error invalid initialization of non-const reference of type int from an rvalue of type int

Question

Wrong form     int  amp z   12    Correct form   int y  int  amp r   y    Question  Why is the first code wrong  What is the  meaning  of the error in the title

User · Answer

12 is a compile-time constant which can not be changed unlike the data referenced by int amp   What you can do is  const int amp  z   12

User · Answer

Non-const and const reference binding follow different rules  These are the rules of the C   language    an expression consisting of a literal number  12  is a  rvalue  it is not permitted to create a non-const reference with a rvalue  int  amp ri   12  is ill-formed it is permitted to create a const reference with a rvalue  in this case  an unnamed object is created by the compiler  this object will persist as long as the reference itself exist    You have to understand that these are C   rules  They just are   It is easy to invent a different language  say C     with slightly different rules  In C     it would be permitted to create a non-const reference with a rvalue  There is nothing inconsistent or impossible here   But it would allow some risky code where the programmer might not get what he intended  and C   designers rightly decided to avoid that risk

User · Answer

C  03 3 10 1 says   Every expression is either an lvalue or an rvalue    It s important to remember that lvalueness versus rvalueness is a property of expressions  not of objects   Lvalues name objects that persist beyond a single expression   For example  obj    ptr   ptr index    and   x are all lvalues   Rvalues are temporaries that evaporate at the end of the full-expression in which they live   at the semicolon     For example  1729   x   y   std  string  meow     and x   are all rvalues   The address-of operator requires that its  operand shall be an lvalue   if we could take the address of one expression  the expression is an lvalue  otherwise it s an rvalue      amp obj      valid   amp 12     invalid

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References are  hidden pointers   non-null  to things which can change  lvalues   You cannot define them to a constant  It should be a  variable  thing   EDIT    I am thinking of  int  amp x   y    as almost equivalent of  int    px    amp y   define x     px    where   px is a fresh name  and the  define x works only inside the block containing the declaration of x reference

User · Answer

int  amp z   12   On the right hand side  a temporary object of type int is created from the integral literal 12  but the temporary cannot be bound to non-const reference  Hence the error  It is same as  int  amp z   int 12     still same error  Why a temporary gets created  Because a reference has to  refer to an object in the memory  and for an object to exist  it has to be created first  Since the object is unnamed  it is a temporary object  It has no name  From this explanation  it became pretty much clear why the second case is fine  A temporary object can be bound to const reference  which means  you can do this  const int  amp z   12    ok   C  11 and Rvalue Reference  For the sake of the completeness  I would like to add that C  11 has introduced rvalue-reference  which can bind to temporary object  So in C  11  you can write this  int  amp  amp  z   12    C 11 only   Note that there is  amp  amp  intead of  amp   Also note that const is not needed anymore  even though the object which z binds to is a temporary object created out of integral-literal 12  Since C  11 has introduced rvalue-reference  int amp  is now henceforth called lvalue-reference

[c++] error: invalid initialization of non-const reference of type ‘int&’ from an rvalue of type ‘int’

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