From http://en.cppreference.com/w/cpp/language/constexpr:
A constexpr specifier used in an object declaration implies const.
However, We can not change from
static constexpr const char* TIME_FORMAT = "%a %b %d %I:%M:%S %Z %Y";
to
static constexpr char* TIME_FORMAT = "%a %b %d %I:%M:%S %Z %Y";
The compiler complains:
In file included from json/formatter.cc:22:0:
json/formatter.hh:132:42: error: deprecated conversion from string
constant to ‘char*’ [-Werror=write-strings]
static constexpr char* TIME_FORMAT = "%a %b %d %I:%M:%S %Z %Y";
Since, unlike const, constexpr does not modify a type. It just applies
to an object (or function), and incidentally implies const to the
top-level type.
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