Const and References

This is a discussion on Const and References within the C++ Programming forums, part of the General Programming Boards category; First Code : - Code: #include<iostream> using namespace std; int main() { int i = 10; const int &j = ...

  1. #1
    Registered User
    Join Date
    Jun 2007
    Posts
    99

    Const and References

    First Code : -

    Code:
    #include<iostream>
    using namespace std;
    
    int main()
    {
       int i = 10;
       const int &j = i;
    
    }
    how can i create a reference to a const, even when the variable of which the reference i am creating is not const. and thats where my problem starts

    Second Code :-

    Code:
    #include<iostream>
    using namespace std;
    
    int main()
    {
       int i = 10;
       const int &j = i;
       cout<<i<<j;
       j = 20;
       cout<<i<<j;
       getchar();    
    }
    now, i get error at j=20

    Third Code:-

    Code:
    #include<iostream>
    using namespace std;
    
    int main()
    {
       int i = 10;
       const int &j = i;
       cout<<i<<j;
       i = 20;
       cout<<i<<j;
       getchar();    
    }
    and this is working

    please i need explaination!

  2. #2
    Registered User
    Join Date
    Jan 2005
    Posts
    7,340
    The reference to const just means that you promise not to change the value through that reference. The original variable is not affected in any way. That is why you can make j a reference to const when i is not const. The const in the j declaration only applies to j.

    It's like if a husband and wife are watching the TV and the husband has the remote. The wife can watch the TV all she wants, it doesn't stop the husband from changing the channel.

    That is also why the third code works. The i variable is not const, so you can change it. The fact that you created a reference to const named j and had it refer to i is irrelevent.

  3. #3
    The larch
    Join Date
    May 2006
    Posts
    3,573
    i is not const, so you can change it. j is an alias of i, but you may not change it, because it is const (you may not use the alias to modify the original - it's a read-only alias). If you want to catch attempts of modifying i, make it const too.

    const references usually come to play, when you want to pass a reference to a function for performance considerations (no copy) but want to be sure that the function won't modify the original value (as a non-const reference could). But that's only as far as this particular function is concerned.
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  4. #4
    Registered User
    Join Date
    Jun 2007
    Posts
    99
    wow!, awsome explaination !

    and now just for confirmation

    isnt this code non-sense?

    Code:
    #include<iostream>
    using namespace std;
    
    int main()
    {
       const int i = 10;
       int &j = i;
    
    }

  5. #5
    Registered User
    Join Date
    Jan 2005
    Posts
    7,340
    Correct, it shouldn't compile.

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