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In 1968 Shirley Chisholm became the first Black woman elected to the United States Congress. In 1972 she launched a campaign for the Democratic presidential nomination, breaking another political barrier.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
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| Polymorphism |
|---|
| Ad hoc polymorphism |
| Parametric polymorphism |
| Subtyping |
In computing, static dispatch is a form of polymorphism fully resolved during compile time. It is a form of method dispatch, which describes how a language or environment will select which implementation of a method or function to use.[1]
Examples are templates in C++, and generic programming in Fortran and other languages, in conjunction with function overloading (including operator overloading). Code is said to be monomorphised, with specific data types deduced and traced through the call graph, in order to instantiate specific versions of generic functions, and select specific function calls based on the supplied definitions.
This contrasts with dynamic dispatch, which is based on runtime information (such as vtable pointers and other forms of run time type information).
Static dispatch is possible because there is a guarantee of there only ever being a single implementation of the method in question. Static dispatch is typically faster than dynamic dispatch which by nature has higher overhead.
Consider the following Rust program:[2]
trait Pet {
fn speak(&self);
}
struct Cat {
name: String
}
impl Cat {
fn new(name: String) -> Self {
Cat { name }
}
}
impl Pet for Cat {
fn speak(&self) {
println!("{} says Meow!", self.name);
}
}
fn talk<T: Pet>(pet: T) {
pet.speak();
}
fn main() {
let pet = Cat::new(String::from("Simba"));
talk(pet);
}
the Rust Compiler will monomorphize the program's code at compile time into:
// [...]
// struct Cat, Cat's impl, the Pet impl for struct Cat, and the Pet trait remain the same.
fn talk_cat(pet: Cat) {
Cat::speak(&pet)
}
fn main() {
let pet = Cat::new(String::from("Simba"));
talk_cat(pet); // talk(pet) gets replaced with the more specialized talk_cat
}
fn main() {
let pet = Cat::new(String::from("Simba"));
Cat::speak(&pet);
}
In which case "talk_cat" and "talk" will be omitted from the final optimized output due to dead code elimination.
 | This programming-language-related article is a stub. You can help Wikipedia by adding missing information. |
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
In computing, static dispatch is a form of polymorphism fully resolved during compile time. It is a form of method dispatch, which describes how a language or environment will select which implementation of a method or function to use. Examples are templates in C++, and generic programming in Fortran and other languages, in conjunction with function overloading (including operator overloading). Code is said to be monomorphised, with specific data types deduced and traced through the call graph, in order to instantiate specific versions of generic functions, and select specific function calls based on the supplied definitions. This contrasts with dynamic dispatch, which is based on runtime information (such as vtable pointers and other forms of run time type information). Static dispatch is possible because there is a guarantee of there only ever being a single implementation of the method in question. Static dispatch is typically faster than dynamic dispatch which by nature has higher overhead.
In computer science, dynamic dispatch is the process of selecting which implementation of a polymorphic operation (method or function) to call at run time. It is commonly employed in, and considered a prime characteristic of, object-oriented programming (OOP) languages and systems. Object-oriented systems model a problem as a set of interacting objects that enact operations referred to by name. Polymorphism is the phenomenon wherein somewhat interchangeable objects each expose an operation of the same name but possibly differing in behavior. As an example, a File object and a Database object both have a StoreRecord method that can be used to write a personnel record to storage. Their implementations differ. A program holds a reference to an object which may be either a File object or a Database object. Which one it is may have been determined by a run-time setting, and at this stage, the program may not know or care which. When the program calls StoreRecord on the object, something needs to choose which behavior gets enacted. If one thinks of OOP as sending messages to objects, then in this example the program sends a StoreRecord message to an object of unknown type, leaving it to the run-time support system to dispatch the message to the right object. The object enacts whichever behavior it implements. Dynamic dispatch contrasts with static dispatch, in which the implementation of a polymorphic operation is selected at compile time. The purpose of dynamic dispatch is to defer the selection of an appropriate implementation until the run time type of a parameter (or multiple parameters) is known. Dynamic dispatch is different from late binding (also known as dynamic binding). Name binding associates a name with an operation. A polymorphic operation has several implementations, all associated with the same name. Bindings can be made at compile time or (with late binding) at run time. With dynamic dispatch, one particular implementation of an operation is chosen at run time. While dynamic dispatch does not imply late binding, late binding does imply dynamic dispatch, since the implementation of a late-bound operation is not known until run time.
