Introduction

Publisher Summary

This chapter defines an Application Programming Interface (API), which provides an abstraction for a problem and specifies how clients should interact with software components that implement a solution to that problem. API development is ubiquitous in modern software development. Its purpose is to provide a logical interface to the functionality of a component while also hiding any implementation details. It can be as small as a single function or involve hundreds of classes, methods, free functions, data types, enumerations, and constants. Its implementation can be proprietary or open source. The important underlying concept is that an API is a well-defined interface that provides a specific service to other pieces of software. An API is an interface designed for developers, in much the same way that a Graphical User Interface (GUI) is an interface designed for end users.

Defines what an Application Programming Interface (API) is with various examples and describes how an API is represented in C++. This chapter details the difference between API development and standard application development, as well as enumerating the pros and cons of APIs in software development. I then illustrate the various layers of APIs that a modern application is built on, with specific reference to a large open source client/server program. The relationship of the terms API and SDK is explained, as is the relationship among APIs, file formats, and network protocols.

1.1 What are Application Programming Interfaces?

An Application Programming Interface (API) provides an abstraction for a problem and specifies how clients should interact with software components that implement a solution to that problem. The components themselves are typically distributed as a software library, allowing them to be used in multiple applications. In essence, APIs define reusable building blocks that allow modular pieces of functionality to be incorporated into end-user applications.

An API can be written for yourself, for other engineers in your organization, or for the development community at large. It can be as small as a single function or involve hundreds of classes, methods, free functions, data types, enumerations, and constants. Its implementation can be proprietary or open source. The important underlying concept is that an API is a well-defined interface that provides a specific service to other pieces of software.

A modern application is typically built on top of many APIs, where some of these can also depend on further APIs. This is illustrated in Figure 1.1, which shows an example application that depends directly on the API for three libraries (1-3), where two of those APIs depend on the API for a further two libraries (4 and 5). For instance, an image viewing application may use an API for loading GIF images, and that API may itself be built upon a lower-level API for compressing and decompressing data.

API development is ubiquitous in modern software development. Its purpose is to provide a logical interface to the functionality of a component while also hiding any implementation details. For example, our API for loading GIF images may simply provide a LoadImage() method that accepts a filename and returns a 2D array of pixels. All of the file format and data compression details are hidden behind this simple interface. This concept is also illustrated in Figure 1.1, where client code only accesses an API via its public interface, shown as the dark section at the top of each box.

1.1.1 Contracts and Contractors

As an analogy, consider the task of building your own home. If you were to build a house entirely on your own, you would need to possess a thorough understanding of architecture, plumbing, electronics, carpentry, masonry, and many other trades. You would also need to perform every task yourself and keep track of the minutest of details for every aspect of the project, such as whether you have enough wood for your floorboards or whether you have the right fasteners to fit the screws that you've bought. Finally, because you are the only person working on the project, you can only perform a single task at any point in time and hence the total time to complete the project could be very large.

An alternative strategy is to hire professional contractors to perform key tasks for you (Figure 1.2). You could hire an architect to design the plans for the house, a carpenter for all of your woodwork needs, a plumber to install the water pipes and sewage system for your house, and an electrician to set up the power systems. Taking this approach, you negotiate a contract with each of your contractors-telling them what work you want done and agreeing upon a price-they then perform that work for you. If you're lucky, maybe you even have a good friend who is a contractor and he offers you his services for free. With this strategy, you are freed from the need to know everything about all aspects of building a house and instead you can take a higher-level supervisory role to select the best contractors for your purpose and ensure that the work of each individual contractor is assembled together to produce the vision of your ideal home.

The analogy to APIs is probably obvious: the house that you're building equates to a software program that you want to write, and the contractors provide APIs that abstract each of the tasks you need to perform and hide the implementation details of the work involved. Your task then resolves to selecting the appropriate APIs for your application and integrating them into your software. The analogy of having skilled friends who provide contracting services for free is meant to represent the use of freely available open source libraries in contrast to commercial libraries that require a licensing fee to use in your software. The analogy could even be extended by having some of the contractors employing subcontractors, which corresponds to certain APIs depending on other APIs to perform their task.

The contractor analogy is a common one in object-oriented programming. Early practitioners in the field talked about an object defining a binding contract for the services or behavior that it provides. An object then implements those services when asked to by a client program, potentially by subcontracting some of the work out to other objects behind the scenes (Meyer, 1987; Snyder, 1986).

1.1.2 APIs in C++

Strictly speaking, an API is simply a description of how to interact with a component. That is, it provides an abstraction and a functional specification for a component. In fact, many software engineers prefer to expand the acronym API as Abstract Programming Interface instead of Application Programming Interface.

In C++, this is embodied as one or more header (.h) files plus supporting documentation files. An implementation for a given API is often represented as a library file that can be linked into end-user applications. This can be either a static library, such as a .lib file on Windows or .a on Mac OS X and Linux, or a dynamic library such as a .dll file on Windows, .dylib on Mac, or .so on Linux.

A C++ API will therefore generally include the following elements:

1. Headers: A collection of .h header files that define the interface and allow client code to be compiled against that interface. Open source APIs also include the source code (.cpp files) for the API implementation.

2. Libraries: One or more static or dynamic library files that provide an implementation for the API. Clients can link their code against these library files in order to add the functionality to their applications.

3. Documentation: Overview information that describes how to use the API, often including automatically generated documentation for all of the classes and functions in the API.

As an example of a well-known API, Microsoft's Windows API (often referred to as the Win32 API) is a collection of C functions, data types, and constants that enable programmers to write applications that run on the Windows platform. This includes functions for file handling, process and thread management, creating graphical user interfaces, talking to networks, and so on.

The Win32 API is an example of a plain C API rather than a C++ API. While you can use a C API directly from a C++ program, a good example of a specific C++ API is the Standard Template Library (STL). The STL contains a set of container classes, iterators for navigating over the elements in those containers, and various algorithms that act on those containers (Josuttis, 1999). For instance, the collection of algorithms includes high-level operations such as std::search(), std::reverse(), std::sort(), and std::set_intersection(). The STL therefore presents a logical interface to the task of manipulating collections of elements, without exposing any of the internal details for how each algorithm is implemented.

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An API is a logical interface to a software...