How to define and use std::less as a template argument in a function definition?

Question

What is the proper C++ syntax to supply and use std::less as a template argument?

#include <iostream>
#include <functional>

template<typename CMP>
float order(float a, float b)
{
    return CMP(a, b) ? a : b;               // COMPILE ERROR                                                               
    // return CMP::operator()(a,b) ? a : b; //  another try
    // return (a CMP b) ? a : b;            // ¯\_(ツ)_/¯
}

int main() 
{
    const float A = order<std::less<float>>(3.1, 7.2);  // COMPILE ERROR
    const float B = order<std::greater<float>>(3.1, 7.2);
    std::cout << "A=" << A << "\n";
    std::cout << "B=" << B << "\n";
    return 0;
}

COMPILE ERROR:

$ g++ -std=c++17 foo.cpp -o foo

foo.cpp:6:12: error: no matching constructor for initialization of 'std::__1::less<float>'
foo.cpp:12:21: note: in instantiation of function template specialization 'order<std::__1::less<float> >' requested here
const float A = order<std::less<float>>(3.1, 7.2);
                ^

Answer 1

You can just return it like this:

return CMP{}(a,b) ? a : b;

This is because the constructor of std::less does not take any parameters.

CMP is a type, so you need to create an object of this type first, CMP{}, before you can invoke the () operator on the temporary CMP to do the comparison.

What the above solution does is: It instantiates an std::less object and then invokes operator() with the parameters a and b. This operator function then takes the two parameters and returns true if a is the lesser one (a boolean expresion). Then, via your ternary check, the smaller value between a and b is returned.

Answer 2

What is the proper C++ syntax to supply and use std::less as a template argument?

The std::less or std::greater are functors (i.e. function object). This means, prior to the operator() call, you need to default constructed them. Therefore, the return statement must be

return CMP{}(a,b) ? a : b;
//     ^^^^^^ ----> default construct the function object

However, if you want to save some typing, you might use template-template parameters for the order. Optionally, you can provide a template parameter(i.e typename T), to make it more generic (If intended) :

template<template<typename> class CMP, typename T>
//       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ --> template-template argument
constexpr auto order(T a, T b) 
{
    return CMP{}(a, b) ? a : b;
}

Now you call the order as

const auto A = order<std::less>(3.1, 7.2); 
//                  ^^^^^^^^^^^ -----> only specify the funnctor
const auto B = order<std::greater>(3.1, 7.2);
//                  ^^^^^^^^^^^^^^ --> only specify the funnctor

Answer 3

A well-designed interface would not prevent the use of a stateful comparison predicate. And for free, you get a nicer implementation of the other alternatives.

template<typename CMP> float order(float a, float b, CMP cmp = {}) {
   return cmp(a, b) ? a : b;
}

NOTE 1:

Despite popular belief, if CMP is not default constructible, the syntax will still work; but of course, you will need to pass a way to construct it. Which is good.

test:

#include <iostream>
#include <functional>

template<typename CMP>
float order(float a, float b, CMP cmp = {}) {
    return cmp(a,b) ? a : b;                                            
}

template<class T>
struct LESS : std::less<T> {
    LESS() = delete;
    LESS(const char*) {}
};

int main() {
    const float A = order<std::less   <float>>(3.1, 7.2);
    const float B = order<std::greater<float>>(3.1, 7.2);
    const float C = order<     LESS   <float>>(3.1, 7.2, "less");
    // const float D = order<     LESS   <float>>(3.1, 7.2);  // compile error, good!

    std::cout << "A=" << A << "\n";
    std::cout << "B=" << B << "\n";
    return 0;
}

NOTE 2:

A better-designed interface will have a default order:

template<typename CMP = std::less<float> >
float order(float a, float b, CMP cmp = {}) {
    return cmp(a,b) ? a : b;                                            
}

NOTE 3:

Technically, if the class has an explicit default constructor, one needs something more verbose for this to work, but I wouldn't implement this way because, if the default constructor is explicit, there must be a good reason for it, and we are basically overriding the intended behavior of the class designer.

template<typename CMP = std::less<float> >
float order(float a, float b, CMP cmp = CMP{}) {  // MMM, good idea?
    return cmp(a,b) ? a : b;                                            
}

---

Bonus:

Some people (including STL, https://en.cppreference.com/w/cpp/algorithm/sort) do not tolerate default parameters:

template<typename CMP>
float order(float a, float b, CMP cmp) {
    return cmp(a,b) ? a : b;                                            
}

template<typename CMP>
float order(float a, float b) {
    return order<CMP>(a, b, {}); // or CMP{});
}

float order(float a, float b) {
    return order<std::less<float>>(a, b);
}