C++ Reference Material
Standard I/O, File I/O and Run Time Type Identification (RTTI)

Sometimes, when a program runs, it does essentially the same thing with different kinds of data. Or, it may need to modify its behavior slightly, depending on the kind of data it is dealing with. Rather than write a separate routine to deal with each kind of data, it may be more programmer-efficient if only one routine is used, but that routine will have to be able to recognize (at runtime) just what kind of data it is currently dealing with, so that it can, in fact, respond "on the fly" in an appropriate manner. This is where the need for Run Time Type Identification comes in.

Here are three typical situations (among many) where such a need might arise:

• A routine is reading from an input stream and always reads data in the same format. But, if the input stream is the standard input, it needs to prompt the user to enter the data, something that is not necessary if the routine is reading from a file input stream.
• A routine in the form of a template function can read data of various types from the keyboard, but needs to prompt the user in different ways, depending on the particular data type of the value requested at any given time.
• A routine is printing data to an output stream, the data may be integers or real values, and the formatting instructions need to be different, depending on which of the two is actually being output.

Some programming languages, including C++, allow the programmer to examine the data type of a variable or object at runtime, and take appropriate action depending on the outcome of such a test. This facility may have to be "switched on", or it may be the default behavior. Since all run time tests are "costly", this too is a "costly" operation, and the better default may be to require its explicit invocation.

C++ has the keyword typeid, which is an operator that may take an expression or a typename as an argument. It returns a reference to an object of the type_info class, which requires that we #include the <typeinfo> header in order to use the typeid operator. A typical construct using typeid might look like this:

if (typeid(someObject) == typeid(SomeType))
{
   //Do whatever needs to be done if that object has that type ...
}