Select template argument at runtime in C++

Suppose I have a set of functions and classes which are templated to use single (float) or double precision. Of course I could write just two pieces of bootstrap code, or mess with macros. But can I just switch template argument at runtime?

From stackoverflow

• Templates are a compile-time mechanism. BTW, macros are as well (strictly speaking - a preprocessing mechanism - that comes even before compilation).

• No, you can't switch template arguments at runtime, since templates are instantiated by the compiler at compile-time. What you can do is have both templates derive from a common base class, always use the base class in your code, and then decide which derived class to use at runtime:

  class Base
  {
     ...
  };
  
  template <typename T>
  class Foo : public Base
  {
      ...
  };
  
  Base *newBase()
  {
      if(some condition)
          return new Foo<float>();
      else
          return new Foo<double>();
  }
  

  Macros have the same problem as templates, in that they are expanded at compile-time.

• Templates are purely a compile time construct, the compiler will expand a template and create your class/function with the specified argument and directly translate that to code.

  If you are trying to switch between foo<float> and foo<double> at runtime, you'll either need to use some metaprogramming trickery or just have seperate code paths for each.