Double dispatch

In software engineering, double dispatch is a special form of multiple dispatch, and a mechanism that dispatches a function call to different concrete functions depending on the runtime types of two objects involved in the call. In most object-oriented systems, the concrete function that is called from a function call in the code depends on the dynamic type of a single object and therefore they are known as single dispatch calls, or simply virtual function calls.

Double dispatch is useful in situations where the choice of computation depends on the runtime types of its arguments. For example, a programmer could use double dispatch in the following situations:

The common idiom, as in the examples presented above, is that the selection of the appropriate algorithm is based on the call's argument types at runtime. The call is therefore subject to all the usual additional performance costs that are associated with dynamic resolution of calls, usually more than in a language supporting only single method dispatch. In C++, for example, a dynamic function call is usually resolved by a single offset calculation - which is possible because the compiler knows the location of the function in the object's method table and so can statically calculate the offset. In a language supporting double dispatch, this is slightly more costly, because the compiler must generate code to calculate the method's offset in the method table at runtime, thereby increasing the overall instruction path length (by an amount that is likely to be no more than the total number of calls to the function, which may not be very significant).

At first glance, double dispatch appears to be a natural result of function overloading. Function overloading allows the function called to depend on the type of the argument. Function overloading, however, is done at compile time using "name mangling" where the internal name of the function encodes the argument's type. For example, a function foo(int) may internally be called __foo_i and the function foo(double) may be called __foo_d. Thus, there is no name collision, and no virtual table lookup. By contrast, dynamic dispatch is based on the type of the calling object, meaning it uses virtual functions (overriding) instead of function overloading, and does result in a vtable lookup. Consider the following example, written in C++, of collisions in a game:

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