Lexicographic ordering
12345678910111213141516171819 | #include <tuple>
class foo
{
public:
foo(int n_, char c_, double d_)
: n{n_}, c{c_}, d{d_}
{}
friend bool operator<(const foo& lh, const foo& rh)
{
return std::tie(lh.n, lh.c, lh.d) <
std::tie(rh.n, rh.c, rh.d);
}
private:
int n;
char c;
double d;
}; |
This pattern is licensed under the CC0 Public Domain Dedication.
Intent
Implement a lexicographic ordering over class members.
Description
The class foo, on lines 3–19, has three member variables n, c and
d declared on lines 16–18. We wish to implement an ordering relation
for foo where these members are compared lexicographically.
Getting an ordering relation right with 3 elements or more is
tedious and error-prone. Fortunately, the standard library
provides a lexicographic ordering over
std::tuple, which we can utilise.
The less-than operator for foo defined on lines 10–14 compares the
member n first, then c if the ns are equal, and finally the
member d if both the ns and cs are equal. To achieve this,
we use std::tie on line 12 and line 13 to
create std::tuples containing references to the members of
the left operand and right operand respectively. The comparison
of these std::tuples provides a lexicographic ordering over
these members.
