ch02.htm

c++语言操作手册

      a template separately and then use its declaration, the template has to 
      be <i>exported</i>. This is done by preceding the template's definition 
      with the keyword <tt>export</tt>.</p>
    <h3> <a name="Heading23">The Standard Template Library</a></h3>
    <p>According to Bjarne Stroustrup, the most important change in C++ since 
      1991 is not a language change; it is the addition of the standard library. 
      The <i>Standard Template Library</i>, or <i>STL</i>, comprises a substantial 
      part of the Standard Library. STL is collection of generic containers -- 
      such as vector, list, and stack -- and a rich collection of generic algorithms 
      for sorting, finding, merging, and transforming these containers. Chapter 
      10, "STL and Generic Programming," is dedicated to STL.</p>
    <h3> <a name="Heading24">Internationalization and Localization</a></h3>
    <p>The current C++ Standard is an international Standard approved by ISO. 
      To qualify as such, Standard C++ has been fully internationalized. Internationalization 
      consists of several modifications and extensions. These include the addition 
      of the keyword <tt>wchar_t</tt> (<tt>wchar_t</tt> was already defined in 
      ISO C as a <tt>typedef</tt> but it wasn't a reserved keyword). In addition, 
      the standard stream and string classes have been templatized to support 
      both narrow and wide characters. Finally, the <tt>&lt;locale&gt;</tt> library 
      defines template classes and declares functions that encapsulate and manipulate 
      locale-related information such as monetary, numeric, and time conventions. 
      The locale feature sets are encapsulated in classes (or <i>facets</i>) that 
      the users can extend. </p>
    <h4> Wide Character Streams</h4>
    <p>C++ provides four standard I/O streams that are automatically instantiated 
      before a program's outset. They are defined in the header <tt>&lt;iostream&gt;</tt>:</p>
    <pre>
<tt>cin    // standard input stream of char</tt>
<tt>cout  // standard output stream of char</tt>
<tt>cerr  // standard unbuffered output stream for error messages</tt>
<tt>clog  // standard output stream for error messages</tt>
</pre>
    <p>Each of these four streams now has a corresponding wide-character version:</p>
    <pre>
<tt>wcin</tt>
<tt>wcout</tt>
<tt>wcerr</tt>
<tt>wclog</tt>
</pre>
    <h3> <a name="Heading25">Miscellaneous</a></h3>
    <p>Two additional extensions were recently added to the Standard: the initialization 
      of <tt>const static</tt> data members inside the class body and the <tt>mutable</tt> 
      storage specifier. Both of these extensions are discussed in the following 
      sections.</p>
    <h4> Initialization of const static Data Members</h4>
    <p><tt>const static</tt> data members of an integral type can now be initialized 
      inside their class. In this case, the initialization is also a definition, 
      so no further definitions are required outside the class body. For example</p>
    <pre>
<tt>#include &lt;string&gt;</tt>
<tt>class Buff</tt>
<tt>{</tt>
<tt>private:</tt>
<tt>  static const int MAX = 512; // initialization +definition</tt>
<tt>  static const char flag = 'a'; // initialization +definition</tt>
<tt>  static const std::string msg; //non-integral type; must be defined outside</tt>
<tt>                                //the class body</tt>
<tt>  //..</tt>
<tt>};</tt>
<tt>const std::string Buff::msg = "hello";</tt>
</pre>
    <h4> A mutable Object Member</h4>
    <p>A <tt>const</tt> member function cannot modify the state of its object. 
