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Is the address of a local variable a constexpr?



Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 17/18, 2019 at 00:00UTC (8:00pm US/Eastern)
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Data science time! April 2019 and salary with experience
Should we burninate the [wrap] tag?What are the differences between a pointer variable and a reference variable in C++?When should you use constexpr capability in C++11?Undefined reference to static constexpr char[]const vs constexpr on variablesDoes static constexpr variable make sense?Difference between `constexpr` and `const`Unique address for constexpr variableenum vs constexpr for actual static constants inside classesConstexpr placement new?constexpr unique id, compiles with clang but not with gcc



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9















In Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 (Section 10.4.5 Address Constant Expressions), he uses a code example where the address of a local variable is set to a constexpr variable. I thought this looked odd, so I tried running the example with g++ version 7.3.0 and was unable to get the same results. Here is his code example verbatim (although slightly abridged):



extern char glob;

void f(char loc)
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope



When I run this, I get:



error: ‘(const char*)(& loc)’ is not a constant expression


Is something happening with g++ that I'm not aware of, or is there something more to Bjarne's example?










share|improve this question



















  • 2





    Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?

    – doug
    1 hour ago







  • 2





    char loc is a locally declared character that is not static. The next time f() is called there is no guarantee loc will have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf

    – David C. Rankin
    1 hour ago







  • 1





    @DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.

    – johnnyodonnell
    1 hour ago






  • 2





    Yes, I was kinda scratching my head as to where &glob came from, but &loc was identifiable.

    – David C. Rankin
    1 hour ago






  • 1





    @DavidC.Rankin Good point, I should show that in my question. But yeah, I think this is because our versions are different

    – johnnyodonnell
    1 hour ago

















9















In Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 (Section 10.4.5 Address Constant Expressions), he uses a code example where the address of a local variable is set to a constexpr variable. I thought this looked odd, so I tried running the example with g++ version 7.3.0 and was unable to get the same results. Here is his code example verbatim (although slightly abridged):



extern char glob;

void f(char loc)
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope



When I run this, I get:



error: ‘(const char*)(& loc)’ is not a constant expression


Is something happening with g++ that I'm not aware of, or is there something more to Bjarne's example?










share|improve this question



















  • 2





    Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?

    – doug
    1 hour ago







  • 2





    char loc is a locally declared character that is not static. The next time f() is called there is no guarantee loc will have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf

    – David C. Rankin
    1 hour ago







  • 1





    @DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.

    – johnnyodonnell
    1 hour ago






  • 2





    Yes, I was kinda scratching my head as to where &glob came from, but &loc was identifiable.

    – David C. Rankin
    1 hour ago






  • 1





    @DavidC.Rankin Good point, I should show that in my question. But yeah, I think this is because our versions are different

    – johnnyodonnell
    1 hour ago













9












9








9


1






In Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 (Section 10.4.5 Address Constant Expressions), he uses a code example where the address of a local variable is set to a constexpr variable. I thought this looked odd, so I tried running the example with g++ version 7.3.0 and was unable to get the same results. Here is his code example verbatim (although slightly abridged):



extern char glob;

void f(char loc)
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope



When I run this, I get:



error: ‘(const char*)(& loc)’ is not a constant expression


Is something happening with g++ that I'm not aware of, or is there something more to Bjarne's example?










share|improve this question
















In Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 (Section 10.4.5 Address Constant Expressions), he uses a code example where the address of a local variable is set to a constexpr variable. I thought this looked odd, so I tried running the example with g++ version 7.3.0 and was unable to get the same results. Here is his code example verbatim (although slightly abridged):



extern char glob;

void f(char loc)
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope



When I run this, I get:



error: ‘(const char*)(& loc)’ is not a constant expression


Is something happening with g++ that I'm not aware of, or is there something more to Bjarne's example?







c++ c++11 constexpr memory-address






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 1 hour ago







johnnyodonnell

















asked 1 hour ago









johnnyodonnelljohnnyodonnell

393114




393114







  • 2





    Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?

