What Constitutes ODR-Use in C Templates and How Does It Relate to Instantiation?

Understanding the Concept of "odr-use"

In the realm of C programming, a crucial aspect related to the One Definition Rule (ODR) is "odr-use." This definition signifies when a particular entity, such as a member function within a class template, must have a corresponding definition provided in the code.

To clarify, mere declaration is not sufficient; an entity must be explicitly used in a way that triggers ODR-use. This concept arises from the fact that "use" can be interpreted differently depending on the context. Hence, the standard employs the term "odr-use" to convey a specific requirement.

Notable to mention, the existence of a pure virtual function automatically results in ODR-use, regardless of whether it is actually utilized within the codebase. This behavior stems from the unique properties associated with pure virtual functions.

ODR-Use and Template Instantiation

In the arena of templates, ODR-use plays a pivotal role alongside the process of instantiation. Section 14.7 of the C standard sheds light on the conditions that necessitate template instantiation. However, a fundamental principle that emerges is the direct correlation between template instantiation and ODR-use.

To explicate further, a template is instantiated only when it is employed within the code. "Employed" in this context refers specifically to ODR-use. Consequently, a member function of a class template is instantiated either when it is invoked or when the class itself is instantiated and the member function is designated as virtual.

By embracing this principle, the standard empowers developers to instantiate a list over an element type that lacks support for the less-than operator (<) while precluding the use of the sort method on that specific list. This approach ensures that the underlying element type does not pose a hindrance to the instantiation of the list itself.

The above is the detailed content of What Constitutes ODR-Use in C Templates and How Does It Relate to Instantiation?. For more information, please follow other related articles on the PHP Chinese website!