We begin this book by exploring some of the reasons for using C++ today. In short, C++ is a highly portable language which offers zero-cost abstractions. Furthermore, we believe that C++ provides programmers with the ability to write and manage large, expressive, and robust code bases. Let's explore the meaning of each of these properties.
Why C++?
Zero-cost abstractions
Active code bases grow. The more developers working on a code base, the larger the code base becomes. We need abstractions such as functions, classes, data structures, layers and so on in order to manage the complexity of a large-scale code base. But constantly adding abstractions and new levels of indirection comes at a price — efficiency. This is where zero-cost abstractions plays its role. A lot of the abstractions offered by C++ comes at a very low price. At a minimum, C++ offers efficient alternatives at hot spots where performance really is a concern.
With C++ you are free to talk about memory addresses and other computer related low-level terms when needed. However, in a large-scale software project it is desirable to express code in terms that deals with whatever the application is doing, and let the libraries handle the computer related terminology. The source code of a graphics application may deal with pencils, colors, and filters, whereas a game may deal with mascots, castles, and mushrooms. Low-level computer-related terms such as memory addresses can stay hidden in C++ library code where performance is critical.
By library code, we refer to code whose concepts are not strictly related to the application. The line between library code and application code can be blurry though, and libraries are often built upon other libraries. An example could be the container algorithms provided in the Standard Template Library (STL) of C++ or a general-purpose math library.
Programming languages and machine code abstractions
In order to relieve programmers from dealing with computer-related terms, modern programming languages use abstractions so that a list of strings, for example, can be handled and thought of as a list of strings rather than a list of addresses that we may easily lose track of if we make the slightest typo. Not only do the abstractions relieve the programmers from bugs, they also make the code more expressive by using concepts from the domain of the application. In other words, the code is expressed in terms that are closer to a spoken language than if expressed with abstract programming keywords.
C++ and C are nowadays two completely different languages. Still, C++ is highly compatible with C and has inherited a lot of its syntax and idioms from C. To give you some examples of C++ abstractions we will here show how a problem can be solved in both C and C++.
Take a look at the following C/C++ code snippets, which correspond to the question: "How many copies of Hamlet is in the list of books?". We begin with the C version:
// C version
struct string_elem_t { const char* str_; string_elem_t* next_; };
int num_hamlet(string_elem_t* books) {
const char* hamlet = "Hamlet";
int n = 0;
string_elem_t* b;
for (b = books; b != 0; b = b->next_)
if (strcmp(b->str_, hamlet) == 0)
++n;
return n;
}
The equivalent version using C++ would look something like this:
// C++ version
int num_hamlet(const std::list<std::string>& books) {
return std::count(books.begin(), books.end(), "Hamlet");
}
Although the C++ version is still more of a robot language than a human language, a lot of programming lingo is gone. Here are some of the noticeable differences between the preceding two code snippets:
- The pointers to raw memory addresses are not visible at all
- The std::list<std::string> container is an abstraction of string_elem_t
- The std::count() method is an abstraction of both the for loop and the if condition
- The std::string class is (among other things) an abstraction of char* and strcmp
Basically, both versions of num_hamlet() translate to roughly the same machine code, but the language features of C++ makes it possible to let the libraries hide computer related terminology such as pointers. Many of the modern C++ language features can be seen as abstractions of basic C functionality and, on top of that, basic C++ functionality:
- C++ classes are abstractions of C-structs and regular functions
- C++ polymorphism is the abstraction of function pointers
On top of that, some recent C++ features are abstractions of former C++ features:
- C++ lambda functions are abstractions of C++ classes
- Templates are abstractions of generating C++ code
Abstractions in other languages
Most programming languages are based on abstractions, which are transformed into machine code to be executed by the CPU. C++ has evolved into a highly expressive language just like many of the other popular programming languages of today. What distinguishes C++ from most other languages is that, while the other languages have implemented these abstractions at the cost of runtime performance, C++ has always strived to implement its abstractions at zero cost at runtime. This doesn't mean that an application written in C++ is by default faster than the equivalent in, say, C#. Rather, it means that by using C++, you'll have explicit control of the emitted machine code instructions and memory footprint if needed.
To be fair, optimal performance is very rarely required today and compromising performance for lower compilation times, garbage collection, or safety, like other languages do, is in many cases more reasonable.
Portability
C++ has been a popular and comprehensive language for a long time. It's highly compatible with C and very little has been deprecated in the language, for better or worse. The history and design of C++ has made it to a highly portable language, and the evolution of modern C++ has ensured that it will stay that way for a long time to come. C++ is a living language and compiler vendors are currently doing a remarkable job to implement new language features rapidly.
Robustness
In addition to performance, expressiveness, and portability, C++ offers a set of language features that gives the programmer the ability to write robust code.
In our experience, robustness does not refer to strength in the programming language itself – it's possible to write robust code in any language. However, strict ownership of resources, const correctness, value semantics, type safety, and deterministic destruction of objects are some of the features offered by C++ that makes it easier to write robust code. That is, the ability to write functions, classes, and libraries that are easy to use and hard to misuse.
C++ of today
To sum it up, C++ of today provides programmers the ability to write an expressive and robust code base while still having the ability to target almost any hardware platform or real-time requirements. Of the most commonly used languages today, C++ is the only one that gives all of these properties.
