1.6. When Metaprogramming?

You've just seen some examples of the why of template metaprogramming, and you've had a tiny glimpse of the how, but we haven't discussed when metaprogramming is appropriate. However, we've touched on most of the relevant criteria for using template metaprogramming already. As a guideline, if any three of the following conditions apply to you, a metaprogrammed solution may be appropriate.

• You want the code to be expressed in terms of the abstractions of the problem domain. For example, you might want a parser to be expressed by something that looks like a formal grammar rather than as tables full of numbers or as a collection of subroutines; you might want array math to be written using operator notation on matrix and vector objects rather than as loops over sequences of numbers.

• You would otherwise have to write a great deal of boilerplate implementation code.

• You need to choose component implementations based on the properties of their type parameters.

• You want to take advantage of valuable properties of generic programming in C++ such as static type checking and behavioral customization, without loss of efficiency.

• You want to do it all within the C++ language, without an external tool or custom source code generator.