[cfe-dev] std::pair not trivially copyable?

Robinson, Paul via cfe-dev cfe-dev at lists.llvm.org
Tue Aug 25 05:51:51 PDT 2020

We had someone trip over a case that reduces to this:

F:\scratch>type tc6347.cpp
#include <utility>
std::pair<unsigned, bool> data[10];
void bar() {
  for (const auto item : data)

F:\scratch>%wo%\clang -c -Wall tc6347.cpp
tc6347.cpp:4:19: warning: loop variable 'item' of type 'const std::pair<unsigned
      int, bool>' creates a copy from type 'const std::pair<unsigned int, bool>'
  for (const auto item : data)
tc6347.cpp:4:8: note: use reference type 'const std::pair<unsigned int, bool> &'
      to prevent copying
  for (const auto item : data)
1 warning generated.


This used to compile cleanly.  It turns out that the diagnostic
used to be suppressed for POD types, and now it is suppressed for
trivially-copyable types.

So... a std::pair of two scalars is not trivially copyable?
I'd have thought it would be, but then C++ is full of surprises.


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