When Should You Use `class` vs. `typename` for Template Parameters?

Choosing between 'class' and 'typename' for Template Parameters

In C , when declaring a function or class template, one can use either class T or typename T to parameterize the template. This raises the question: are there any compelling reasons to prefer one form over the other?

Equivalence and Syntax

Both class T and typename T are generally equivalent from a compiler's perspective. However, starting with C 17, the keyword typename is strictly recommended for use when specifying nested template template parameters, effectively replacing the previous requirement to use class in such cases.

Usage Preferences

Despite their underlying equivalence, some programmers prefer to use typename or class consistently for various reasons:

• TypeName Preference:

  • Enhances clarity by explicitly indicating that the parameter is a type.

• Class Preference:

  • Maintains compatibility with legacy code that predates the introduction of typename.

• Dual Usage:

  • Some programmers use typename for template types and class for template template parameters, which aligns with the historical naming convention.

• No Preference:

  • Others simply don't find any meaningful distinction between the two forms.

Historical Background

The distinction between class and typename emerged in the evolution of C templates. Initially, only class was used, but concerns about potential confusion led to the creation of typename as a separate keyword specifically for specifying template types. However, to avoid breaking backward compatibility, class continued to be allowed for this purpose.

In conclusion, while both class and typename can be used interchangeably for most practical purposes, the choice between them often comes down to personal preference or adherence to certain conventions. The most recent C 17 revision clarifies the preferred usage of typename for nested template template parameters, ensuring a consistent and unambiguous syntax for these complex constructs.

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