What does do in a C declaration

Question

I am a C guy and I m trying to understand some C   code  I have the following function declaration   int foo const string  amp myname      cout  lt  lt   called foo for     lt  lt  myname  lt  lt  endl    return 0      How does the function signature differ from the equivalent C   int foo const char  myname    Is there a difference between using string  myname vs string  amp myname  What is the difference between  amp  in C   and   in C to indicate pointers   Similarly   const string  amp GetMethodName             What is the  amp  doing here  Is there some website that explains how  amp  is used differently in C vs C

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One way to look at the  amp   reference  operator in c   is that is merely a syntactic sugar to a pointer  For example  the following are roughly equivalent   void foo int  amp x        x   x   1     void foo int  x         x    x   1      The more useful is when you re dealing with a class  so that your methods turn from x- bar   to x bar     The reason I said roughly is that using references imposes additional compile-time restrictions on what you can do with the reference  in order to protect you from some of the problems caused when dealing with pointers  For instance  you can t accidentally change the pointer  or use the pointer in any way other than to reference the singular object you ve been passed

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Your function declares a constant reference to a string   int foo const string  amp myname      cout  lt  lt   called foo for     lt  lt  myname  lt  lt  endl    return 0      A reference has some special properties  which make it a safer alternative to pointers in many ways    it can never be NULL it must always be initialised it cannot be changed to refer to a different variable once set it can be used in exactly the same way as the variable to which it refers  which means you do not need to deference it like a pointer       How does the function signature differ from the equivalent C    int foo const char  myname    There are several differences  since the first refers directly to an object  while const char  must be dereferenced to point to the data      Is there a difference between using string  myname vs string  amp myname    The main difference when dealing with parameters is that you do not need to dereference  amp myname   A simpler example is   int add ptr int  x  int  y        return  x    y    int add ref int  amp x  int  amp y        return x   y      which do exactly the same thing   The only difference in this case is that you do not need to dereference x and y as they refer directly to the variables passed in   const string  amp GetMethodName                What is the  amp  doing here  Is there some website that explains how  amp  is used differently in C vs C      This returns a constant reference to a string   So the caller gets to access the returned variable directly  but only in a read-only sense   This is sometimes used to return string data members without allocating extra memory   There are some subtleties with references - have a look at the C   FAQ on References for some more details

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In this context  amp  is causing the function to take stringname by reference  The difference between references and pointers is    When you take a reference to a variable  that reference is the variable you referenced  You don t need to dereference it or anything  working with the reference is sematically equal to working with the referenced variable itself  NULL is not a valid value to a reference and will result in a compiler error  So generally  if you want to use an output parameter  or a pointer reference in general  in a C   function  and passing a null value to that parameter should be allowed  then use a pointer  or smart pointer  preferably   If passing a null value makes no sense for that function  use a reference  You cannot  re-seat  a reference  While the value of a pointer can be changed to point at something else  a reference has no similar functionality  Once you take a variable by reference  you are effectively dealing with that variable directly  Just like you can t change the value of a by writing b   4   A reference s value is the value of whatever it referenced

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The   amp   denotes a reference instead of a pointer to an object  In your case a constant reference     The advantage of having a function such as   foo string const amp  myname     over   foo string const  myname    is that in the former case you are guaranteed that myname is non-null  since C   does not allow NULL references  Since you are passing by reference  the object is not copied  just like if you were passing a pointer   Your second example   const string  amp GetMethodName             Would allow you to return a constant reference to  for example  a member variable  This is useful if you do not wish a copy to be returned  and again be guaranteed that the value returned is non-null  As an example  the following allows you direct  read-only access   class A     public    int bar   const  return someValue       Big  expensive to copy class    class B   public   A const amp  getA     return mA   private   A mA    void someFunction      B b   B       Access A  ability to call const functions on A    No need to check for null  since reference is guaranteed to be valid   int value   b getA   bar         You have to of course be careful to not return invalid references  Compilers will happily compile the following  depending on your warning level and how you treat warnings   int const amp  foo       int a      This is very bad  returning reference to something on the stack  This will    crash at runtime   return a       Basically  it is your responsibility to ensure that whatever you are returning a reference to is actually valid

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include lt iostream gt  using namespace std  int add int  amp number    int main                  int number              int result              number 5              cout  lt  lt   The value of the variable number before calling the function      lt  lt  number  lt  lt  endl              result add  amp number               cout  lt  lt   The value of the variable number after the function is returned      lt  lt  number  lt  lt  endl              cout  lt  lt   The value of result      lt  lt  result  lt  lt  endl              return 0      int add int  amp p                 p  p 100              return  p       This is invalid code on several counts  Running it through g   gives   crap cpp  In function    int main       crap cpp 11  error  invalid initialization of non-const reference of type    int amp     from a temporary of type    int     crap cpp 3  error  in passing argument 1 of    int add int amp      crap cpp  In function    int add int amp       crap cpp 19  error  invalid type argument of    unary      crap cpp 19  error  invalid type argument of    unary      crap cpp 20  error  invalid type argument of    unary        A valid version of the code reads    include lt iostream gt  using namespace std  int add int  amp number    int main                  int number              int result              number 5              cout  lt  lt   The value of the variable number before calling the function      lt  lt  number  lt  lt  endl              result add number               cout  lt  lt   The value of the variable number after the function is returned      lt  lt  number  lt  lt  endl              cout  lt  lt   The value of result      lt  lt  result  lt  lt  endl              return 0      int add int  amp p                p p 100              return p      What is happening here is that you are passing a variable  as is  to your function  This is roughly equivalent to   int add int  p           p  p 100        return  p      However  passing a reference to a function ensures that you cannot do things like pointer arithmetic with the reference  For example   int add int  amp p                 p  p 100              return p      is invalid   If you must use a pointer to a reference  that has to be done explicitly   int add int  amp p                        int  i    amp p              i i 100L              return  i      Which on a test run gives  as expected  junk output   The value of the variable number before calling the function   5 The value of the variable number after the function is returned   5 The value of result   1399090792

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Here   amp  is not used as an operator  As part of function or variable declarations   amp  denotes a reference  The C   FAQ Lite has a pretty nifty chapter on references

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string   and string amp  differ in a couple of ways  First of all  the pointer points to the address location of the data  The reference points to the data  If you had the following function   int foo string  param1     You would have to check in the function declaration to make sure that param1 pointed to a valid location  Comparatively   int foo string  amp param1     Here  it is the caller s responsibility to make sure the pointed to data is valid  You can t pass a  NULL  value  for example  int he second function above   With regards to your second question  about the method return values being a reference  consider the following three functions   string  amp foo    string  foo    string foo      In the first case  you would be returning a reference to the data  If your function declaration looked like this   string  amp foo         string localString    Hello        return localString      You would probably get some compiler errors  since you are returning a reference to a string that was initialized in the stack for that function  On the function return  that data location is no longer valid  Typically  you would want to return a reference to a class member or something like that   The second function above returns a pointer in actual memory  so it would stay the same  You would have to check for NULL-pointers  though   Finally  in the third case  the data returned would be copied into the return value for the caller  So if your function was like this   string foo         string localString    Hello        return localString      You d be okay  since the string  Hello  would be copied into the return value for that function  accessible in the caller s memory space

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