorithm(96) : see reference to function template instantiation '_InIt std::_Find_if(_InIt,_InIt,_Pr)' being compiled
1>        with
1>        [
1>            _InIt=int *,
1>            _Pr=bool (__thiscall test::* )(int)
1>        ]
1>        c:\testprogram\nomfc\main.cpp(32) : see reference to function template instantiation '_InIt std::find_if(_InIt,_InIt,_Pr)' being compiled
1>        with
1>        [
1>            _InIt=int *,
1>            _Pr=bool (__thiscall test::* )(int)
1>        ]

Answer 1

test::equals is a member function, which has different pointer syntax from an ordinary function pointer. In particular, to call it, find_if would need an object of type test, which it doesn't have (it won't, for example, automatically call it on this, which I'm guessing is what you have in mind).

You can move the function equals outside the class test, and it should work.

Answer 2

The find_if function expects an object which is callable as a function with no parameters. This is something like a free function, a function object or a static class function. You passed in the address of the equals member function which is none of these. You could resolve this by making the equals function a free function or a static function, since it does not require any members of the test instance.

// static
class test
{
  public:
    static bool equals(int p); // etc
};
int* p = std::find_if(a, a+10, &test::equals);

// free
bool equals(int p)
{
    return p == 9;
}
int* p = std::find_if(a, a+10, equals);

If your real code example requires that it be a member function, then you need to pass in a function object that acts as a closure over the class instance. I favour using the Boost bind method for this, but there are other methods as well.

int* p = std::find_if(a, a+10, boost::bind(&test::equals, this, _1));

Answer 3

The third argument to find_if must be a (pointer to a) function or functor taking one argument, not a (pointer to an) instance-method which is what you're using. For example, an appropriate functor might be (loosely adapted from [this thread][1]):

template <typename PType, typename ArgType>
class is_good_test : unary_function<PType, bool>
{ public:
is_good_test(const ArgType & arg) : _val(arg) { }
~is_good_test() { }

bool operator()(const PType p) const
{
return p->equals(_val);
}

private:
ArgType _val;
};

which lets you do calls like:

std::find_if(a, a+10, is_good_test<test*, int>(10))


  [1]: http://www.velocityreviews.com/forums/t288980-functors-and-stl-findif.html

Answer 4

int *p = find_if(a, a+10, bind1st(mem_fun(&test::equals), this));

Or better yet, get rid of that test::equals() member function and then

int *p = find_if(a, a+10, bind2nd(equals(), 9));

where equals is in fact std::equals(), a binary functor defined in header <functional>.