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<title>Effective C++, 2E | Item 27: Explicitly disallow use of implicitly generated member functions you don't want</TITLE>
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<A NAME="6406"></A>
<DIV ALIGN="CENTER"><FONT SIZE="-1">Back to <A HREF="./EI26_FR.HTM" TARGET="_top">Item 26: Guard against potential ambiguity. </A> <BR> Continue to <A HREF="./EI28_FR.HTM" TARGET="_top">Item 28: Partition the global namespace.</A></FONT></DIV>
<P><A NAME="dingp1"></A><FONT ID="eititle">Item 27: Explicitly disallow use of implicitly generated member functions you don't want.</FONT><SCRIPT>create_link(1);</SCRIPT>
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<A NAME="6407"></A>
<P><A NAME="dingp2"></A>
Suppose you want to write a class template, <CODE>Array</CODE>, whose generated classes behave like built-in C++ arrays in every way, except they perform bounds checking. One of the design problems you would face is how to prohibit assignment between <CODE>Array</CODE> objects, because assignment isn't legal for C++ <NOBR>arrays:<SCRIPT>create_link(2);</SCRIPT>
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<UL><PRE>double values1[10];
double values2[10];
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<UL><PRE>values1 = values2; // error!
</PRE>
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<P><A NAME="dingp3"></A>
For most functions, this wouldn't be a problem. If you didn't want to allow a function, you simply wouldn't put it in the class. However, the assignment operator is one of those distinguished member functions that C++, always the helpful servant, writes for you if you neglect to write it yourself (see <A HREF="./EI45_FR.HTM#8160" TARGET="_top">Item 45</A>). What then to <NOBR>do?<SCRIPT>create_link(3);</SCRIPT>
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<P><A NAME="dingp4"></A>
The solution is to declare the function, <CODE>operator=</CODE> in this case, <I>private</I>. By declaring a member function explicitly, you prevent compilers from generating their own version, and by making the function private, you keep people from calling <NOBR>it.<SCRIPT>create_link(4);</SCRIPT>
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<P><A NAME="dingp5"></A>
However, the scheme isn't foolproof; member and friend functions can still call your private function. <I>Unless</I>, that is, you are clever enough not to <I>define</I> the function. Then if you inadvertently call the function, you'll get an error at link-time (see <A HREF="./EI46_FR.HTM#195225" TARGET="_top">Item 46</A>).<SCRIPT>create_link(5);</SCRIPT>
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<P><A NAME="dingp6"></A>
For <CODE>Array</CODE>, your template definition would start out like <NOBR>this:<SCRIPT>create_link(6);</SCRIPT>
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<UL><PRE>template<class T>
class Array {
private:
// Don't define this function!
Array& operator=(const Array& rhs);
</PRE>
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<UL><PRE> ...
</PRE>
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<UL><PRE>};
</PRE>
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<P><A NAME="dingp7"></A>
<A NAME="p117"></A>Now if a client tries to perform assignments on <CODE>Array</CODE> objects, compilers will thwart the attempt, and if you inadvertently try it in a member or a friend function, the linker will <NOBR>yelp.<SCRIPT>create_link(7);</SCRIPT>
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<P><A NAME="dingp8"></A>
Don't assume from this example that this Item applies only to assignment operators. It doesn't. It applies to each of the compiler-generated functions described in <A HREF="./EI45_FR.HTM#8160" TARGET="_top">Item 45</A>. In practice, you'll find that the behavioral similarities between assignment and copy construction (see Items <A HREF="./EI11_FR.HTM#2042" TARGET="_top">11</A> and <A HREF="./EI16_FR.HTM#2225" TARGET="_top">16</A>) almost always mean that anytime you want to disallow use of one, you'll want to disallow use of the other, <NOBR>too.<SCRIPT>create_link(8);</SCRIPT>
</NOBR></P>
<DIV ALIGN="CENTER"><FONT SIZE="-1">Back to <A HREF="./EI26_FR.HTM" TARGET="_top">Item 26: Guard against potential ambiguity. </A> <BR> Continue to <A HREF="./EI28_FR.HTM" TARGET="_top">Item 28: Partition the global namespace.</A></FONT></DIV>
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