std::compare_three_way not compiling

[JUANDENT]

Hi,

I have this code:

template<typename T>
struct Value
{
	T val{};

	auto operator<=>(const Value& rhs) const noexcept(noexcept(val<=>val))
	{
		return std::compare_three_way{}(val <=> rhs.val);
	}
};

It does not compile!!

struct BB
{
	int num;

	bool operator==(const BB&) const = default;
	auto operator<=>(const BB& rhs) const noexcept -> std::strong_ordering
	{
		return num <=> rhs.num;
	}
};

void useValue()
{
	Value<BB> a{ 10 }, b{ 14 };
	bool less = a < b;
}

What am I doing wrong?

Thanks!

[seeplus]

Are you compiling as C++20 ?

[malibor]

This works:

auto operator<=>(const Value& rhs) const noexcept(noexcept(val <=> val))
{
	return val <=> rhs.val;
}

[JUANDENT]

yes I am compiling with C++20
the problem is with

std::compare_three_way

it says:

call to object of class type 'std::compare_three_way': no matching call operator found

[malibor]

JUANDENT wrote:

it says: call to object of class type 'std::compare_three_way': no matching call operator found

Operator expects arguments, not comparison, therefore:

return std::compare_three_way{}(val, rhs.val);

[JUANDENT]

thank you!!

[George P]

cppreference has excellent examples on how to use C++ stdlib functionality.

https://en.cppreference.com/w/cpp/utility/compare/compare_three_way

Admittedly the site isn't designed to learn C++ from 'cold', a bit of determination and prior knowledge is required for best results.