C++ :: Sorting Data Based On Some Metric - Function Pointer Syntax

I have a piece of code that sorts data based on some metric. The some metric is something I now want to make flexible so I can easily switch between and compare metrics. To do this, I want to pass the function to use as a parameter to the method that does the sorting (and other stuff). However, I'm having problems figuring out the syntax. Also, I think my [temporary] organization of code is violating a lot of basic code design principles.

To make the function pointer passable, I defined the "typename" in the header where the function is located (it is part of a struct, "Data"):

// Below the struct definition of Data
typedef double (Data::*CostF)(unsigned l, double W) const;

The two example functions I want to use are defined in that struct ("Data"):

// Inside the struct definition
inline double someExampleCost(unsigned l, double W) const {
// Returns some basic calculation
}

The function that uses it is part of a different class (that holds a reference to the first class, in case that matters; I feel like I'm doing something odd here, because I'm binding a member function in the definition/passing, but never referencing the object). It looks like this:

// Inside another class ("Foo")
inline void DoSomeStuff(double& ECost, double& TCost, CostF cost) {
// Irrelevant stuff here
std::sort(vector.begin(), vector.end(), [&](unsigned a, unsigned b){
return (*cost)(a, W) < (*cost)(b, W);
});
// More irrelevant stuff here
}

The error shown is "operand of "*" must be a pointer". If I remove the '*':
[code]return cost(A, W) < cost(b, W);

the error becomes: "expression must have a (pointer-to-)function type."

The call to this function is, currently, just in the main function, as I'm just testing before I wrap it into real code. It looks like this:

// In main
Foo bar; // Make an object of the struct that has the "sorting" function
CostF costFunction = &Data::someExampleCost;
// Bind to the Cost function bar.DoSomeStuff(varA, varB, costFunction);

This bit shows no errors by itself. So, my questions:

a) Clearly I'm missing the insight into Function Pointers. I'm comfortable with regular pointer stuff, but I can't wrap my head around FPs, partly due to the awkward syntax.

b) I'm very uncomfortable with the fact that I'm binding a member function of a class, but never try to reference an actual object of that class. This is probably a big part of why it's not working, but I can't seem to bind a function belonging to a specific object. I thought of doing

// In the main again
Data d; // Construct the object, which contains big lookup tables
Foo F(d); // Construct the object, which only holds a reference to a Data object
CostF costFunction = &d.someExampleCost; // Bind to the Cost function of that object

but that doesn't work ("a pointer to a bound function may only be used to call the function").