C Double Address Operator

Question

I m reading STL source code and I have no idea what  amp  amp  address operator is supposed to do  Here is a code example from stl vector h   vector amp  operator  vector amp  amp    x      lt -- Note double ampersands here          NB  DR 675      this- gt clear        this- gt swap   x        return  this      Does  Address of Address  make any sense  Why does it have two address operators instead of just one

User · Answer

As other answers have mentioned  the  amp  amp  token in this context is new to C  0x  the next C   standard  and represent an  rvalue reference    Rvalue references are one of the more important new things in the upcoming standard  they enable support for  move  semantics on objects and permit perfect forwarding of function calls   It s a rather complex topic - one of the best introductions  that s not merely cursory  is an article by Stephan T  Lavavej   Rvalue References  C  0x Features in VC10  Part 2   Note that the article is still quite heavy reading  but well worthwhile   And even though it s on a Microsoft VC   Blog  all  or nearly all  the information is applicable to any C  0x compiler

User · Answer

I believe that is is a move operator  operator  is the assignment operator  say vector x   vector y  The clear   function call sounds like as if it is deleting the contents of the vector to prevent a memory leak  The operator returns a pointer to the new vector   This way   std  vector lt int gt  a 100  10   std  vector lt int gt  b   a  for unsigned int i   0  i  lt  b size    i          std  cout  lt  lt  b i   lt  lt           Even though we gave vector a values  vector b has the values  It s the magic of the operator      MSDN -- How to create a move constructor

User · Answer

This is C  11 code   In C  11  the  amp  amp  token can be used to mean an  rvalue reference

User · Answer

amp  amp  is new in C  11  and it signifies that the function accepts an RValue-Reference -- that is  a reference to an argument that is about to be destroyed

User · Answer

amp  amp  is new in C  11  int amp  amp  a means  a  is an r-value reference   amp  amp  is normally only used to declare a parameter of a function  And it only takes a r-value expression  If you don t know what an r-value is  the simple explanation is that it doesn t have a memory address  E g  the number 6  and character  v  are both r-values  int a  a is an l-value  however  a 2  is an r-value  For example   void foo int amp  amp  a          Some magical code       int main         int b      foo b     Error  An rValue reference cannot be pointed to a lValue      foo 5     Compiles with no error      foo b 3     Compiles with no error       int amp  amp  c   b    Error  An rValue reference cannot be pointed to a lValue      int amp  amp  d   5    Compiles with no error      Hope that is informative

[c++] C++ Double Address Operator? (&&)

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