How Does C Determine Partial Ordering in Template Deduction?

Partial Ordering Procedure in Template Deduction

When determining the partial ordering of templates for type deduction, the C standard employs a specific procedure involving the comparison of transformed function types. For each template involved, the original function type is transformed by replacing each template parameter with a unique, fictitious type. This transformed function type is then used as the argument template in the deduction process, while the original function type of the other template serves as the parameter template.

This comparison is performed in both directions:

• Using the transformed type of the first template as the argument template and the original type of the second template as the parameter template.
• Using the transformed type of the second template as the argument template and the original type of the first template as the parameter template.

If a match can be established in one direction but not the other, then one template is determined to be more specialized than the other. Otherwise, neither template is considered more specialized.

This procedure aids in resolving ambiguities in template deduction, where multiple templates may seem equally applicable. By comparing the transformed types, the compiler can determine which template can be more precisely matched to the specific arguments being passed.

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