📄 ec7.htm
字号:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Frameset//EN" "http://www.w3.org/TR/REC-html40/frameset.dtd">
<HTML LANG="EN">
<HEAD>
<title>Effective C++, 2E | Chapter 7: Miscellany</TITLE>
<LINK REL=STYLESHEET HREF=../INTRO/ECMEC.CSS>
<SCRIPT LANGUAGE="Javascript" SRC="../JAVA/COOKIE.JS"></SCRIPT>
<SCRIPT LANGUAGE="Javascript">var imagemax = 2; setCurrentMax(2);</SCRIPT>
<SCRIPT LANGUAGE="Javascript" SRC="../JAVA/IMGDOC.JS"></SCRIPT>
<SCRIPT LANGUAGE="Javascript" SRC="../JAVA/NSIMGDOC.JS"></SCRIPT>
<SCRIPT LANGUAGE="Javascript" SRC="../JAVA/DINGBATS.JS"></SCRIPT>
<SCRIPT LANGUAGE="Javascript">
var dingbase = "EC7_DIR.HTM";
var dingtext = "EC++ Miscellany, P";
if (self == top) {
top.location.replace(dingbase + this.location.hash);
}
</SCRIPT>
</HEAD>
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" ONLOAD="setResize()">
<!-- one image occurring in Chapter 7 -->
<!-- SectionName="EC++ Chapter Intro: Miscellany" -->
<A NAME="8158"></A><A NAME="p212"></A><A NAME="8156"></A><A NAME="8157"></A><DIV ALIGN="CENTER"><FONT SIZE="-1">Back to <A HREF="./EC6_FR.HTM#8113" TARGET="_top">Item 44: Say what you mean; understand what you're saying.</A>
<BR>Continue to <A HREF="#8160">Item 45: Know what functions C++ silently writes and calls.</A></FONT></DIV>
<P><A NAME="dingp1"></A><FONT ID="egtitle">Miscellany</FONT><SCRIPT>create_link(1);</SCRIPT>
</P>
<P><A NAME="dingp2"></A>
Some guidelines for effective C++ programming defy convenient categorization. This section is where such guidelines come to roost. Not that that diminishes their importance. If you are to write effective software, you must understand what compilers are doing for you (to you?) behind your back, how to ensure that non-local static objects are initialized before they are used, what you can expect from the standard library, and where to go for insights into the language's underlying design philosophy. In this final section of the book, I expound on these issues, and <NOBR>more.<SCRIPT>create_link(2);</SCRIPT>
</NOBR></P>
<!-- SectionName="E45: Know the implicitly generated functions" -->
<A NAME="8160"></A><A NAME="8163"></A>
<DIV ALIGN="CENTER"><FONT SIZE="-1">Back to <A HREF="#8156">Miscellany</A>
<BR>Continue to <A HREF="#195225">Item 46: Prefer compile-time and link-time errors to runtime errors.</A></FONT></DIV>
<P><A NAME="dingp3"></A><FONT ID="eititle">Item 45: Know what functions C++ silently writes and calls.</FONT><SCRIPT>create_link(3);</SCRIPT>
</P>
<P><A NAME="dingp4"></A>
When is an empty class not an empty class? When C++ gets through with it. If you don't declare them yourself, your thoughtful compilers will declare their own versions of a copy constructor, an assignment operator, a destructor, and a pair of address-of operators. Furthermore, if you don't declare any constructors, they will declare a default constructor for you, too. All these functions will be public. In other words, if you write <NOBR>this,<SCRIPT>create_link(4);</SCRIPT>
</NOBR></P>
<A NAME="8166"></A>
<UL><PRE>class Empty{};
</PRE>
</UL><A NAME="8167"></A><p><A NAME="dingp5"></A>it's the same as if you'd written <NOBR>this:<SCRIPT>create_link(5);</SCRIPT>
</NOBR></P>
<A NAME="8169"></A>
<UL><PRE>class Empty {
public:
Empty(); // default constructor
Empty(const Empty& rhs); // copy constructor
</PRE>
</UL><A NAME="23223"></A>
<UL><PRE>
~Empty(); // destructor — see
// below for whether
// it's virtual
Empty&
operator=(const Empty& rhs); // assignment operator
</PRE>
</UL><A NAME="8171"></A>
<UL><PRE>
Empty* operator&(); // address-of operators
const Empty* operator&() const;
};
</PRE>
</UL></P><A NAME="8174"></A>
<P><A NAME="dingp6"></A>
<A NAME="p213"></A>Now these functions are generated only if they are needed, but it doesn't take much to need them. The following code will cause each function to be <NOBR>generated:<SCRIPT>create_link(6);</SCRIPT>
</NOBR></P>
<A NAME="8175"></A>
<UL><PRE>
const Empty e1; // default constructor;
// destructor
</PRE>
</UL><A NAME="23271"></A>
<UL><PRE>
Empty e2(e1); // copy constructor
</PRE>
</UL><A NAME="23272"></A>
<UL><PRE>
e2 = e1; // assignment operator
</PRE>
</UL><A NAME="8176"></A>
