The following code works well with gcc:
struct S {
int i, j;
auto operator<(const S& s) const {
return i < s.i;
};
};
std::vector<S> v;
std::make_heap(v.begin(), v.end());
But when I switch to C++20's range algorithm:
std::ranges::make_heap(v);
I got this compiler error:
source>:14:27: error: no match for call to '(const std::ranges::__make_heap_fn) (std::vector<S>&)'
14 | std::ranges::make_heap(v);
|
^
It seems struct S
doesn't satisfy the requirements of the ranges::make_heap
, but I don't know what exactly it is, can someone help?
std::ranges::make_heap
uses std::ranges::less
, which has a constraint:
Unlike
std::less
,std::ranges::less
requires all six comparison operators<
,<=
,>
,>=
,==
and!=
to be valid (via thetotally_ordered_with
constraint).
Your type S
does not have an equality operator; the spaceship operator only provides the other comparison operators.*
To fix this, provide an operator==
for your type:
constexpr auto operator==(const S& s) const {
return i == s.i;
}
Godbolt Link: https://godbolt.org/z/cGfrxs
* operator<=>
does not imply operator==
for performance reasons, as operator==
can short circuit over collections whereas operator<=>
cannot. However, from https://en.cppreference.com/w/cpp/language/default_comparisons , we see that a defaulted operator<=>
will also implicitly default an operator==
.
How did I figure this out? The error message for your code includes the following (trimmed and word wrapped by me):
note: the expression 'is_invocable_v<_Fn, _Args ...> [with _Fn = std::ranges::less&; _Args = {value_type&, value_type&}]' evaluated to 'false' 338 | concept invocable = is_invocable_v<_Fn, _Args...>; | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This means that std::ranges::make_heap
finds that it can't call std::ranges::less
for our type. Repeating this error message investigation for std::ranges::less
on the value_type
of the vector yields:
note: no operand of the disjunction is satisfied 123 | requires totally_ordered_with<_Tp, _Up> | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 124 | || __detail::__less_builtin_ptr_cmp<_Tp, _Up>
At this point, the compiler is trying hard to tell us that we aren't satisfying totally_ordered_with
, which means that it's time to hit the documentation for the concept and for std::ranges::less
.
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