ei49.htm

一个非常适合初学者入门的有关c++的文档

 Effective C++, 2E | Item 49: Familiarize yourself with the standard library Back to Item 48: Pay attention to compiler warnings.
Continue to Item 50: Improve your understanding of C++.
Item 49: Familiarize yourself with the standard library.
C++'s standard library is big. Very big. Incredibly big. How big? Let me put it this way: the specification takes over 300 closely-packed pages in the C++ standard, and that all but excludes the standard C library, which is included in the C++ library "by reference." (That's the term they use, honest.)
Bigger isn't always better, of course, but in this case, bigger is better, because a big library contains lots of functionality. The more functionality in the standard library, the more functionality you can lean on as you develop your applications. The C++ library doesn't offer everything (support for concurrency and for graphical user interfaces is notably absent), but it does offer a lot. You can lean almost anything against it.
Before summarizing what's in the library, I need to tell you a bit about how it's organized. Because the library has so much in it, there's a reasonable chance you (or someone like you) may choose a class or function name that's the same as a name in the standard library. To shield you from the name conflicts that would result, virtually everything in the standard library is nestled in the namespace std (see Item 28). But that leads to a new problem. Gazillions of lines of existing C++ rely on functionality in the pseudo-standard library that's been in use for years, e.g., functionality declared in the headers <iostream.h>, <complex.h>, <limits.h>, etc. That existing software isn't designed to use namespaces, and it would be a shame if wrapping the standard library by std caused the existing code to break. (Authors of the broken code would likely use somewhat harsher language than "shame" to describe their feelings about having the library rug pulled out from underneath them.)
Mindful of the destructive power of rioting bands of incensed programmers, the standardization committee decided to create new header names for the std-wrapped components. The algorithm they chose for generating the new header names is as trivial as the results it produces are jarring: the .h on the existing C++ headers was simply dropped. So <iostream.h> became <iostream>, <complex.h> became <complex>, etc. For C headers, the same algorithm was applied, but a c was prepended to each result. Hence C's <string.h> became <cstring>, <stdio.h> became <cstdio>, etc. For a final twist, the old C++ headers were officially deprecated (i.e., listed as no longer supported), but the old C headers were not (to maintain C compatibility). In practice, compiler vendors have no incentive to disavow their customers' legacy software, so you can expect the old C++ headers to be supported for many years.
Practically speaking, then, this is the C++ header situation: Old C++ header names like <iostream.h> are likely to continue to be supported, even though they aren't in the official standard. The contents of such headers are not in namespace std. New C++ header names like <iostream> contain the same basic functionality as the corresponding old headers, but the contents of the headers are in namespace std. (During standardization, the details of some of the library components were modified, so there isn't necessarily an exact match between the entities in an old C++ header and those in a new one.) Standard C headers like <stdio.h> continue to be supported. The contents of such headers are not in std. New C++ headers for the functionality in the C library have names like <cstdio>. They offer the same contents as the corresponding old C headers, but the contents are in std.
All this seems a little weird at first, but it's really not that hard to get used to. The biggest challenge is keeping all the string headers straight: <string.h> is the old C header for char*-based string manipulation functions, <string> is the std-wrapped C++ header for the new string classes (see below), and <cstring> is the std-wrapped version of the old C header. If you can master that (and I know you can), the rest of the library is easy.
The next thing you need to know about the standard library is that almost everything in it is a template. Consider your old friend iostreams. (If you and iostreams aren't friends, turn to Item 2 to find out why you should cultivate a relationship.) Iostreams help you manipulate streams of characters, but what's a character? Is it a char? A wchar_t? A Unicode character? Some other multi-byte character? There's no obviously right answer, so the library lets you choose. All the stream classes are really class templates, and you specify the character type when you instantiate a stream class. For example, the standard library defines the type of cout to be ostream, but ostream is really a typedef for basic_ostream<char>.
Similar considerations apply to most of the other classes in the standard library. string isn't a class, it's a class template: a type parameter defines the type of characters in each string class. complex isn't a class, it's a class template: a type parameter defines the type of the real and imaginary components in each complex class. vector isn't a class, it's a class template. On and on it goes.
You can't escape the templates in the standard library, but if you're used to working with only streams and strings of chars, you can mostly ignore them. That's because the library defines typedefs for char instantiations for these components of the library, thus letting you continue to program in terms of the objects cin, cout, cerr, etc., and the types istream, ostream, string, etc., without having to worry about the fact that cin's real type is basic_istream<char> and string's is basic_string<char>.
Many components in the standard library are templatized much more than this suggests. Consider again the seemingly straightforward notion of a string. Sure, it can be parameterized based on the type of characters it holds, but different character sets differ in details, e.g., special end-of-file characters, most efficient way of copying arrays of them, etc. Such characteristics are known in the standard as traits, and they are specified for string instantiations by an additional template parameter. In addition, string objects are likely to perform dynamic memory allocation and deallocation, but there are lots of different ways to approach that task (see Item 10). Which is best? You get to choose: the string template takes an Allocator parameter, and objects of type Allocator are used to allocate and deallocate the memory used by string objects.
Here's a full-blown declaration for the basic_string template and the string typedef that builds on it; you can find this (or something equivalent to it) in the header <string>: namespace std {

  template<class charT,
           class traits = char_traits<charT>,
           class Allocator = allocator<charT> >
     class basic_string;

  typedef basic_string<char> string;

}


Notice how basic_string has default values for its traits and Allocator parameters. This is typical of the standard library. It offers flexibility to those who need it, but "typical" clients who just want to do the "normal" thing can ignore the complexity that makes possible the flexibility. In other words, if you just want string objects that act more or less like C strings, you can use string objects and remain merrily ignorant of the fact that you're really using objects of type basic_string<char, char_traits<char>, allocator<char> >.
Well, usually you can. Sometimes you have to peek under the hood a bit. For example, Item 34 discusses the advantages of declaring a class without providing its definition, and it remarks that the following is the wrong way to declare the string type: 
class string;                   // this will compile, but
                                // you don't want to do it


Setting aside namespace considerations for a moment, the real problem here is that string isn't a class, it's a typedef. It would be nice if you could solve the problem this way: typedef basic_string<char> string;