Template Constraints

The fact that templates can be instantiated with any argument yet not every argument is compatible with every template brings an inconvenience. If a template argument is not compatible with a particular template, the incompatibility is necessarily detected during the compilation of the template code for that argument. As a result, the compilation error points at a line inside the template implementation:

struct A
{
    int doSomething() { return 42; }
}

void fun(T)(T a)
{
    a.doSomething();    // <--- error here
}

void main()
{
    A a;
    int b;
    fun(a);
    fun(b);
}

Compiling this code will issue an error due to the fact that int does not have a method doSomething. The error will be issued after the code for fun!int was generated, during the semantical analysis phase, at the call to doSomething. Imagine that fun was defined in a library and the user does not have access to the source code; in this situation it is hard to spot the exact bug, however using template constraints eases this process:

struct A
{
    int doSomething() { return 42; }
}

void fun(T)(T a)
if(is(typeof(a.doSomething)))    // <--- error here
{
    a.doSomething();
}

void main()
{
    A a;
    int b;
    fun(a);
}

Template constraints have the same syntax as an if statement (without else).

Practice

1. Implement a generic partitioning search algorithm.
   • The algorithm will receive an array/associative array and an element and returns the number of elements that are lesser than the element in the array/associative array list of keys.
   • The element type may be struct, class or builtin type.
   • Use templated function(s) and template constraints or static ifs to implement the various cases.
   • Use these helper traits.
2. Follow this link. That is the official D language standard library implementation of the ordered function. Read the documentation and the unittests, then try to understand the implementation. This is how real life meta-programming looks like. Ask the lab rats about clarifications on any misunderstandings.