How to initialize const member variable in a class

Question

include  lt iostream gt   using namespace std  class T1     const int t   100    public     T1            cout  lt  lt   T1 constructor     lt  lt  t  lt  lt  endl           When I am trying to initialize the const member variable t with 100  But it s giving me the following error   test cpp 21  error  ISO C   forbids initialization of member    t    test cpp 21  error  making    t    static   How can I initialize a const value

User · Answer

you can add static to make possible the initialization of this class member variable   static const int i   100    However  this is not always a good practice to use inside class declaration  because all objects instacied from that class will shares the same static variable which is stored in internal memory outside of the scope memory of instantiated objects

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If you don t want to make the const data member in class static  You can initialize the const data member using the constructor of the class  For example   class Example        const int x      public        Example int n       Example  Example int n  x n       if there are multiple const data members in class you can use the following syntax to initialize the members   Example  Example int n  int z  x n  someOtherConstVariable z

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There are couple of ways to initialize the const members inside the class    Definition of const member in general  needs initialization of the variable too    1  Inside the class   if you want to initialize the const the syntax is like this  static const int a   10    at declaration   2  Second way can be   class A     static const int a    declaration     const int A  a   10    defining the static member outside the class   3  Well if you don t want to initialize at declaration  then the other way is to through constructor  the variable needs to be initialized in the initialization list not in the body of the constructor   It has to be like this  class A     const int b    A int c    b c       const member initialized in initialization list

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The const variable specifies whether a variable is modifiable or not  The constant value assigned will be used each time the variable is referenced  The value assigned cannot be modified during program execution   Bjarne Stroustrup s explanation sums it up briefly      A class is typically declared in a header file and a header file is typically included into many translation units  However  to avoid complicated linker rules  C   requires that every object has a unique definition  That rule would be broken if C   allowed in-class definition of entities that needed to be stored in memory as objects    A const variable has to be declared within the class  but it cannot be defined in it  We need to define the const variable outside the class   T1     t  100       Here the assignment t   100 happens in initializer list  much before the class initilization occurs

User · Answer

Well  you could make it static   static const int t   100    or you could use a member initializer   T1     t 100           Other constructor stuff here

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If a member is a Array it will be a little bit complex than the normal is   class C       static const int ARRAY 10    public      C         const unsigned int C  ARRAY 10     0 1 2 3 4 5 6 7 8 9     or  int  a   new int N      fill a  class C     const std  vector lt int gt  v  public    C   v a  a N

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You can upgrade your compiler to support C  11 and your code would work perfectly  Use initialization list in constructor   T1     t  100

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Another solution is  class T1       enum               t   100             public      T1         So t is initialised to 100 and it cannot be changed and it is private

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Another possible way are namespaces    include  lt iostream gt   namespace mySpace      static const int T   100      using namespace std   class T1      public     T1               cout  lt  lt   T1 constructor     lt  lt  mySpace  T  lt  lt  endl            The disadvantage is that other classes can also use the constants if they include the header file

User · Answer

This is the right way to do  You can try this code       include  lt iostream gt   using namespace std   class T1       const int t       public          T1   t 100                cout  lt  lt   T1 constructor     lt  lt  t  lt  lt  endl                int main         T1 obj      return 0      if you are using C  10 Compiler or below then you can not initialize the cons member at the time of declaration  So here it is must to make constructor to initialise the const data member  It is also must to use initialiser list T1   t 100  to get memory at instant

User · Answer

In C   you cannot initialize any variables directly while the declaration  For this we ve to use the concept of constructors  See this example -  include  lt iostream gt   using namespace std   class A       public    const int x         A   x 0    initializing the value of x to 0             constructor         int main         A a    creating object    cout  lt  lt   quot Value of x -  quot   lt  lt a x lt  lt endl          return 0     Hope it would help you

[c++] How to initialize const member variable in a class?

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