Template basics

As a barebones definition, we might say that a template is a block of code that doesn't exist until it is used. A template can be declared in a source module, but if it is never instantiated, it doesn't get compiled into the final binary. Further, there are different ways to declare a template and several ways to control how it is compiled into the binary. In this section, we're going to explore the former.

Templates as code blocks

A template declaration looks somewhat like a function declaration. It opens with the template keyword, followed by an identifier, a parameter list, and then a pair of braces for the body. The body may contain any valid D declaration except module declarations, as the following example demonstrates:

template MyTemplate(T) {
  T val;
  void printVal() {
    import std.stdio : writeln;
    writeln("The type is ", typeid(T));
    writeln("The value is ", val);
  }
}

The first line declares a template named MyTemplate that takes one parameter, T. This isn...