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

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));