Meaning of const last in a function declaration of a class

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What is the meaning of const in declarations like these  The const confuses me   class foobar     public       operator int    const       const char  foo   const

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The const keyword used with the function declaration specifies that it is a const member function and it will not be able to change the data members of the object

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Blair s answer is on the mark   However note that there is a mutable qualifier which may be added to a class s data members   Any member so marked can be modified in a const method without violating the const contract   You might want to use this  for example  if you want an object to remember how many times a particular method is called  whilst not affecting the  logical  constness of that method

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The const qualifier means that the methods can be called on any value of foobar  The difference comes when you consider calling a non-const method on a const object  Consider if your foobar type had the following extra method declaration   class foobar           const char  bar        The method bar   is non-const and can only be accessed from non-const values   void func1 const foobar amp  fb1  foobar amp  fb2      const char  v1   fb1 bar        won t compile   const char  v2   fb2 bar        works     The idea behind const though is to mark methods which will not alter the internal state of the class  This is a powerful concept but is not actually enforceable in C     It s more of a promise than a guarantee  And one that is often broken and easily broken     foobar amp  fbNonConst   const cast lt foobar amp  gt  fb1

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Here const means that at that function any variable s value can not change class Test  private      int a  public      void test  const          a   10            And like this example  if you try to change the value of a variable in the test function you will get an error

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when you use const in the method signature  like your said  const char  foo   const   you are telling the compiler that memory pointed to by this can t be changed by this method  which is foo here

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The const means that the method promises not to alter any members of the class  You d be able to execute the object s members that are so marked  even if the object itself were marked const   const foobar fb  fb foo      would be legal   See How many and which are the uses of    const    in C    for more information

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Meaning of a Const Member Function in C   Common Knowledge  Essential Intermediate Programming gives a clear explanation        The type of the this pointer in a non-const member function of a class   X is X   const  That is  it   s a constant pointer to a non-constant X    see Const Pointers and Pointers to Const  7  21    Because the object   to which this refers is not const  it can be modified  The type of   this in a const member function of a class X is const X   const  That   is  it   s a constant pointer to a constant X  Because the object to   which this refers is const  it cannot be modified  That   s the   difference between const and non-const member functions    So in your code     class foobar     public       operator int    const       const char  foo   const       You can think it as this     class foobar     public       operator int  const foobar   const this  const       const char  foo const foobar   const this  const

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When you add the const keyword to a method the this pointer will essentially become a pointer to const object  and you cannot therefore change any member data   Unless you use mutable  more on that later    The const keyword is part of the functions signature which means that you can implement two similar methods  one which is called when the object is const  and one that isn t    include  lt iostream gt   class MyClass   private      int counter  public      void Foo                  std  cout  lt  lt   Foo   lt  lt  std  endl                 void Foo   const               std  cout  lt  lt   Foo const   lt  lt  std  endl             int main         MyClass cc      const MyClass amp  ccc   cc      cc Foo        ccc Foo        This will output  Foo Foo const   In the non-const method you can change the instance members  which you cannot do in the const version  If you change the method declaration in the above example to the code below you will get some errors       void Foo                 counter      this works         std  cout  lt  lt   Foo   lt  lt  std  endl                 void Foo   const               counter      this will not compile         std  cout  lt  lt   Foo const   lt  lt  std  endl          This is not completely true  because you can mark a member as mutable and a const method can then change it  It s mostly used for internal counters and stuff  The solution for that would be the below code    include  lt iostream gt   class MyClass   private      mutable int counter  public       MyClass     counter 0          void Foo                 counter            std  cout  lt  lt   Foo   lt  lt  std  endl                 void Foo   const               counter          This works because counter is  mutable          std  cout  lt  lt   Foo const   lt  lt  std  endl             int GetInvocations   const               return counter            int main void        MyClass cc      const MyClass amp  ccc   cc      cc Foo        ccc Foo        std  cout  lt  lt   Foo has been invoked    lt  lt  ccc GetInvocations    lt  lt    times   lt  lt  std  endl      which would output  Foo Foo const Foo has been invoked 2 times

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These const mean that compiler will Error if the method  with const  changes internal data   class A   public      A   member                     int hashGetter   const               state    1          return member             int goodGetter   const               return member             int getter   const                 member    2     error         return member             int badGetter                 return member         private      mutable int state       int member        The test  int main         const A a1      a1 badGetter       doesn t work     a1 goodGetter       works     a1 hashGetter       works      A a2      a2 badGetter       works     a2 goodGetter       works     a2 hashGetter       works     Read this for more information

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https   isocpp org wiki faq const-correctness const-member-fns  What is a  quot const member function quot   A member function that inspects  rather than mutates  its object  A const member function is indicated by a const suffix just after the member function   s parameter list  Member functions with a const suffix are called    const member functions    or    inspectors     Member functions without a const suffix are called    non-const member functions    or    mutators     class Fred   public    void inspect   const       This member promises NOT to change  this   void mutate                This member function might change  this    void userCode Fred amp  changeable  const Fred amp  unchangeable      changeable inspect         Okay  doesn t change a changeable object   changeable mutate          Okay  changes a changeable object   unchangeable inspect       Okay  doesn t change an unchangeable object   unchangeable mutate        ERROR  attempt to change unchangeable object    The attempt to call unchangeable mutate   is an error caught at compile time  There is no runtime space or speed penalty for const  and you don   t need to write test-cases to check it at runtime  The trailing const on inspect   member function should be used to mean the method won   t change the object   s abstract  client-visible  state  That is slightly different from saying the method won   t change the    raw bits    of the object   s struct  C   compilers aren   t allowed to take the    bitwise    interpretation unless they can solve the aliasing problem  which normally can   t be solved  i e   a non-const alias could exist which could modify the state of the object   Another  important  insight from this aliasing issue  pointing at an object with a pointer-to-const doesn   t guarantee that the object won   t change  it merely promises that the object won   t change via that pointer

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I would like to add the following point   You can also make it a const  amp  and const  amp  amp   So   struct s      void val1   const            this is const here  Hence this function cannot modify any member of  this           void val2   const  amp            this is const amp  here           void val3   const  amp  amp           The object calling this function should be const rvalue only            void val4    amp  amp           The object calling this function should be rvalue reference only             int main      s a    a val1      okay   a val2      okay      a val3   not okay  a is not rvalue will be okay if called like   std  move a  val3       okay  move makes it a rvalue      Feel free to improve the answer  I am no expert

[c++] Meaning of 'const' last in a function declaration of a class?

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