Why can t I make a vector of references

Question

When I do this   std  vector lt int gt  hello    Everything works great  However  when I make it a vector of references instead   std  vector lt int  amp  gt  hello    I get horrible errors like      error C2528   pointer    pointer to reference is illegal   I want to put a bunch of references to structs into a vector  so that I don t have to meddle with pointers  Why is vector throwing a tantrum about this  Is my only option to use a vector of pointers instead

User · Answer

boost  ptr vector lt int gt  will work   Edit  was a suggestion to use std  vector lt  boost  ref lt int gt   gt   which will not work because you can t default-construct a boost  ref

User · Answer

TL  DR  Use std  reference wrapper like this    include  lt functional gt   include  lt string gt   include  lt vector gt   include  lt iostream gt   int main         std  string hello    Hello         std  string world    everyone        typedef std  vector lt std  reference wrapper lt std  string gt  gt  vec t      vec t vec    hello  world       vec 1  get      world        std  cout  lt  lt  hello  lt  lt  world  lt  lt  std  endl      return 0      Demo  Long answer  As standard suggests  for a standard container X containing objects of type T  T must be Erasable from X    Erasable means that the following expression is well formed   allocator traits lt A gt   destroy m  p    A is container s allocator type  m is allocator instance and p is a pointer of type  T  See here for Erasable definition   By default  std  allocator lt T gt  is used as vector s allocator  With the default allocator  the requirement is equivalent to the validity of p- gt  T    Note the T is a reference type and p is pointer to a reference   However  pointer to a reference is illegal  hence the expression is not well formed

User · Answer

The component type of containers like vectors must be assignable  References are not assignable  you can only initialize them once when they are declared  and you cannot make them reference something else later   Other non-assignable types are also not allowed as components of containers  e g  vector lt const int gt  is not allowed

User · Answer

By their very nature  references can only be set at the time they are created  i e   the following two lines have very different effects   int  amp  A   B       makes A an alias for B A   C             assigns value of C to B    Futher  this is illegal   int  amp  D           must be set to a int variable    However  when you create a vector  there is no way to assign values to it s items at creation   You are essentially just making a whole bunch of the last example

User · Answer

As other have mentioned  you will probably end up using a vector of pointers instead   However  you may want to consider using a ptr vector instead

User · Answer

yes you can  look for std  reference wrapper  that mimics a reference but is assignable and also can be  reseated

User · Answer

It s a flaw in the C   language   You can t take the address of a reference  since attempting to do so would result in the address of the object being referred to  and thus you can never get a pointer to a reference   std  vector works with pointers to its elements  so the values being stored need to be able to be pointed to   You ll have to use pointers instead

User · Answer

As the other comments suggest  you are confined to using pointers  But if it helps  here is one technique to avoid facing directly with pointers   You can do something like the following   vector lt int  gt  iarray  int default item   0     for handling out-of-range exception  int amp  get item as ref unsigned int idx          handling out-of-range exception    if idx  gt   iarray size           return default item     return reinterpret cast lt int amp  gt   iarray idx

User · Answer

Ion Todirel already mentioned an answer YES using std  reference wrapper  Since C  11 we have a mechanism to retrieve object from std  vector  and remove the reference by using std  remove reference  Below is given an example compiled using g   and clang with option -std c  11 and executed successfully    include  lt iostream gt   include  lt vector gt   include lt functional gt   class MyClass   public      void func             std  cout  lt  lt   I am func  n              MyClass int y    x y          int getval                 return x         private           int x      int main         std  vector lt std  reference wrapper lt MyClass gt  gt  vec       MyClass obj1 2       MyClass obj2 3        MyClass amp  obj ref1   std  ref obj1       MyClass amp  obj ref2   obj2       vec push back obj ref1       vec push back obj ref2        for  auto obj3   vec                std  remove reference lt MyClass amp  gt   type obj3  func                  std  cout  lt  lt  std  remove reference lt MyClass amp  gt   type obj3  getval    lt  lt    n

[c++] Why can't I make a vector of references?

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