constexpr可在Ubuntu上使用，但不适用于MacOS

Question

I have this code which compiles fine on Ubuntu, but when I try to compile it on MacOS I get this error:

Constexpr variable 'HeuristicNames' must be initialized by a constant expression

#define LSHPair std::pair<const char *, LISTSCHED_HEURISTIC>
static constexpr LSHPair HeuristicNames[] = {
    LSHPair("CP", LSH_CP),    LSHPair("LUC", LSH_LUC),
    LSHPair("UC", LSH_UC),    LSHPair("NID", LSH_NID),
    LSHPair("CPR", LSH_CPR),  LSHPair("ISO", LSH_ISO),
    LSHPair("SC", LSH_SC),    LSHPair("LS", LSH_LS),
    LSHPair("LLVM", LSH_LLVM)};

LISTSCHED_HEURISTIC is an enum .

I take this error to mean that some part of the right hand side of the assignment is not a constexpr , so the resulting variable can't be a constexpr . However I don't have a firm enough grasp of the rules around constexpr to understand why, or how to fix it.

I also don't get why this is different on MacOS than on Ubuntu. Can anyone shed some light on this?

Answer 1

First of all you do not need macro. You could define type

using LSHPair =  std::pair<const char *, LISTSCHED_HEURISTIC>;

Or just use brace initialization, what is much more clean:

using LSHPair =  std::pair<const char *, LISTSCHED_HEURISTIC>;

static constexpr LSHPair HeuristicNames[] = {
    {"CP", LSH_CP},
    {"LUC", LSH_LUC},
    {"UC", LSH_UC},   
    {"NID", LSH_NID},
    {"CPR", LSH_CPR},
    {"ISO", LSH_ISO},
    {"SC", LSH_SC}, 
    {"LS", LSH_LS},
    {"LLVM", LSH_LLVM}
};

As @CuriouslyRecurringThoughts point out constructor of std::pair is a constexpr since c++14 .

I've test this on my Mac OS and apparently clang there works a bit differently then on Linux (as shown on compiler explorer ):

Marek R$ g++ main.cpp -std=c++11
main.cpp:17:26: error: constexpr variable 'HeuristicNames' must be initialized by a constant expression
static constexpr LSHPair HeuristicNames[] = {
                         ^                  ~
main.cpp:18:5: note: non-constexpr constructor 'pair<char const (&)[3], LISTSCHED_HEURISTIC, false>' cannot be used in a constant expression
    {"CP", LSH_CP},
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/utility:446:5: note: declared here
    pair(_U1&& __u1, _U2&& __u2)
    ^
1 error generated.
Marek R$ g++ main.cpp -std=c++14
Marek R$ 

So MacOS clang is right.

Problem must be on header files and versioning them depending on C++ standard. Most probably same header files of standard library are used for clang and gcc are used on Linux. On Mac std::pair constructor is prefixed by macro _LIBCPP_CONSTEXPR_AFTER_CXX11 which definition changes deeding on C++ standard enabled. On Linux you have to check by yourself how it is done.