Difference of keywords typename and class in templates

Question

For templates I have seen both declarations   template  lt  typename T  gt  template  lt  class T  gt    What s the difference    And what exactly do those keywords mean in the following example  taken from the German Wikipedia article about templates    template  lt  template  lt  typename  typename  gt  class Container  typename Type  gt  class Example        Container lt  Type  std  allocator  lt  Type  gt   gt  baz

User · Answer

There is no difference between using OR ; i.e. it is a convention used by C++ programmers. I myself prefer as it more clearly describes it use; i.e. defining a template with a specific type :)

Note: There is one exception where you do have to use class (and not typename) when declaring a template template parameter:

template <template class T> class C { }; // valid!

template <template typename T> class C { }; // invalid!

In most cases, you will not be defining a nested template definition, so either definition will work -- just be consistent in your use...

User · Answer

typename and class are interchangeable in the basic case of specifying a template   template lt class T gt  class Foo        and  template lt typename T gt  class Foo        are equivalent   Having said that  there are specific cases where there is a difference between typename and class   The first one is in the case of dependent types  typename is used to declare when you are referencing a nested type that depends on another template parameter  such as the typedef in this example   template lt typename param t gt  class Foo       typedef typename param t  baz sub t       The second one you actually show in your question  though you might not realize it   template  lt  template  lt  typename  typename  gt  class Container  typename Type  gt    When specifying a template template  the class keyword MUST be used as above -- it is not interchangeable with typename in this case  note  since C  17 both keywords are allowed in this case    You also must use class when explicitly instantiating a template   template class Foo lt int gt     I m sure that there are other cases that I ve missed  but the bottom line is  these two keywords are not equivalent  and these are some common cases where you need to use one or the other

User · Answer

While there is no technical difference  I have seen the two used to denote slightly different things   For a template that should accept any type as T  including built-ins  such as an array    template lt typename T gt  class Foo           For a template that will only work where T is a real class   template lt class T gt  class Foo           But keep in mind that this is purely a style thing some people use  Not mandated by the standard or enforced by compilers

User · Answer

For naming template parameters  typename and class are equivalent    14 1 2      There is no semantic difference   between class and typename in a   template-parameter    typename however is possible in another context when using templates - to hint at the compiler that you are referring to a dependent type    14 6 2      A name used in a template declaration   or definition and that is dependent on   a template-parameter is assumed not to   name a type unless the applicable name   lookup finds a type name or the name   is qualified by the keyword typename    Example   typename some template lt T gt   some type   Without typename the compiler can t tell in general whether you are referring to a type or not

User · Answer

This piece of snippet is from c   primer book  Although I am sure this is wrong   Each type parameter must be preceded by the keyword class or typename      error  must precede U with either typename or class template  lt typename T  U gt  T calc const T amp   const U amp      These keywords have the same meaning and can be used interchangeably inside a template parameter list  A template parameter list can use both keywords      ok  no distinction between typename and class in a template parameter list template  lt typename T  class U gt  calc  const T amp   const U amp      It may seem more intuitive to use the keyword typename rather than class to designate a template type parameter  After all  we can use built-in  nonclass  types as a template type argument  Moreover  typename more clearly indicates that the name that follows is a type name  However  typename was added to C   after templates were already in widespread use  some programmers continue to use class exclusively

User · Answer

No difference Template type parameter Container is itself a template with two type parameters

[c++] Difference of keywords 'typename' and 'class' in templates?

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