Difference between const reference and normal parameter

Question

void DoWork int n   void DoWork const int  amp n     What s the difference

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With   void DoWork int n     n is a copy of the value of the actual parameter  and it is legal to change the value of n within the function  With   void DoWork const int  amp n     n is a reference to the actual parameter  and it is not legal to change its value

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The difference is more prominent when you are passing a big struct class   struct MyData       int a b c d e f g h      long array 1234      void DoWork MyData md   void DoWork const MyData amp  md     when you use use  normal  parameter  you pass the parameter by value and hence creating a copy of the parameter you pass  if you are using const reference  you pass it by reference and the original data is not copied   in both cases  the original data cannot be modified from inside the function   EDIT  In certain cases  the original data might be able to get modified as pointed out by Charles Bailey in his answer

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The first method passes n by value  i e  a copy of n is sent to the function  The second one passes n by reference which basically means that a pointer to the n with which the function is called is sent to the function   For integral types like int it doesn t make much sense to pass as a const reference since the size of the reference is usually the same as the size of the reference  the pointer   In the cases where making a copy is expensive it s usually best to pass by const reference

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There are three methods you can pass values in the function    Pass by value  void f int n       n   n   10     int main        int x   3      f x       cout  lt  lt  x  lt  lt  endl      Output  3  Disadvantage  When parameter x pass through f function then compiler creates a copy in memory in of x  So wastage of memory  Pass by reference   void f int amp  n       n   n   10     int main        int x   3      f x       cout  lt  lt  x  lt  lt  endl      Output  13  It eliminate pass by value disadvantage  but if programmer do not want to change the value then use constant reference Constant reference  void f const int amp  n       n   n   10     Error  assignment of read-only reference     n       int main        int x   3      f x       cout  lt  lt  x  lt  lt  endl      Output  Throw error at n   n   10 because when we pass const reference parameter argument then it is read-only parameter  you cannot change value of n

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The important difference is that when passing by const reference  no new object is created  In the function body  the parameter is effectively an alias for the object passed in   Because the reference is a const reference the function body cannot directly change the value of that object  This has a similar property to passing by value where the function body also cannot change the value of the object that was passed in  in this case because the parameter is a copy   There are crucial differences  If the parameter is a const reference  but the object passed it was not in fact const then the value of the object may be changed during the function call itself   E g   int a   void DoWork const int  amp n        a   n   2      If n was a reference to a  n will have been doubled       f        Might change the value of whatever n refers to     int main         DoWork a       Also if the object passed in was not actually const then the function could  even if it is ill advised  change its value with a cast   e g   void DoWork const int  amp n        const cast lt int amp  gt  n    22      This would cause undefined behaviour if the object passed in was actually const   When the parameter is passed by const reference  extra costs include dereferencing  worse object locality  fewer opportunities for compile optimizing   When the parameter is passed by value and extra cost is the need to create a parameter copy  Typically this is only of concern when the object type is large

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Firstly  there is no concept of cv-qualified references  So the terminology  const reference  is not correct and is usually used to describle  reference to const   It is better to start talking about what is meant       8 3 2 1-  Cv-qualified references are ill-formed except when the   cv-qualifiers are introduced through the use of a typedef  7 1 3  or   of a template type argument  14 3   in which case the cv-qualifiers   are ignored     Here are the differences      13 1 -  Only the const and volatile type-specifiers at the outermost   level of the parameter type specification are ignored in this fashion    const and volatile type-specifiers buried within a parameter type   specification are significant and can be used to distinguish   overloaded function declarations 112   In particular  for any type T       pointer to T        pointer to const T     and    pointer to volatile T    are   considered distinct parameter types  as are    reference to T          reference to const T     and    reference to volatile T       void f int  amp n      cout  lt  lt  1      n       void f int const  amp n      cout  lt  lt  2       n       Error   Non modifiable lvalue    int main       int x   2      f x             Calls overload 1  after the call x is 3    f 2             Calls overload 2    f 2 2           Calls overload 2  a temporary of double is created  8 5 3

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Since none of you mentioned nothing about the const keyword        The const keyword modifies the type of a type declaration or the type of a function parameter  preventing the value from varying   Source  MS    In other words  passing a parameter by reference exposes it to modification by the callee  Using the const keyword prevents the modification

[c++] Difference between const reference and normal parameter

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