What is the difference between typedef and using in C 11

Question

I know that in C  11 we can now use using to write type alias  like typedefs  typedef int MyInt   Is  from what I understand  equivalent to  using MyInt   int   And that new syntax emerged from the effort to have a way to express  quot template typedef quot   template lt  class T  gt  using MyType   AnotherType lt  T  MyAllocatorType  gt    But  with the first two non-template examples  are there any other subtle differences in the standard  For example  typedefs do aliasing in a  quot weak quot  way  That is it does not create a new type but only a new name  conversions are implicit between those names   Is it the same with using or does it generate a new type  Are there any differences

User · Answer

They are essentially the same but using provides alias templates which is quite useful  One good example I could find is as follows   namespace std    template lt typename T gt  using add const t   typename add const lt T gt   type      So  we can use std  add const t lt T gt  instead of typename std  add const lt T gt   type

User · Answer

The using syntax has an advantage when used within templates  If you need the type abstraction  but also need to keep template parameter to be possible to be specified in future  You should write something like this   template  lt typename T gt  struct whatever      template  lt typename T gt  struct rebind     typedef whatever lt T gt  type     to make it possible to substitue the whatever in future      rebind lt int gt   type variable   template  lt typename U gt  struct bar   typename rebind lt U gt   type  var member      But using syntax simplifies this use case   template  lt typename T gt  using my type   whatever lt T gt    my type lt int gt  variable  template  lt typename U gt  struct baz   my type lt U gt   var member

User · Answer

All standard references below refers to N4659  March 2017 post-Kona working draft C  17 DIS   Typedef declarations can  whereas alias declarations cannot  be used as initialization statements  But  with the first two non-template examples  are there any other subtle differences in the standard    Differences in semantics  none  Differences in allowed contexts  some 1     1  In addition to the examples of alias templates  which has already been mentioned in the original post  Same semantics As governed by  dcl typedef  2  extract  emphasis mine    dcl typedef  2 A typedef-name can also be introduced by an alias-declaration  The identifier following the using keyword becomes a typedef-name and the optional attribute-specifier-seq following the identifier appertains to that typedef-name  Such a typedef-name has the same semantics as if it were introduced by the typedef specifier         a typedef-name introduced by an alias-declaration has the same semantics as if it were introduced by the typedef declaration  Subtle difference in allowed contexts However  this does not imply that the two variations have the same restrictions with regard to the contexts in which they may be used  And indeed  albeit a corner case  a typedef declaration is an init-statement and may thus be used in contexts which allow initialization statements    C  11  C  03   init  statement in for loop iteration statements   for typedef int Foo  Foo      0          C  17  if and switch initialization statements   if  typedef int Foo  true     void Foo                          init-statement  switch typedef int Foo  0    case 0   void Foo                             init-statement     C  20  range-based for loop initialization statements   std  vector lt int gt  v 1  2  3   for typedef int Foo  Foo f   v     void f                        init-statement  for typedef struct   int x  int y   P                                        init-statement     auto  x  y     P 1  1    1  2    3  5       void x   void y     whereas an alias-declaration is not an init-statement  and thus may not be used in contexts which allows initialization statements    C   11  for using Foo   int  Foo      0                          error  expected expression     C  17  initialization expressions in switch and if statements   if  using Foo   int  true     void Foo                          error  expected expression  switch using Foo   int  0    case 0   void Foo                             error  expected expression     C  20  range-based for loop initialization statements   std  vector lt int gt  v 1  2  3   for using Foo   int  Foo f   v     void f                        error  expected expression

User · Answer

Both keywords are equivalent  but there are a few caveats  One is that declaring a function pointer with using T   int     int  int   is clearer than with typedef int   T  int  int    Second is that template alias form is not possible with typedef  Third is that exposing C API would require typedef in public headers

User · Answer

I know the original poster has a great answer  but for anyone stumbling on this thread like I have there s an important note from the proposal that I think adds something of value to the discussion here  particularly to concerns in the comments about if the typedef keyword is going to be marked as deprecated in the future  or removed for being redundant old      It has been suggested to  re use the keyword typedef     to introduce template aliases   template lt class T gt    typedef std  vector lt T  MyAllocator lt T gt   gt  Vec        That notation has the advantage of using a keyword already known to introduce a type alias  However  it also displays several disavantages  sic  among which the confusion of using a keyword known to introduce an alias for a type-name in a context where the alias does not designate a type  but a template  Vec is not an alias for a type  and should not be taken for a typedef-name  The name Vec is a name for the family std  vector lt      MyAllocator lt     gt   gt      where the bullet is a placeholder for a type-name Consequently we do not propose the    typedef    syntax On the other hand the sentence  template lt class T gt    using Vec   std  vector lt T  MyAllocator lt T gt   gt         can be read interpreted as  from now on  I   ll be using Vec lt T gt  as a synonym for std  vector lt T  MyAllocator lt T gt   gt   With that reading  the new syntax for aliasing seems reasonably logical    To me  this implies continued support for the typedef keyword in C   because it can still make code more readable and understandable   Updating the using keyword was specifically for templates  and  as was pointed out in the accepted answer  when you are working with non-templates using and typedef are mechanically identical  so the choice is totally up to the programmer on the grounds of readability and communication of intent

User · Answer

They are largely the same  except that      The alias declaration is compatible with templates  whereas the C   style typedef is not

User · Answer

They are equivalent  from the standard  emphasis mine   7 1 3 2       A typedef-name can also be introduced by an alias-declaration  The   identifier following the using keyword becomes a typedef-name and the   optional attribute-specifier-seq following the identifier appertains   to that typedef-name  It has the same semantics as if it were   introduced by the typedef specifier  In particular  it   does not define a new type and it shall not appear in the type-id

[c++] What is the difference between 'typedef' and 'using' in C++11?

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