10/31/2022 0 Comments Rust any downcast![]() ![]() In C++ and similar languages, the compiler does this by creating a virtual method table (often called a virtual table or even just a vtable) and adding a hidden member variable to the beginning of BaseClass that points to the vtable. When a function call is made on a polymorphic type (such as our base_class->method(arg1, arg2) example from earlier) the program needs to find the subtype of this instance of base_class in order to find the correct implementation of method to invoke. Who needs to downcast?ĭowncasting is essential to any implementation of dynamic polymorphism. The Rust programming language does not have inheritance per se, but it makes sense to think of trait objects as the base class and concrete types as the most-derived class. I will use it here to refer to casting from a base class to the most-derived class, because that is the most useful class to present to a user of a debugger. In languages with classes and inheritance, "downcasting" refers to casting a reference to an instance of a base class to one of its derived classes. In statically typed languages without garbage collection, dynamic polymorphism is typically implemented with a technique called virtual method tables that I'll discuss a a bit later. What base_class->method(arg1, arg2) does in C++ if BaseClass::method is virtual depends on which subtype of BaseClass is present at run time. Virtual functions, on the other hand, are an example of dynamic polymorphism. ![]() What a + b does in C++ thus depends on the types of a and b. The addition operator can be overloaded to do different things to an integer and a matrix. In C++, operator overloading is an example of static polymorphism. It is split into static and dynamic polymorphism depending on whether the selection of the function happens at compile time (static) or run time (dynamic). #Rust any downcast code#Polymorphism broadly refers to ways in which the same source code in a high level language can run different functions depending on the types of inputs to that function. But how is it actually implemented? What is dynamic polymorphism? You might also be familiar with this technique in gdb via the set print object on command. Our Pernosco Omniscient Debugger automatically downcasts types that use dynamic polymorphism in supported languages (C++, Rust, and Ada). Automatically deducing the most-"derived" type and downcasting to it presents the entire object to developers and makes debugging code that uses dynamic polymorphism much more pleasant. These pose challenges for debuggers, because viewing only fields from the declared type of a variable may not be particularly useful. Many programming languages include mechanisms for dynamic polymorphism. ![]()
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