Use class or typename for template parameters

Question

Possible Duplicate    C   difference of keywords    typename    and    class    in templates       When defining a function template or class template in C    one can write this   template  lt class T gt        or one can write this   template  lt typename T gt        Is there a good reason to prefer one over the other     I accepted the most popular  and interesting  answer  but the real answer seems to be  No  there is no good reason to prefer one over the other     They are equivalent  except as noted below   Some people have reasons to always use typename  Some people have reasons to always use class  Some people have reasons to use both  Some people don t care which one they use    Note  however  that before C  17 in the case of template template parameters  use of class instead of typename was required  See user1428839 s answer below   But this particular case is not a matter of preference  it was a requirement of the language

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As an addition to all above posts  the use of the class keyword is forced  up to and including C  14  when dealing with template template parameters  e g    template  lt template  lt typename  typename gt  class Container  typename Type gt  class MyContainer  public Container lt Type  std  allocator lt Type gt  gt                 In this example  typename Container would have generated a compiler error  something like this   error  expected  class  before  Container

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I prefer to use typename because I m not a fan of overloaded keywords  jeez - how many different meanings does static have for various different contexts

User · Answer

According to Scott Myers  Effective C    3rd ed   item 42  which must  of course  be the ultimate answer   - the difference is  nothing       Advice is to use  class  if it is expected T will always be a class  with  typename   if other types  int  char  whatever  may be expected   Consider it a usage hint

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Just pure history  Quote from Stan Lippman   The reason for the two keywords is historical  In the original template specification  Stroustrup reused the existing class keyword to specify a type parameter rather than introduce a new keyword that might of course break existing programs  It wasn t that a new keyword wasn t considered -- just that it wasn t considered necessary given its potential disruption  And up until the ISO-C   standard  this was the only way to declare a type parameter   But one should use typename rather than class  See the link for more info  but think about the following code  template  lt class T gt  class Demonstration    public  void method        T  A  aObj     oops

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It doesn t matter at all  but class makes it look like T can only be a class  while it can of course be any type  So typename is more accurate  On the other hand  most people use class  so that is probably easier to read generally

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In response to Mike B  I prefer to use  class  as  within a template   typename  has an overloaded meaning  but  class  does not   Take this checked integer type example   template  lt class IntegerType gt  class smart integer   public       typedef integer traits lt Integer gt  traits      IntegerType operator   IntegerType value           typedef typename traits  larger integer t larger t          larger t interm   larger t myValue    larger t value            if interm  gt  traits  max      interm  lt  traits  min                throw overflow            myValue   IntegerType interm             larger integer t is a dependent name  so it requires  typename  to preceed it so that the parser can recognize that larger integer t is a type   class  on the otherhand  has no such overloaded meaning   That    or I m just lazy at heart   I type  class  far more often than  typename   and thus find it much easier to type   Or it could be a sign that I write too much OO code

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There is a difference  and you should prefer class to typename  But why  typename is illegal for template template arguments  so to be consistent  you should use class  template lt template lt class gt  typename MyTemplate  class Bar gt  class Foo                template lt template lt class gt     class MyTemplate  class Bar gt  class Foo

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According to Scott Myers  Effective C    3rd ed   item 42  which must  of course  be the ultimate answer   - the difference is  nothing       Advice is to use  class  if it is expected T will always be a class  with  typename   if other types  int  char  whatever  may be expected   Consider it a usage hint

User · Answer

There is a difference  and you should prefer class to typename  But why  typename is illegal for template template arguments  so to be consistent  you should use class  template lt template lt class gt  typename MyTemplate  class Bar gt  class Foo                template lt template lt class gt     class MyTemplate  class Bar gt  class Foo

User · Answer

According to Scott Myers  Effective C    3rd ed   item 42  which must  of course  be the ultimate answer   - the difference is  nothing       Advice is to use  class  if it is expected T will always be a class  with  typename   if other types  int  char  whatever  may be expected   Consider it a usage hint

User · Answer

Stan Lippman talked about this here   I thought it was interesting   Summary  Stroustrup originally used class to specify types in templates to avoid introducing a new keyword  Some in the committee worried that this overloading of the keyword led to confusion  Later  the committee introduced a new keyword typename to resolve syntactic ambiguity  and decided to let it also be used to specify template types to reduce confusion  but for backward compatibility  class kept its overloaded meaning

User · Answer

As far as I know  it doesn t matter which one you use   They re equivalent in the eyes of the compiler   Use whichever one you prefer   I normally use class

User · Answer

Stan Lippman talked about this here   I thought it was interesting   Summary  Stroustrup originally used class to specify types in templates to avoid introducing a new keyword  Some in the committee worried that this overloading of the keyword led to confusion  Later  the committee introduced a new keyword typename to resolve syntactic ambiguity  and decided to let it also be used to specify template types to reduce confusion  but for backward compatibility  class kept its overloaded meaning

User · Answer

I prefer to use typename because I m not a fan of overloaded keywords  jeez - how many different meanings does static have for various different contexts

User · Answer

As far as I know  it doesn t matter which one you use   They re equivalent in the eyes of the compiler   Use whichever one you prefer   I normally use class

User · Answer

Just pure history  Quote from Stan Lippman   The reason for the two keywords is historical  In the original template specification  Stroustrup reused the existing class keyword to specify a type parameter rather than introduce a new keyword that might of course break existing programs  It wasn t that a new keyword wasn t considered -- just that it wasn t considered necessary given its potential disruption  And up until the ISO-C   standard  this was the only way to declare a type parameter   But one should use typename rather than class  See the link for more info  but think about the following code  template  lt class T gt  class Demonstration    public  void method        T  A  aObj     oops