      However, auxiliary data members (flags, reference counters) sometimes have 
      to be modified by a <tt>const</tt> member function. Such data members can 
      be declared <tt>mutable</tt>. A <tt>mutable</tt> member is never <tt>const</tt>, 
      even if its object is <tt>const</tt>; therefore, it can be modified by a 
      <tt>const</tt> member function. The following example demonstrates the use 
      of this feature:</p>
    <pre>
<tt>class Buffer</tt>
<tt>{</tt>
<tt>private:</tt>
<tt>  void * data;  //raw data buffer to be transmitted on some network</tt>
<tt>  size_t size;</tt>
<tt>  mutable int crc;//used to verify that no errors occurred during transmission</tt>
<tt>public:</tt>
<tt>  int GetCrc() const;</tt>
<tt>  void Transmit() const; //computation of crc is done here</tt>
<tt>};</tt>
<tt>void f()</tt>
<tt>{</tt>
<tt>  Buffer buffer;</tt>
<tt>  //...fill buffer with data</tt>
<tt>  buffer.Transmit(); //crc can be modified here; non-mutable members may not</tt>
<tt>}</tt>
</pre>
    <p>There is no point in calculating the <tt>crc</tt> value every time a few 
      more bytes are appended to <tt>buffer</tt>. Instead, it is computed by the 
      member function <tt>Transmit()</tt> right before <tt>buffer</tt> is sent. 
      However, <tt>Transmit()</tt> is not supposed to modify its object's state 
      so that it is declared as a <tt>const</tt> member function. In order to 
      allow assignment of the correct <tt>crc</tt> value, the data member <tt>crc</tt> 
      is declared <tt>mutable</tt>; hence, it can be modified by a <tt>const</tt> 
      member function.</p>
    <h2> <a name="Heading26">Deprecated Feature</a></h2>
    <p>A <i>deprecated feature</i> is one that the current edition of the Standard 
      regards as normative, but that is not guaranteed to be part of the Standard 
      in future revisions. Again, this is somewhat understating the intent of 
      this definition. One of the consequences of the evolution and standardization 
      of a programming language is the gradual removal of undesirable, dangerous, 
      or redundant features and constructs. By deprecating a feature, the standardization 
      committee expresses the desire to have the feature removed from the language. 
      Removing it from the language altogether is impractical because existing 
      code will not compile anymore. The deprecation gives the user sufficient 
      time to replace a deprecated feature with a Standard-endorsed feature. The 
      Standard lists the features that are discussed in the following sections 
      as deprecated.</p>
    <h3> <a name="Heading27">Use of an Operand of Type bool with the Postfix ++ 
      Operator</a></h3>
    <p>Applying postfix <tt>++</tt> to a variable of type <tt>bool</tt> is still 
      allowed for backward compatibility with old code that uses plain <tt>int</tt> 
      or some <tt>typedef</tt>, as in</p>
    <pre>
<tt>  bool done = false;</tt>
<tt>  while(!done)</tt>
<tt>  {</tt>
<tt>    if(condition)</tt>
<tt>      //...do something</tt>
<tt>    else done++; //deprecated</tt>
<tt>  }</tt>
</pre>
    <p>Incrementing a <tt>bool</tt> always yields <tt>true</tt>, regardless of 
      the original value. However, this practice is deprecated; therefore, it 
      might not be supported in the future. Remember that applying <tt>--</tt> 
      (decrement operator) to a <tt>bool</tt> variable is illegal.</p>
    <h3> <a name="Heading28">Use of static to Declare Objects in Namespace Scope</a></h3>
    <p>The use of the keyword <tt>static</tt> to declare a function or an object 
      local to a translation unit is deprecated. Instead, use an <i>unnamed namespace</i> 
      for that purpose (more on this in Chapter 8).</p>
    <h3> <a name="Heading29">Access Declarations</a></h3>
    <p>The access of a member of a base class can be changed in a derived class 
      by an <i>access declaration</i>. For example</p>
    <pre>
<tt>class A</tt>
<tt>{</tt>
<tt>public:</tt>
<tt> int k;</tt>
<tt> int l;</tt>
<tt>};</tt>
<tt>class D : public A</tt>
<tt>{</tt>
<tt>private:</tt>
<tt>   A::l; // access declaration changes access of A::l to private; deprecated</tt>
<tt>};</tt>
</pre>
    <p>The use of access declarations is deprecated. Use a <tt>using</tt><i> declaration</i> 
      instead:</p>
