Why does this code return a warning
warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
if
A constexpr specifier used in an object declaration or non-static member function (until C++14) implies const. A constexpr specifier used in a function or static member variable (since C++17) declaration implies inline.
(cppreference.com)
#include <cassert> #include <string> #include <iostream> struct A { // warning: ISO C++ forbids converting a string constant to ‘char*’ static constexpr char* name_ = "A"; static constexpr char* name() { return name_; }; }; int main() {};
If I add a const
after constexpr
, the warning is gone:
#include <cassert> #include <string> #include <iostream> struct A { static constexpr const char* name_ = "A"; static constexpr const char* name() { return name_; }; }; int main() {};
With g++ --version = g++ (GCC) 8.2.1 20181127
,
compilation g++ -O3 -std=c++2a -Wall main.cpp -o main
.
Does the constexpr
not imply const
on static
data members?
constexpr
does imply const
, but in this case it applies const
to the "wrong thing".
constexpr char*
is basically the same as
char * const
which is a constant pointer to a non-const char
. This won't work because string literals have the type const char[N]
so it would cast away the constness of the array elements.
constexpr const char*
on the other hand, is basically the same as
char const * const
which is a constant pointer to a constant char
, which is what you want as it preserves the constness of the elements.
There is a usual difference between a constant pointer and a pointer to constant. By making your constexpr char*
you made a pointer itself a constexpr
(and, of course, const
), but it still attempts to point at non-const character - and this is wrong, as string literals are const
. Solution:
constexpr const char* ch = "StackOverflow!";
Which declares a constexpr
pointer to const
.
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