    – doug
    1 hour ago







  • 2





    char loc is a locally declared character that is not static. The next time f() is called there is no guarantee loc will have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf

    – David C. Rankin
    1 hour ago







  • 1





    @DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.

    – johnnyodonnell
    1 hour ago






  • 2





    Yes, I was kinda scratching my head as to where &glob came from, but &loc was identifiable.

    – David C. Rankin
    1 hour ago






  • 1





    @DavidC.Rankin Good point, I should show that in my question. But yeah, I think this is because our versions are different

    – johnnyodonnell
    1 hour ago












  • 2





    Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?

    – doug
    1 hour ago







  • 2





    char loc is a locally declared character that is not static. The next time f() is called there is no guarantee loc will have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf

    – David C. Rankin
    1 hour ago







  • 1





    @DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.

    – johnnyodonnell
    1 hour ago






  • 2





    Yes, I was kinda scratching my head as to where &glob came from, but &loc was identifiable.

    – David C. Rankin
    1 hour ago






  • 1





    @DavidC.Rankin Good point, I should show that in my question. But yeah, I think this is because our versions are different

    – johnnyodonnell
    1 hour ago







2




2





Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?

– doug
1 hour ago






Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?

– doug
1 hour ago





2




2





char loc is a locally declared character that is not static. The next time f() is called there is no guarantee loc will have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf

– David C. Rankin
1 hour ago






char loc is a locally declared character that is not static. The next time f() is called there is no guarantee loc will have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf

– David C. Rankin
1 hour ago





1




1





@DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.

– johnnyodonnell
1 hour ago





@DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.

– johnnyodonnell
1 hour ago




2




2





Yes, I was kinda scratching my head as to where &glob came from, but &loc was identifiable.

– David C. Rankin
1 hour ago





Yes, I was kinda scratching my head as to where &glob came from, but &loc was identifiable.

– David C. Rankin
1 hour ago




1




1





@DavidC.Rankin Good point, I should show that in my question. But yeah, I think this is because our versions are different

– johnnyodonnell
1 hour ago





@DavidC.Rankin Good point, I should show that in my question. But yeah, I think this is because our versions are different

– johnnyodonnell
1 hour ago












3 Answers
3






active

oldest

votes


















6














Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 refers to the following code:



constexpr const char* p1="asdf";


This is OK because "asdf" is stored in a fixed memory location.



void f(char loc) 
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope



However, loc is not in a fixed memory location. it's on the stack and will have varying locations depending on when it is called.






share|improve this answer























  • So, you're saying that Bjarne should not have said that &loc will be "OK", right?

    – johnnyodonnell
    1 hour ago












  • The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.

    – johnnyodonnell
    1 hour ago











  • Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…

    – jackw11111
    1 hour ago











  • @jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5

    – johnnyodonnell
    1 hour ago


















3














It appears that the example from section 10.4.5 provided in my hard-copy of the "The C++ Programming Language (4th Edition)" is incorrect. And so I've concluded that the address of a local variable is not a constexpr.



The example appears to have been updated in some pdf versions as seen here:



enter image description here






share|improve this answer






























    1














    Just to add to other answers that have pointed out the mistake, C++ standard only allows constexpr pointers to objects of static-storage duration, one past the end of such, or nullptr. See [expr.const/8] specifically #8.2;



    It's worth noting that:




    • string-literals have static-storage duration:

    • Based on constraints in declaring extern variables, they'll inherently have static-storage duration or thread local-storage duration.

