How to check for the type of a template parameter

Question

Suppose I have a template function and two classes  class animal     class person      template lt class T gt  void foo       if  T is animal        kill            How do I do the check for T is animal  I don t want to have something that checks during the run time  Thanks

User · Answer

std  is same   is only available since C  11  For pre-C  11 you can use typeid    template  lt typename T gt  void foo         if  typeid T     typeid animal

User · Answer

You can specialize your templates based on what s passed into their parameters like this    template  lt  gt  void foo lt animal gt         Note that this creates an entirely new function based on the type that s passed as T  This is usually preferable as it reduces clutter and is essentially the reason we have templates in the first place

User · Answer

In C  17  we can use variants   To use std  variant  you need to include the header    include  lt variant gt    After that  you may add std  variant in your code like this   using Type   std  variant lt Animal  Person gt    template  lt class T gt  void foo Type type        if  std  is same v lt type  Animal gt                Do stuff          else              Do stuff

User · Answer

I think todays  it is better to use  but only with C  17    include  lt type traits gt   template  lt typename T gt  void foo         if constexpr  std  is same v lt T  animal gt                use type specific operations                If you use some type specific operations in if expression body without constexpr  this code will not compile

User · Answer

Use is same    include  lt type traits gt   template  lt typename T gt  void foo         if  std  is same lt T  animal gt   value                    optimizable        Usually  that s a totally unworkable design  though  and you really want to specialize   template  lt typename T gt  void foo        generic implementation        template  lt  gt  void foo lt animal gt           specific for T   animal        Note also that it s unusual to have function templates with explicit  non-deduced  arguments  It s not unheard of  but often there are better approaches

[c++] How to check for the type of a template parameter?

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