Question

This is a more specific question based on a question I asked earlier…

If I have a function that takes two parameters (one required, one optional):

1. an STL container such as a vector
2. an “optional” comparison function that serves as a relational overload and returns the maximum value, whatever that is, in the vector…

Code:

template <typename Type>
Type FindMax (std::vector<Type> &myVec, int (*cmp)(Type one, Type two) = CallBack)
/.../

WHAT exactly, does the “int (*cmp)(Type one…)” say to the compiler? I want it to say, here’s a function to use when comparing two of type Type…ie when using the relational operators <, >, =, etc. If no function is supplied by the user then use the default, otherwise, use what the user provides…

What exactly does the (*cmp)(Type one, Type two) say? Here’s a pointer to a function that takes two parameters Type one and Type two? Is there any significance as to what comes after the *, ie could I write (*titsmagee)(Type one, Type two)? I’m assuming the naming convention is to help future readers?

For this to work with a “struct” does anything specific to the potential comparisons to be made need to be stored within said struct?

Thanks!

Answer 1

int (*cmp)(Type one, Type two)

The parameter named cmp is a pointer to a function returning int that takes two parameters of type Type.

struct Foo
{
   int x;
};
int compare(Foo x, Foo y)
{
   return (x.x == y.x) ? 0 : (x.x > y.x ? 1 : -1);
}

std::vector<Foo> vec;

FindMax<Foo>(vec, &compare);

You need cmp so you can call the comparison function inside FindMax:

template <typename Type>
Type FindMax (std::vector<Type> &myVec, int (*cmp)(Type one, Type two) = CallBack)
{
    //whatever loop
    max = cmp(myVec[i],myVec[j]) >= 0 ? myVec[i] : myVec[j];
}

EDIT Breaking down the return:

return (x.x == y.x) ? 0 : (x.x > y.x ? 1 : -1);

?: is the ternary conditional operator.

condition ? expression1 : expression2

returns (loosely speaking) expression1 if condition is true, expression2 otherwise.

So what that means is:

if (x.x == y.x) 
   return 0; 
else 
   if (x.x > y.x) 
       return 1; 
   else 
       return -1;

It’s what you expect the comparison function to do. Return 0 for equality, 1 if the first element is bigger than the second, and -1 for the inverse.

EDIT 2

struct Foo
{
   int x;
};
//Foo has a member x.
Foo f;
//Create a Foo object called f.
f.x;
//Access the member x of the object