How Does C Template Deduction Determine Specialization Using Partial Ordering?

Partial Ordering Procedure in Template Deduction

The partial ordering procedure determines the specialization relationship between function templates in C template deduction. It involves creating transformed function types for each template and using them for comparison.

1. Transformed Function Types:
   For each template, a transformed function type is created by substituting unique types for template parameters in its function type.

2. Matching Arguments and Parameters:
   The transformed function type of one template is matched against the original function template of the other template, using the following two cases:

   • Transformed type-1 as argument template and original template-2 as parameter template
   • Transformed type-2 as argument template and original template-1 as parameter template
3. Match Success and Ordering:
   If type deduction can successfully match the transformed function type to the original function template in either case, one template is more specialized than the other. If a successful match occurs in both directions, neither template is more specialized.

Example:

Consider these two function templates:

template<typename T, typename U>
void foo(T, U); // original #1

template<typename T>
void foo(T const*, X<T>); // original #2

Matching the transformed types:

• 1b vs. #2: Type deduction cannot match parameters.

• 2b vs. #1: Type deduction can match parameters (T = char const*, U = X).

Conclusion: Overload #2 is more specialized than #1.

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