使用std :: ptr_fun作为成员函数

Question

Consider the following:

class A
{
    public:
    bool is_odd(int i)
    {
        return (i % 2) != 0;
    }

    void fun()
    {
        std::vector<int> v2;
        v2.push_back(4);
        v2.push_back(5);
        v2.push_back(6);

        // fails here
        v2.erase(std::remove_if(v2.begin(), v2.end(), std::not1(std::ptr_fun(is_odd))), v2.end());
    }
};

The above code fails to negate the effect of is_odd() because it is a member function. The call to std::ptr_fun() fails.

How do I make it to work? Please note that I want is_odd() to be a non-static member function.

Answer 1

There are multiple issues with using A::is_odd(int) as a unary predicate, especially when it needs to be used with std::not1() :

1. A call to A::is_odd(int) takes two arguments: the implicit object (" this ") and the visible int argument.
2. It isn't a function object defining argument_type and result_type .

Properly using this member function as a unary predicate requires two steps:

1. Adapting the member function pointer to be a suitable function object, eg, using one of the std::mem_*fun functions.
2. Binding the first argument to a suitable object, with a non-C++11 compiler probably using std::bind1st() .

With a C++11 compiler things are a lot easier because std::bind() take care of both of these. Assuming it is used from a member of A :

... std::not1(std::bind(&A::is_odd, this, std::placeholders::_1)) ...

The same with a pre-C++11 compiler is somewhat harder. The use in std::remove_if() would look something like this:

v2.erase(
    std::remove_if(v2.begin(),
                   v2.end(),
                   std::not1(std::bind1st(std::mem_fun(&A::is_odd), this))),
    v2.end());

Answer 2

Just make is_odd static, so it won't require an implicit this parameter:

static bool is_odd(int i)

It doesn't use any member variables or other member functions anyway.