What is a constant reference not a reference to a constant

Question

A pretty theoretical question   Why constant references do not behave the same way as constant pointers and I can actually change the object they are pointing to  They really seem like another plain variable declaration  Why would I ever use them  This is a short example that I run which compiles and runs with no errors   int main         int i 0      int y 1          int amp const icr i      icr y              Can change the object it is pointing to so it s not like a const pointer        icr 99             Can assign another value but the value is not assigned to y        int x 9      icr x      cout lt  lt  icr    lt  lt icr lt  lt    y   lt  lt y lt  lt endl

User · Answer

What is a constant reference  not a reference to a constant  A Constant Reference is actually a Reference to a Constant     A constant reference  Reference to a constant is denoted by     int const  amp i   j    or Alternatively const int  amp i   j  i   1               Compilation Error   It basically means  you cannot modify the value of type object to which the Reference Refers  For Example  Trying to modify value assign 1  of variable j through const reference  i will results in error        assignment of read-only reference    i        icr y              Can change the object it is pointing to so it s not like a const pointer    icr 99    Doesn t change the reference  it assigns the value of the type to which the reference refers  References cannot be made to refer any other variable than the one they are bound to at Initialization   First statement assigns the value y to i Second statement assigns the value 99 to i

User · Answer

The clearest answer  Does    X amp  const x    make any sense   No  it is nonsense To find out what the above declaration means  read it right-to-left     x is a const reference to a X     But that is redundant     references are always const  in the sense that you can never reseat a reference to make it refer to a different object  Never  With or without the const  In other words     X amp  const x    is functionally equivalent to    X amp  x     Since you   re gaining nothing by adding the const after the  amp   you shouldn   t add it  it will confuse people     the const will make some people think that the X is const  as if you had said    const X amp  x

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This code is ill-formed   int amp const icr i    Reference  C  17  dcl ref  1      Cv-qualified references are ill-formed except when the cv-qualifiers are introduced   through the use of a typedef-name or decltype-specifier  in which case the cv-qualifiers are ignored    This rule has been present in all standardized versions of C    Because the code is ill-formed    you should not use it  and there is no associated behaviour     The compiler should reject the program  and if it doesn t  the executable s behaviour is completely undefined   NB  Not sure how none of the other answers mentioned this yet    nobody s got access to a compiler

User · Answer

As it mentioned in another answers  a reference is inherently const   int  amp ref   obj    Once you initialized a reference with an object  you can t unbound this reference with its object  it refers to  A reference works just like an alias   When you declare a  const reference  it is nothing but a reference which refers to a const object   const int  amp ref   obj    The declarative sentences above like  const and int is determining the available features of the object which will be referenced by the reference  To be more clear  I want to show you the pointer equivalent of a const reference   const int  const ptr    amp obj    So the above line of code is equivalent to a const reference in its working way  Additionally  there is a one last point which I want to mention      A reference must be initialized only with an object   So when you do this  you are going to get an error   int   amp r   0     Error  a nonconst reference cannot be initialized to a literal   This rule has one exception  If the reference is declared as const  then you can initialize it with literals as well   const int   amp r   0     a valid approach

User · Answer

First I think int amp const icr i  is just int amp  icr   i  Modifier  const  makes no sense It just means you cannot make the reference refer to other variable    const int x   10     int amp  const y   x     Compiler error here   Second  constant reference just means you cannot change the value of variable through reference   const int x   10  const int amp  y   x    y   20     Compiler error here   Third  Constant references can bind right-value  Compiler will create a temp variable to bind the reference   float x   10  const int amp  y   x  const int amp  z   y   10  cout  lt  lt   long long  amp x  lt  lt  endl    print 348791766212 cout  lt  lt   long long  amp y  lt  lt  endl    print 348791766276 cout  lt  lt   long long  amp z  lt  lt  endl    print 348791766340

User · Answer

The statement icr y  does not make the reference refer to y  it assigns the value of y to the variable that icr refers to  i    References are inherently const  that is you can t change what they refer to  There are  const references  which are really  references to const   that is you can t change the value of the object they refer to  They are declared const int amp  or int const amp  rather than int amp  const though

User · Answer

By  constant reference  I am guessing you really mean  reference to constant data   Pointers on the other hand  can be a constant pointer  the pointer itself is constant  not the data it points to   a pointer to constant data  or both

[c++] What is a constant reference? (not a reference to a constant)

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