User · Answer

It doesn t matter at all  but class makes it look like T can only be a class  while it can of course be any type  So typename is more accurate  On the other hand  most people use class  so that is probably easier to read generally

User · Answer

According to Scott Myers  Effective C    3rd ed   item 42  which must  of course  be the ultimate answer   - the difference is  nothing       Advice is to use  class  if it is expected T will always be a class  with  typename   if other types  int  char  whatever  may be expected   Consider it a usage hint

User · Answer

It doesn t matter at all  but class makes it look like T can only be a class  while it can of course be any type  So typename is more accurate  On the other hand  most people use class  so that is probably easier to read generally

User · Answer

As far as I know  it doesn t matter which one you use   They re equivalent in the eyes of the compiler   Use whichever one you prefer   I normally use class

User · Answer

In response to Mike B  I prefer to use  class  as  within a template   typename  has an overloaded meaning  but  class  does not   Take this checked integer type example   template  lt class IntegerType gt  class smart integer   public       typedef integer traits lt Integer gt  traits      IntegerType operator   IntegerType value           typedef typename traits  larger integer t larger t          larger t interm   larger t myValue    larger t value            if interm  gt  traits  max      interm  lt  traits  min                throw overflow            myValue   IntegerType interm             larger integer t is a dependent name  so it requires  typename  to preceed it so that the parser can recognize that larger integer t is a type   class  on the otherhand  has no such overloaded meaning   That    or I m just lazy at heart   I type  class  far more often than  typename   and thus find it much easier to type   Or it could be a sign that I write too much OO code

User · Answer

Stan Lippman talked about this here   I thought it was interesting   Summary  Stroustrup originally used class to specify types in templates to avoid introducing a new keyword  Some in the committee worried that this overloading of the keyword led to confusion  Later  the committee introduced a new keyword typename to resolve syntactic ambiguity  and decided to let it also be used to specify template types to reduce confusion  but for backward compatibility  class kept its overloaded meaning

User · Answer

Stan Lippman talked about this here   I thought it was interesting   Summary  Stroustrup originally used class to specify types in templates to avoid introducing a new keyword  Some in the committee worried that this overloading of the keyword led to confusion  Later  the committee introduced a new keyword typename to resolve syntactic ambiguity  and decided to let it also be used to specify template types to reduce confusion  but for backward compatibility  class kept its overloaded meaning

User · Answer

It doesn t matter at all  but class makes it look like T can only be a class  while it can of course be any type  So typename is more accurate  On the other hand  most people use class  so that is probably easier to read generally

User · Answer

In response to Mike B  I prefer to use  class  as  within a template   typename  has an overloaded meaning  but  class  does not   Take this checked integer type example   template  lt class IntegerType gt  class smart integer   public       typedef integer traits lt Integer gt  traits      IntegerType operator   IntegerType value           typedef typename traits  larger integer t larger t          larger t interm   larger t myValue    larger t value            if interm  gt  traits  max      interm  lt  traits  min                throw overflow            myValue   IntegerType interm             larger integer t is a dependent name  so it requires  typename  to preceed it so that the parser can recognize that larger integer t is a type   class  on the otherhand  has no such overloaded meaning   That    or I m just lazy at heart   I type  class  far more often than  typename   and thus find it much easier to type   Or it could be a sign that I write too much OO code

User · Answer

Extending DarenW s comment   Once typename and class are not accepted to be very different  it might be still valid to be strict on their use  Use class only if is really a class  and typename when its a basic type  such as char   These types are indeed also accepted instead of typename     template lt  char myc           which would be in this case even superior to typename or class    Think of  hintfullness  or intelligibility to other people  And actually consider that 3rd party software scripts might try to use the code information to guess what is happening with the template  consider swig

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Extending DarenW s comment   Once typename and class are not accepted to be very different  it might be still valid to be strict on their use  Use class only if is really a class  and typename when its a basic type  such as char   These types are indeed also accepted instead of typename     template lt  char myc           which would be in this case even superior to typename or class    Think of  hintfullness  or intelligibility to other people  And actually consider that 3rd party software scripts might try to use the code information to guess what is happening with the template  consider swig

User · Answer

As an addition to all above posts  the use of the class keyword is forced  up to and including C  14  when dealing with template template parameters  e g    template  lt template  lt typename  typename gt  class Container  typename Type gt  class MyContainer  public Container lt Type  std  allocator lt Type gt  gt                 In this example  typename Container would have generated a compiler error  something like this   error  expected  class  before  Container

User · Answer

I prefer to use typename because I m not a fan of overloaded keywords  jeez - how many different meanings does static have for various different contexts

User · Answer

In response to Mike B  I prefer to use  class  as  within a template   typename  has an overloaded meaning  but  class  does not   Take this checked integer type example   template  lt class IntegerType gt  class smart integer   public       typedef integer traits lt Integer gt  traits      IntegerType operator   IntegerType value           typedef typename traits  larger integer t larger t          larger t interm   larger t myValue    larger t value            if interm  gt  traits  max      interm  lt  traits  min                throw overflow            myValue   IntegerType interm             larger integer t is a dependent name  so it requires  typename  to preceed it so that the parser can recognize that larger integer t is a type   class  on the otherhand  has no such overloaded meaning   That    or I m just lazy at heart   I type  class  far more often than  typename   and thus find it much easier to type   Or it could be a sign that I write too much OO code

User · Answer

I prefer to use typename because I m not a fan of overloaded keywords  jeez - how many different meanings does static have for various different contexts

User · Answer

As far as I know  it doesn t matter which one you use   They re equivalent in the eyes of the compiler   Use whichever one you prefer   I normally use class

[c++] Use 'class' or 'typename' for template parameters?

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