<UL><PRE>
Empty *pe2 = &e2; // address-of
// operator (non-const)
</PRE>
</UL><A NAME="8177"></A>
<UL><PRE>
const Empty *pe1 = &e1; // address-of
// operator (const)
</PRE>
</UL><A NAME="222409"></A>
<P><A NAME="dingp7"></A>
Given that compilers are writing functions for you, what do the functions do? Well, the default constructor and the destructor don't really do anything. They just enable you to create and destroy objects of the class. (They also provide a convenient place for implementers to place code whose execution takes care of "behind the scenes" behavior — see Items <A HREF="./EC5_FR.HTM#6729" TARGET="_top">33</A> and <A HREF="../MEC/MC4_FR.HTM#41284" TARGET="_top">M24</A>.) Note that the generated destructor is nonvirtual (see <A HREF="./EC3_FR.HTM#223029" TARGET="_top">Item 14</A>) unless it's for a class inheriting from a base class that itself declares a virtual destructor. The default address-of operators just return the address of the object. These functions are effectively defined like <NOBR>this:<SCRIPT>create_link(7);</SCRIPT>
</NOBR></P>
<A NAME="8180"></A>
<UL><PRE>inline Empty::Empty() {}
</PRE>
</UL><A NAME="23286"></A>
<UL><PRE>inline Empty::~Empty() {}
</PRE>
</UL><A NAME="8182"></A>
<UL><PRE>inline Empty * Empty::operator&() { return this; }
</PRE>
</UL><A NAME="8184"></A>
<UL><PRE>inline const Empty * Empty::operator&() const
{ return this; }
</PRE>
</UL><A NAME="8187"></A>
<P><A NAME="dingp8"></A>
As for the copy constructor and the assignment operator, the official rule is this: the default copy constructor (assignment operator) performs memberwise copy construction (assignment) of the nonstatic data members of the class. That is, if <CODE>m</CODE> is a nonstatic data member of type <CODE>T</CODE> in a class <CODE>C</CODE> and <CODE>C</CODE> declares no copy constructor (assignment operator), <CODE>m</CODE> will be copy constructed (assigned) using the copy constructor (assignment operator) defined for <CODE>T</CODE>, if there is one. If there isn't, this rule will be recursively applied to <CODE>m</CODE>'s data members until a copy constructor (assignment operator) or built-in type (e.g., <CODE>int</CODE>, <CODE>double</CODE>, pointer, etc.) is found. By default, objects of built-in types are copy constructed (assigned) using bitwise copy from the source object to the destination object. For classes that inherit from other classes, this rule is applied to each level of the inheritance hierarchy, so user-defined copy constructors and assignment operators are called at whatever level they are <NOBR>declared.<SCRIPT>create_link(8);</SCRIPT>
</NOBR></P>
<A NAME="8191"></A>
<P><A NAME="dingp9"></A>
I hope that's crystal <NOBR>clear.<SCRIPT>create_link(9);</SCRIPT>
</NOBR></P>
<A NAME="23653"></A>
<P><A NAME="dingp10"></A>
<A NAME="p214"></A>But just in case it's not, here's an example. Consider the definition of a <CODE>NamedObject</CODE> template, whose instances are classes allowing you to associate names with <NOBR>objects:<SCRIPT>create_link(10);</SCRIPT>
</NOBR></P>
<A NAME="8197"></A>
<UL><PRE>template<class T>
class NamedObject {
public:
NamedObject(const char *name, const T& value);
NamedObject(const string& name, const T& value);
</PRE>
</UL><A NAME="8199"></A>
<UL><PRE> ...
</PRE>
</UL><A NAME="23315"></A>
<UL><PRE>private:
string nameValue;
T objectValue;
};
</PRE>
</UL><A NAME="8201"></A>
<P><A NAME="dingp11"></A>
Because the <CODE>NamedObject</CODE> classes declare at least one constructor, compilers won't generate default constructors, but because the classes fail to declare copy constructors or assignment operators, compilers will generate those functions (if they are <NOBR>needed).<SCRIPT>create_link(11);</SCRIPT>
</NOBR></P>
<A NAME="24864"></A>
<P><A NAME="dingp12"></A>
Consider the following call to a copy <NOBR>constructor:<SCRIPT>create_link(12);</SCRIPT>
</NOBR></P>
<A NAME="8202"></A>
<UL><PRE>NamedObject<int> no1("Smallest Prime Number", 2);
</PRE>
</UL><A NAME="8203"></A>
<UL><PRE>NamedObject<int> no2(no1); // calls copy constructor
</PRE>
</UL></P><A NAME="8204"></A>
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -