Re: [libgpiod] Fix for non-const bitset_cmp operator ()

[Michael Beach <michaelb@xxxxxxxx>]

Hi all, when trying to build libgpiod with clang 6.0 and libc++ I was
running into a problem with bitset_cmp in line_bulk.cpp. Clang was
complaining that the operator () defined by bitset_cmp was non-const.
Although some readings of the C++ standard allow this, it was a bit of
a grey area and generally considered to be undesirable, and has since
been fixed in C++17 (it is now required to be const) -- see
https://cplusplus.github.io/LWG/issue2542.
The following patch resolves the problem...

diff --git a/bindings/cxx/line_bulk.cpp b/bindings/cxx/line_bulk.cpp
index c93f364..e52d33d 100644
--- a/bindings/cxx/line_bulk.cpp
+++ b/bindings/cxx/line_bulk.cpp
@@ -29,7 +29,7 @@ const ::std::map<int, int> reqtype_mapping = {

 struct bitset_cmp
 {
-       bool operator()(const ::std::bitset<32>& lhs, const
::std::bitset<32>& rhs)
+       bool operator()(const ::std::bitset<32>& lhs, const
::std::bitset<32>& rhs) const
        {
                return lhs.to_ulong() < rhs.to_ulong();
        }