    Hence this is valid:



    #include <string>

    extern char glob;
    std::string boom = "Haha";

    void f(char loc)
    constexpr const char* p1 = &glob;
    constexpr std::string* p2 = nullptr;
    constexpr std::string* p3 = &boom;






    share|improve this answer























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      3 Answers
      3






      active

      oldest

      votes








      3 Answers
      3






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      6














      Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 refers to the following code:



      constexpr const char* p1="asdf";


      This is OK because "asdf" is stored in a fixed memory location.



      void f(char loc) 
      constexpr const char* p0 = &glob; // OK: &glob's is a constant
      constexpr const char* p2 = &loc; // OK: &loc is constant in its scope



      However, loc is not in a fixed memory location. it's on the stack and will have varying locations depending on when it is called.






      share|improve this answer























      • So, you're saying that Bjarne should not have said that &loc will be "OK", right?

        – johnnyodonnell
        1 hour ago












      • The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.

        – johnnyodonnell
        1 hour ago











      • Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…

        – jackw11111
        1 hour ago











      • @jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5

        – johnnyodonnell
        1 hour ago















      6














      Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 refers to the following code:



      constexpr const char* p1="asdf";


      This is OK because "asdf" is stored in a fixed memory location.



      void f(char loc) 
      constexpr const char* p0 = &glob; // OK: &glob's is a constant
      constexpr const char* p2 = &loc; // OK: &loc is constant in its scope



      However, loc is not in a fixed memory location. it's on the stack and will have varying locations depending on when it is called.






      share|improve this answer























      • So, you're saying that Bjarne should not have said that &loc will be "OK", right?

        – johnnyodonnell
        1 hour ago












      • The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.

        – johnnyodonnell
        1 hour ago











      • Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…

        – jackw11111
        1 hour ago











      • @jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5

        – johnnyodonnell
        1 hour ago













      6












      6








      6







      Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 refers to the following code:



      constexpr const char* p1="asdf";


      This is OK because "asdf" is stored in a fixed memory location.



      void f(char loc) 
      constexpr const char* p0 = &glob; // OK: &glob's is a constant
      constexpr const char* p2 = &loc; // OK: &loc is constant in its scope



      However, loc is not in a fixed memory location. it's on the stack and will have varying locations depending on when it is called.






      share|improve this answer













      Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 refers to the following code:



      constexpr const char* p1="asdf";


      This is OK because "asdf" is stored in a fixed memory location.



      void f(char loc) 
      constexpr const char* p0 = &glob; // OK: &glob's is a constant
      constexpr const char* p2 = &loc; // OK: &loc is constant in its scope



      However, loc is not in a fixed memory location. it's on the stack and will have varying locations depending on when it is called.







      share|improve this answer












      share|improve this answer



      share|improve this answer










      answered 1 hour ago









      dougdoug

      8621410




      8621410












      • So, you're saying that Bjarne should not have said that &loc will be "OK", right?

        – johnnyodonnell
        1 hour ago












      • The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.

        – johnnyodonnell
        1 hour ago











      • Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…

        – jackw11111
        1 hour ago











      • @jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5

        – johnnyodonnell
        1 hour ago

















      • So, you're saying that Bjarne should not have said that &loc will be "OK", right?

        – johnnyodonnell
        1 hour ago












      • The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.

        – johnnyodonnell
        1 hour ago











      • Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…

        – jackw11111
        1 hour ago











      • @jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5

        – johnnyodonnell
        1 hour ago
















      So, you're saying that Bjarne should not have said that &loc will be "OK", right?

      – johnnyodonnell
      1 hour ago






      So, you're saying that Bjarne should not have said that &loc will be "OK", right?

      – johnnyodonnell
      1 hour ago














      The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.

      – johnnyodonnell
      1 hour ago





      The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.

      – johnnyodonnell
      1 hour ago













      Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…

      – jackw11111
      1 hour ago





      Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…

      – jackw11111
      1 hour ago













      @jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5

      – johnnyodonnell
      1 hour ago





      @jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5

      – johnnyodonnell
      1 hour ago













      3














      It appears that the example from section 10.4.5 provided in my hard-copy of the "The C++ Programming Language (4th Edition)" is incorrect. And so I've concluded that the address of a local variable is not a constexpr.