    <pre>
<tt>class D : public A // using-declaration version</tt>
<tt>{</tt>
<tt>private:</tt>
<tt>   using A::l; // using declaration changes the access of A::l to private</tt>
<tt>};</tt>
<tt>void func()</tt>
<tt>{</tt>
<tt>  D d;</tt>
<tt>  d.l = 0; //error; cannot access private member</tt>
<tt>}</tt>
</pre>
    <h3> <a name="Heading30">Implicit Conversion from const to non-const Qualification 
      for String Literals</a></h3>
    <p>A string literal can be implicitly converted from type pointer to <tt>const 
      char</tt> to type <tt>pointer to char</tt>. Similarly, a wide string literal 
      can be implicitly converted from type pointer to <tt>const wchar_t</tt> 
      to pointer to <tt>wchar_t</tt>. For example</p>
    <pre>
<tt>char *s = "abc"; //string literal implicitly converted to non-const; deprecated</tt>
</pre>
    <p>The type of the literal <tt>"abc"</tt> is <tt>const char[]</tt>, but <tt>s</tt> 
      is a pointer to non-<tt>const char</tt>. Such implicit conversions from 
      <tt>const</tt> to non-<tt>const</tt> qualifications for string literals 
      are deprecated because they might lead to the following erroneous code:</p>
    <pre>
<tt>strcpy(s, "cde"); //undefined behavior</tt>
</pre>
    <p>The preferred form is</p>
    <pre>
<tt>const char *s = "abc"; //OK</tt>
</pre>
    <h3> <a name="Heading31">Standard C Headers in the form &lt;name.h&gt;</a></h3>
    <p>For compatibility with the Standard C library, C++ still supports the naming 
      convention of C headers in the form <tt>&lt;xxx.h&gt;</tt> -- but this naming 
      convention is now deprecated (this is discussed in more detail in Chapter 
      8). For example</p>
    <pre>
<tt>#include &lt;stdlib.h&gt; //deprecated</tt>
</pre>
    <p>Use the newer naming convention, <tt>&lt;cxxx&gt;</tt>, instead:</p>
    <pre>
<tt>#include &lt;cstdlib&gt; //OK,  'c' prefixed and ".h" omitted</tt>
</pre>
    <p>The reason for this is that <tt>".h"</tt> headers inject all their names 
      into the global namespace, whereas the newer convention headers, <tt>&lt;cname&gt;</tt>, 
      keep their names in namespace <tt>std</tt>.</p>
    <h3> <a name="Heading32">Implicit int Declarations</a></h3>
    <p>The default type for missing declarations such as the following is <tt>int</tt>:</p>
    <pre>
<tt>static k =0; //'int' type deduced; deprecated</tt>
<tt>const c =0; //'int' type deduced; deprecated</tt>
</pre>
    <p>This convention is now considered deprecated. Instead, use explicit type 
      names:</p>
    <pre>
<tt>static int k =5;</tt>
<tt>const int c = 0;</tt>
</pre>
    <h3> <a name="Heading33">Other Deprecated Features</a></h3>
    <p>The Standard deprecates some of the members of old <tt>iostream</tt> classes 
      (article 4.6). In addition, it deprecates three types in the header <tt>&lt;strstream&gt;</tt> 
      that associate stream buffers with character array objects (article 4.7).</p>
    <p>From a user's point of view, deprecated features are not to be used in 
      new code because future versions of the language will flag it as an error. 
      In the interim period, compilers can issue a warning about its use.</p>
    <h2> <a name="Heading34">Conclusions</a></h2>
    <p>As you have seen, standard C++ is quite different from what it used to 
      be four, five, or ten years ago. Many of the changes and modifications have 
      been initiated by software vendors (the <tt>bool</tt> data type, for instance). 
      Others have been initiated by the Standardization committee (for example, 
      the new cast operators and STL). During the hectic standardization process, 
      complaints about "randomly accepted changes" on the one hand, and "missing 
      features" on the other hand, were often heard. However, the Standardization 
      committee has been very thoughtful and prudent in selecting these extensions 
      and modifications. Fortunately, the approval of the Standard by ISO in 1998 
      ensures its stability for at least five years. This freeze period enables 
      both compiler vendors and language users to digest the current Standard 
      and use it effectively. In doing so, one usually finds that C++ is better 
      today than it ever was. </p>
    <p>&nbsp;</p>
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