FAQ 27.04 What is the basic problem with dynamic type checking?

The basic problem with dynamic type checking is that it uses code to find code, creating extensibility problems later.

With dynamic type checking, code has to be written to check the type of an object to see if it supports a particular set of member functions (this is the code that is doing the finding). Accessing the member functions may require a down cast to the appropriate pointer type (this is the code that is being searched for).

When the user code uses code to find server code, the user code is more complex and fragile. OO programming is supposed to encapsulate complexity, and the in appropriate use of dynamic type checking can undo this benefit. Often dynamic type-checking tests require the user code to know the server's inheritance hierarchy, in which case changing the server's inheritance hierarchy breaks the user code. This is unfortunate, considering that one of the main goals of object-oriented technology is to reduce maintenance costs.

Dynamic type checking also requires a runtime check to ensure that the object supports the requested member function. This is usually implemented using control flow, such as an if or switch statement. These runtime tests are frequently avoidable if the design exploits the static type-checking capabilities of the C++ compiler.

Finally, it is much more expensive to catch an error at runtime than it is to find the same error at compile time. Don't use dynamic type checking without a good reason.