C++ functors, any effective aproach? - c++

Question

In my C++ code I wrote two functors that take to arguments one returns the sum and the other returns the subtract so I could use them as arguments to functions. like this:
template<class T>
class AddValues{
public:
T operator()(const T &value1, const T &value2) {
return value1 + value2;
}
};
template<class T>
class SubstractValues{
public:
T operator()(const T &value1, const T &value2) {
return value1 - value2;
}
};
But now I am looking to write like 6 functors that each one of them takes two arguments and returns true/false wither the first value is <,<=,>,>=,==,!= than the other.
Is there a clearer way to do that rather than defining 6 classes?
I'm working with C++11

Answer 1

Note: This post is a refinement of my comments to the original post.
First, you should be aware that the STL already defines a set of functors. See https://en.cppreference.com/w/cpp/header/functional under Comparators for <,<=,>,>=,==,!=, and Arithmetic operations for +,- (which you have redefined). It is good practice to know the STL and how to use it.
How to use them
Functors are objects like any other object and are to be used with value semantics. For a functional look, they define the function operator (operator()) and can be called () on the object directly.
std::less is_less;
bool is_0_less_than_0 = is_less(0,1); // Calls bool operator()(int, int) and evaluates to true
The functors are usually used in combination with the algorithms. For a not so pretty use case to compare two arrays of integers:
std::array<int,4> low_numbers = {1,2,3,4};
std::array<int,4> high_numbers = {5,6,7,8};
std::array<bool,4> is_number_greater;
// compares low_numbers and high_numbers element wise and stores result in is_number_greater.
std::transform(low_numbers.begin(),
low_numbers_low.end(),
high_numbers.begin(),
is_number_greater.begin(),
std::greater{});
How to write your own functors
So you have already (functionality wise) redefined std::plus (as AddValues) and std::minus (as SubtractValues). Note that I say functionality wise, since it is more flexible to only templatize the function operator:
struct AddValues{
template<class T>
T operator()(const T &value1, const T &value2) {
return value1 + value2;
}
};
And as the member method operator() does not modify any members of AddValues, it should be marked const:
struct AddValues{
template<class T>
T operator()(const T &value1, const T &value2) const {
return value1 + value2;
}
};
Then you do not need to specify the type when instantiating the object. Compare template class:
AddValues<int> add_values; // templated type has to be explicitly written.
add_values(1,2); //=3
with templated method:
AddValues add_values;
add_values(1,2); //=3, types deduced when calling method.
.
Anyway, you would have to do the same for <,<=,>,>=,==,!=, since you need a wrapper around each operator. The difference would only be that now you return booleans instead of a type.
struct MyLess
{
template<typename T>
bool operator()(const T& lhs, const T& rhs) const { return lhs < rhs; }
};