Virtual inheritance is a C++ technique that ensures only one copy of a base class's member variables are inherited by grandchild derived classes. Without virtual inheritance, if two classes B and C inherit from a class A, and a class D inherits from both B and C, then D will contain two copies of A's member variables: one via B, and one via C. These will be accessible independently, using scope resolution. Instead, if classes B and C inherit virtually from class A, then objects of class D will contain only one set of the member variables from class A. This feature is most useful for multiple inheritance, as it makes the virtual base a common subobject for the deriving class and all classes that are derived from it. This can be used to avoid the diamond problem by clarifying ambiguity over which ancestor class to use, as from the perspective of the deriving class (D in the example above) the virtual base (A) acts as though it were the direct base class of D, not a class derived indirectly through a base (B or C). It is used when inheritance represents restriction of a set rather than composition of parts. In C++, a base class intended to be common throughout the hierarchy is denoted as virtual with the virtual keyword. Consider the following class hierarchy. As declared above, a call to bat.eat() is ambiguous because there are two Animal (indirect) base classes in Bat, so any Bat object has two different Animal base class subobjects. So, an attempt to directly bind a reference to the Animal subobject of a Bat object would fail, since the binding is inherently ambiguous: To disambiguate, one would have to explicitly convert bat to either base class subobject: In order to call eat(), the same disambiguation, or explicit qualification is needed: static_cast(bat).eat() or static_cast(bat).eat() or alternatively bat.Mammal::eat() and bat.WingedAnimal::eat(). Explicit qualification not only uses an easier, uniform syntax for both pointers and objects but also allows for static dispatch, so it would arguably be the preferable method. In this case, the double inheritance of Animal is probably unwanted, as we want to model that the relation (Bat is an Animal) exists only once; that a Bat is a Mammal and is a WingedAnimal, does not imply that it is an Animal twice: an Animal base class corresponds to a contract that Bat implements (the "is a" relationship above really means "implements the requirements of"), and a Bat only implements the Animal contract once. The real world meaning of "is a only once" is that Bat should have only one way of implementing Eat, not two different ways, depending on whether the Mammal view of the Bat is eating, or the WingedAnimal view of the Bat. (In the first code example we see that Eat is not overridden in either Mammal or WingedAnimal, so the two Animal subobjects will actually behave the same, but this is just a degenerate case, and that does not make a difference from the C++ point of view.) This situation is sometimes referred to as diamond inheritance (see Diamond problem) because the inheritance diagram is in the shape of a diamond. Virtual inheritance can help to solve this problem.
The fragile base class problem is a fundamental architectural problem of object-oriented programming systems where base classes (superclasses) are considered "fragile" because seemingly safe modifications to a base class, when inherited by the derived classes, may cause the derived classes to malfunction. The programmer cannot determine whether a base class change is safe simply by examining in isolation the methods of the base class. One possible solution is to make instance variables private to their defining class and force subclasses to use accessors to modify superclass states. A language could also make it so that subclasses can control which inherited methods are exposed publicly. These changes prevent subclasses from relying on implementation details of superclasses and allow subclasses to expose only those superclass methods that are applicable to themselves. An alternative solution is to have an interface instead of superclass. The fragile base class problem has been blamed on open recursion (dynamic dispatch of methods on this), with the suggestion that invoking methods on this default to closed recursion (static dispatch, early binding) rather than open recursion (dynamic dispatch, late binding), only using open recursion when it is specifically requested; external calls (not using this) would be dynamically dispatched as usual.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Joe Louis.