error passing xxx as this argument of xxx discards qualifiers

Question

include  lt iostream gt   include  lt set gt   using namespace std   class StudentT    public      int id      string name  public      StudentT int  id  string  name    id  id   name  name              int getId             return id            string getName             return name            inline bool operator lt   StudentT s1  StudentT s2        return  s1 getId    lt  s2 getId       int main          set lt StudentT gt  st      StudentT s1 0   quot Tom quot        StudentT s2 1   quot Tim quot        st insert s1       st insert s2       set lt StudentT gt     iterator itr      for  itr   st begin    itr    st end    itr              cout  lt  lt  itr- gt getId    lt  lt   quot   quot   lt  lt  itr- gt getName    lt  lt  endl            return 0     In line  cout  lt  lt  itr- gt getId    lt  lt   quot   quot   lt  lt  itr- gt getName    lt  lt  endl   It give an error that      main cpp 35  error  passing  const StudentT  as  this  argument of  int StudentT  getId    discards qualifiers    main cpp 35  error  passing  const StudentT  as  this  argument of  std  string StudentT  getName    discards qualifiers  What s wrong with this code  Thank you

User · Answer

Actually the C   standard  i e  C   0x draft  says  tnx to  Xeo  amp   Ben Voigt for pointing that out to me       23 2 4 Associative containers   5 For set and multiset the value type   is the same as the key type  For map   and multimap it is equal to pair  Keys in an associative   container are immutable    6 iterator of   an associative container is of the   bidirectional iterator category  For   associative containers where the value   type is the same as the key type  both   iterator and const iterator are   constant iterators  It is unspecified   whether or not iterator and   const iterator are the same type    So VC   2008 Dinkumware implementation is faulty     Old answer   You got that error because in certain implementations of the std lib the set  iterator is the same as set  const iterator     For example libstdc    shipped with g    has it  see here for the entire source code    typedef typename  Rep type  const iterator            iterator  typedef typename  Rep type  const iterator            const iterator    And in SGI s docs it states   iterator       Container  Iterator used to iterate through a set  const iterator Container  Const iterator used to iterate through a set   Iterator and const iterator are the same type     On the other hand VC   2008 Express compiles your code without complaining that you re calling non const methods on set  iterators

User · Answer

The objects in the std  set are stored as const StudentT  So when you try to call getId   with the const object the compiler  detects a problem  mainly you re calling a non-const member function on const object which is not allowed because non-const member functions make NO PROMISE not to modify the object  so the compiler is going to make a safe assumption that getId   might attempt to modify the object but at the same time  it also notices that the object is const  so any attempt to modify the const object should be an error  Hence compiler generates an error message    The solution is simple  make the functions const as   int getId   const       return id    string getName   const       return name      This is necessary because now you can call getId   and getName   on const objects as   void f const StudentT  amp  s         cout  lt  lt  s getId        now okay  but error with your versions      cout  lt  lt  s getName      now okay  but error with your versions     As a sidenote  you should implement operator lt  as     inline bool operator lt   const StudentT  amp  s1  const StudentT  amp  s2        return  s1 getId    lt  s2 getId        Note parameters are now const reference

User · Answer

Member functions that do not modify the class instance should be declared as const   int getId   const       return id    string getName   const       return name      Anytime you see  discards qualifiers   it s talking about const or volatile

User · Answer

Let s me give a more detail example  As to the below struct   struct Count      uint32 t c       Count uint32 t i 0  c i         uint32 t getCount            return c             uint32 t add const Count amp  count           uint32 t total   c   count getCount            return total               As you see the above  the IDE CLion   will give tips Non-const function  getCount  is called on the const object  In the method add count is declared as const object  but  the method getCount is not const method  so count getCount   may change the members in count   Compile error as below core message in my compiler    error  passing  const xy stl  Count  as  this  argument discards qualifiers  -fpermissive    To solve the above problem  you can    change the method uint32 t getCount        to  uint32 t getCount   const        So count getCount   won t change the members in count    or   change uint32 t add const Count amp  count       to uint32 t add Count amp  count        So count don t care about changing members in it    As to you problem  objects in the std  set are stored as const StudentT  but the method getId and getName are not const  so you give the above error   You can also see this question Meaning of   39 const  39  last in a function declaration of a class  for more detail

[c++] error: passing xxx as 'this' argument of xxx discards qualifiers

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