Warning: ISO C++ forbids converting a string constant to ‘char*’ for a static `constexpr char*` data...












19















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?






c++ c++11 static constexpr







share|improve this question

























  • stackoverflow.com/questions/28845058/…

    – Mat
    14 hours ago











  • stackoverflow.com/questions/14116003/…

    – Brandon Lyons
    14 hours ago
















19















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?






c++ c++11 static constexpr







share|improve this question

























  • stackoverflow.com/questions/28845058/…

    – Mat
    14 hours ago











  • stackoverflow.com/questions/14116003/…

    – Brandon Lyons
    14 hours ago














19












19








19


1






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?






c++ c++11 static constexpr







share|improve this question
















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?






c++ c++11 static constexpr




c++ c++11 static constexpr






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 14 hours ago







tmaric

















asked 14 hours ago









tmarictmaric

2,6651949




2,6651949













  • stackoverflow.com/questions/28845058/…

    – Mat
    14 hours ago











  • stackoverflow.com/questions/14116003/…

    – Brandon Lyons
    14 hours ago



















  • stackoverflow.com/questions/28845058/…

    – Mat
    14 hours ago











  • stackoverflow.com/questions/14116003/…

    – Brandon Lyons
    14 hours ago

















stackoverflow.com/questions/28845058/…

– Mat
14 hours ago





stackoverflow.com/questions/28845058/…

– Mat
14 hours ago













stackoverflow.com/questions/14116003/…

– Brandon Lyons
14 hours ago





stackoverflow.com/questions/14116003/…

– Brandon Lyons
14 hours ago












2 Answers
2






active

oldest

votes


















36














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.






share|improve this answer

































    10














    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.






    share|improve this answer



















    • 1





      I remember the rule const binds to the thing to its immediate left (unless its first, in which case it binds to the thing to its immediate right). In my code, I tend to favor the general case, rather than the except-to-the-general rule in the parentheses. constexpr, when I use it (not frequently), I always put on the left as the first thing. I've not been bitten by the OP's situation yet.

      – Eljay
      14 hours ago











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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    36














    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.






    share|improve this answer






























      36














      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.






      share|improve this answer




























        36












        36








        36







        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.






        share|improve this answer















        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.







        share|improve this answer














        share|improve this answer



        share|improve this answer








        edited 13 hours ago









        Remy Lebeau

        332k18251446




        332k18251446










        answered 14 hours ago









        NathanOliverNathanOliver

        89.3k15121188




        89.3k15121188

























            10














            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.






            share|improve this answer



















            • 1





              I remember the rule const binds to the thing to its immediate left (unless its first, in which case it binds to the thing to its immediate right). In my code, I tend to favor the general case, rather than the except-to-the-general rule in the parentheses. constexpr, when I use it (not frequently), I always put on the left as the first thing. I've not been bitten by the OP's situation yet.

              – Eljay
              14 hours ago
















            10














            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.






            share|improve this answer



















            • 1





              I remember the rule const binds to the thing to its immediate left (unless its first, in which case it binds to the thing to its immediate right). In my code, I tend to favor the general case, rather than the except-to-the-general rule in the parentheses. constexpr, when I use it (not frequently), I always put on the left as the first thing. I've not been bitten by the OP's situation yet.

              – Eljay
              14 hours ago














            10












            10








            10







            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.






            share|improve this answer













            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.







            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered 14 hours ago









            SergeyASergeyA

            42.1k53784




            42.1k53784








            • 1





              I remember the rule const binds to the thing to its immediate left (unless its first, in which case it binds to the thing to its immediate right). In my code, I tend to favor the general case, rather than the except-to-the-general rule in the parentheses. constexpr, when I use it (not frequently), I always put on the left as the first thing. I've not been bitten by the OP's situation yet.

              – Eljay
              14 hours ago














            • 1





              I remember the rule const binds to the thing to its immediate left (unless its first, in which case it binds to the thing to its immediate right). In my code, I tend to favor the general case, rather than the except-to-the-general rule in the parentheses. constexpr, when I use it (not frequently), I always put on the left as the first thing. I've not been bitten by the OP's situation yet.

              – Eljay
              14 hours ago








            1




            1





            I remember the rule const binds to the thing to its immediate left (unless its first, in which case it binds to the thing to its immediate right). In my code, I tend to favor the general case, rather than the except-to-the-general rule in the parentheses. constexpr, when I use it (not frequently), I always put on the left as the first thing. I've not been bitten by the OP's situation yet.

            – Eljay
            14 hours ago





            I remember the rule const binds to the thing to its immediate left (unless its first, in which case it binds to the thing to its immediate right). In my code, I tend to favor the general case, rather than the except-to-the-general rule in the parentheses. constexpr, when I use it (not frequently), I always put on the left as the first thing. I've not been bitten by the OP's situation yet.

            – Eljay
            14 hours ago


















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