Is it possible to completely turn off part of the code of a template class method in C++?

Question

Programming

Good day.
I have been developing in C++ for a long time, I don’t experience any special problems, but I ran into a problem that I don’t understand how to solve, although it should be easy to solve. Please tell me if there is any way to completely disable the generation of a certain section of code in the template function (as it would be when using the preprocessor). Example:

template <classT, bool alt = false>
Shablon
{

  void foo()
  {
      if (alt)
      {
         ((T*)this)->m_member.foo(); // ошибка компиляции если alt == false, и у класса T нет члена с методом foo.
         ((T*)this)->someFoo();
      }
      ((T*)this)->someFoo2();
  }

}

Answer 1

In C++, you can't just take it so that everything is fine, if without macros, of course. Simple solution with enable_if:

#include <boost/utility/enable_if.hpp>

template <class T, bool alt = false>
struct Shablon
{
  template <bool C>
  typename boost::enable_if_c<C>::type doSomething() {
    ((T*)this)->m_member.foo();
    ((T*)this)->someFoo();
  }

  template <bool C>
  typename boost::disable_if_c<C>::type doSomething() {
  }

  void foo()
  {
    doSomething<alt>();
    ((T*)this)->foo2();
  }
};

Answer 2

You can use template specialization, in your case partial specialization, and write the necessary code there. Since you want (so simple) it will not work.
You can also make enable_if on foo a function within the same class.

Answer 3

It seems to me that you need SFINAE, there are articles on the wiki and habré. I advise you to use type_traits. Visual c++ has __if_exists, but it's not cross-platform.

Answer 4

Look aside en.cppreference.com/w/cpp/types
The main mechanisms are: template specializations and SFINAE.
Checking for the existence of a method can only be done with SFINAE (enable_if is based on SFINAE). In general, checking for the presence of a method by templates may often not be the best solution, because there may be inheritance.
But in general, take a look: stackoverflow.com/questions/87372/check-if-a-class-has-a-member-function-of-a-given-signature for a specific method existence check.

Answer 5

In your case, you can just use a partial specialization of your class, or make an additional template class that runs the required methods, and specialize only it (something like this paste.kde.org/p29e289bb/ ). You can go further and use SFINAE to determine if the given methods are in class members, then you can hide your optional alt template parameter.

Answer 6

template <int v>
struct Int2Type
{
        enum { value = v };
};

template <classT, bool alt = false>
class Shablon
{
  void doSomething(Int2Type<true>)
  {
     ((T*)this)->m_member.foo();
     ((T*)this)->someFoo();
  }

  void doSomething(Int2Type<false>)
  {
  }

  void foo()
  {
      DoSomething(alt);

      ((T*)this)->someFoo2();
  }
};

Answer 7

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