queryinterface

QueryInterface is a C++ template class found in the Qt framework. It provides a mechanism for objects to expose different functionalities or interfaces. When an object supports multiple interfaces, QueryInterface allows other objects to query for the presence of a specific interface and obtain a pointer to it if it is supported. This is a form of runtime polymorphism, enabling objects to dynamically reveal capabilities.

The primary use case for QueryInterface is in scenarios where an object might implement a set of distinct behaviors, and clients need to interact with these behaviors without knowing the object's exact concrete type at compile time. For example, a graphical widget might implement interfaces for rendering, event handling, and data manipulation. A client could then use QueryInterface to ask if the widget supports the rendering interface and, if so, get a pointer to that specific rendering functionality.

This pattern is analogous to the QueryInterface method in COM (Component Object Model). In Qt, QueryInterface is typically implemented by inheriting from QObject and using the meta-object system. Objects that wish to expose custom interfaces often define a specific method, usually named `queryInterface`, that returns a pointer to the requested interface type or null if the interface is not supported. This allows for flexible and extensible object designs within the Qt ecosystem.