      The example appears to have been updated in some pdf versions as seen here:



      enter image description here






      share|improve this answer



























        3














        It appears that the example from section 10.4.5 provided in my hard-copy of the "The C++ Programming Language (4th Edition)" is incorrect. And so I've concluded that the address of a local variable is not a constexpr.



        The example appears to have been updated in some pdf versions as seen here:



        enter image description here






        share|improve this answer

























          3












          3








          3







          It appears that the example from section 10.4.5 provided in my hard-copy of the "The C++ Programming Language (4th Edition)" is incorrect. And so I've concluded that the address of a local variable is not a constexpr.



          The example appears to have been updated in some pdf versions as seen here:



          enter image description here






          share|improve this answer













          It appears that the example from section 10.4.5 provided in my hard-copy of the "The C++ Programming Language (4th Edition)" is incorrect. And so I've concluded that the address of a local variable is not a constexpr.



          The example appears to have been updated in some pdf versions as seen here:



          enter image description here







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 1 hour ago









          johnnyodonnelljohnnyodonnell

          393114




          393114





















              1














              Just to add to other answers that have pointed out the mistake, C++ standard only allows constexpr pointers to objects of static-storage duration, one past the end of such, or nullptr. See [expr.const/8] specifically #8.2;



              It's worth noting that:




              • string-literals have static-storage duration:

              • Based on constraints in declaring extern variables, they'll inherently have static-storage duration or thread local-storage duration.

              Hence this is valid:



              #include <string>

              extern char glob;
              std::string boom = "Haha";

              void f(char loc)
              constexpr const char* p1 = &glob;
              constexpr std::string* p2 = nullptr;
              constexpr std::string* p3 = &boom;






              share|improve this answer



























                1














                Just to add to other answers that have pointed out the mistake, C++ standard only allows constexpr pointers to objects of static-storage duration, one past the end of such, or nullptr. See [expr.const/8] specifically #8.2;



                It's worth noting that:




                • string-literals have static-storage duration:

                • Based on constraints in declaring extern variables, they'll inherently have static-storage duration or thread local-storage duration.

                Hence this is valid:



                #include <string>

                extern char glob;
                std::string boom = "Haha";

                void f(char loc)
                constexpr const char* p1 = &glob;
                constexpr std::string* p2 = nullptr;
                constexpr std::string* p3 = &boom;






                share|improve this answer

























                  1












                  1








                  1







                  Just to add to other answers that have pointed out the mistake, C++ standard only allows constexpr pointers to objects of static-storage duration, one past the end of such, or nullptr. See [expr.const/8] specifically #8.2;



                  It's worth noting that:




                  • string-literals have static-storage duration:

                  • Based on constraints in declaring extern variables, they'll inherently have static-storage duration or thread local-storage duration.

                  Hence this is valid:



                  #include <string>

                  extern char glob;
                  std::string boom = "Haha";

                  void f(char loc)
                  constexpr const char* p1 = &glob;
                  constexpr std::string* p2 = nullptr;
                  constexpr std::string* p3 = &boom;






                  share|improve this answer













                  Just to add to other answers that have pointed out the mistake, C++ standard only allows constexpr pointers to objects of static-storage duration, one past the end of such, or nullptr. See [expr.const/8] specifically #8.2;



                  It's worth noting that:




                  • string-literals have static-storage duration:

                  • Based on constraints in declaring extern variables, they'll inherently have static-storage duration or thread local-storage duration.

                  Hence this is valid:



                  #include <string>

                  extern char glob;
                  std::string boom = "Haha";

                  void f(char loc)
                  constexpr const char* p1 = &glob;
                  constexpr std::string* p2 = nullptr;
                  constexpr std::string* p3 = &boom;







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered 51 mins ago









                  WhiZTiMWhiZTiM

                  18.1k33153




                  18.1k33153



























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