How Does the Lambda Library Improve Your Programs?

• Adapts functions and function objects for use with Standard Library algorithms

• Binds arguments to function calls

• Transforms arbitrary expressions into function objects compatible with the Standard Library algorithms

• Defines unnamed functions at the call site, thereby improving readability and maintainability of the code

• Implements predicates when and where needed

When using the Standard Library, or any library employing a similar design that relies on algorithmic configuration by the means of functions and function objects, one often ends up writing lots of small function objects that perform quite trivial operations. As we saw in "Library 9: Bind 9," this can quickly become a problem, because an explosion of small classes that are scattered through the code base is not easily maintained. Also, understanding the code where the function objects are actually invoked is harder, because part of the functionality is defined elsewhere. A perfect solution to this problem is a way to define these functions or function objects directly at the call site. This typically makes the code faster to write, easier to read, and more readily maintained, as the definition of the functionality then resides in the location where it is used. This is what the Boost.Lambda library offers, unnamed functions defined at the call site. Boost.Lambda works by creating function objects that can be defined and invoked directly, or stored for later invocation. This is similar to the offerings from the Boost.Bind library, but Boost.Lambda does both argument binding and much more, by adding control structures, automatic conversions of expressions into function objects, and even support for exception handling in lambda expressions.

The term lambda expression, or lambda function, originates from functional programming and lambda calculus. A lambda abstraction defines an unnamed function. Although lambda abstractions are ubiquitous in functional programming languages, that's not the case for most imperative programming languages, such as C++. But, using advanced techniques such as expression templates, C++ makes it possible to augment the language with a form of lambda expressions.

The first and foremost motivation for creating the Lambda library is to enable unnamed functions for use with the Standard Library algorithms. Because the use of the Standard Library has virtually exploded since the first C++ Standard in 1998, our knowledge of what's good and what's missing has rapidly increasedand one of the parts that can be problematic is the definition of numerous small function objects, where a simple expression would seem to suffice. The function object issue is obviously addressed by this library, but there is still room for exploration of the uses of lambda functions. Now that lambda functions are available, we have the opportunity to apply them to problems that previously required totally different solutions. It's both fascinating and exciting that it is possible to explore new programming techniques in a language as mature as C++. What new idioms and ways of solving problems will arise from the presence of unnamed functions and expression templates? The truth is that we don't know, because we have yet to try them all out! Still, the focus here is on the practical problems that the library explicitly addressesavoiding code bloat and scattered functionality through lambda expressionsfunctions defined at the call site. We can do many wonderful things with thisand we can be really terse about it, which should satisfy both programmers, who can focus more on the problem at hand, and their managers, who can reap the benefits of a higher production rate (and, hopefully, more easily maintained code!).