Using a non-static class member inside a comparison function

Question

I'm currently developing a syntaxic analyser class that needs, at a point of the code, to sort structs holding info about operators. Each operator has a priority, which is user-defined through public member functions of my analyser class. Thus, when sorting, I need my sorting function to order elements based on the priority of the corresponding operator. I'm using the following code to compare elements:

bool parser::op_comp(const op_info& o1, const op_info& o2) {
    op_def& op1 = operators[o1.op_char];
    op_def& op2 = operators[o2.op_char];

    return op1.priority > op2.priority;
}

Note that I had to make this function static, since it's defined inside of a class.

In fact, my compare function compares elements of type op_char, and I retrieve the operator def from a map which contain elements of type op_def, which have a field "priority".

The problem I'm facing is that I can't manage to use std::sort(ops.begin(), ops.end(), std::mem_fun_ref(&parser::op_comp)) (where ops is a vector of op_info) method. I get the following error, which sounds quite logical :

error: invalid use of member `parser::operators' in static member function

Here is my question : how can I force std::sort to use a comp function that makes use of elements from non-static members of the class ? Obviously the function should be non-static, but I can't manage to use it if I don't make it static...

Answer 1

Use a functor instead of a function:

struct op_comp : std::binary_function<op_info, op_info, bool>
    {
    op_comp(parser * p) : _parser(p) {}
    bool operator() (const op_info& o1, const op_info& o2) {
        return _parser->op_comp(o1, o2);
    }
    parser * _parser;
};

This way the method op_comp can stay non-static. However the caller needs an instance of the parser, where are all the operators are stored. This is the usage of our new functor:

std::sort(ops.begin(), ops.end(), op_comp(&my_parser));

Where my_parser is the instance of parser you are using. Alternatively, if you are calling std::sort from the parser, you can simply write:

std::sort(ops.begin(), ops.end(), op_comp(this));