part2.htm.kbk
来自「c++设计思想」· KBK 代码 · 共 439 行 · 第 1/2 页
KBK
439 行
copy(l.begin(), l.end(), out);
cout << <font color=#004488>"\n Reversing the list:"</font> << endl;
l.reverse();
copy(l.begin(), l.end(), out);
cout << <font color=#004488>"\n Sorting the list:"</font> << endl;
l.sort();
copy(l.begin(), l.end(), out);
cout << <font color=#004488>"\n Swapping two elements:"</font> << endl;
list<Noisy>::iterator it1, it2;
it1 = it2 = l.begin();
it2++;
swap(*it1, *it2);
cout << endl;
copy(l.begin(), l.end(), out);
cout << <font color=#004488>"\n Using generic reverse(): "</font> << endl;
reverse(l.begin(), l.end());
cout << endl;
copy(l.begin(), l.end(), out);
cout << <font color=#004488>"\n Cleanup"</font> << endl;
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Operations as seemingly radical as
reversing and sorting the list require no copying of objects, because instead of
moving the objects, the links are simply changed. However, notice that
<B>sort( )</B> and <B>reverse( )</B> are member functions of
<B>list</B>, so they have special knowledge of the internals of <B>list</B> and
can perform the pointer movement instead of copying. On the other hand, the
<B>swap( )</B> function is a generic algorithm, and doesn’t know
about <B>list</B> in particular and so it uses the copying approach for swapping
two elements. There are also generic algorithms for <B>sort( )</B> and
<B>reverse( )</B>, but if you try to use these you’ll discover that
the generic <B>reverse( )</B> performs lots of copying and destruction (so
you should never use it with a <B>list</B>) and the generic <B>sort( )</B>
simply doesn’t work because it requires random-access iterators that
<B>list</B> doesn’t provide (a definite benefit, since this would
certainly be an expensive way to sort compared to <B>list</B>’s own<B>
sort( )</B>). The generic <B>sort( )</B> and <B>reverse( )</B>
should only be used with arrays, <B>vector</B>s and
<B>deque</B>s.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">If you have large and complex objects you
may want to choose a <B>list</B> first, especially if construction, destruction,
copy-construction and assignment are expensive and if you are doing things like
sorting the objects or otherwise reordering them a
lot.</FONT><A NAME="_Toc519042030"></A><BR></P></DIV>
<A NAME="Heading217"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H3 ALIGN="LEFT">
Special list operations</H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The <B>list</B> has some special
operations that are built-in to make the best use of the structure of the
<B>list</B>. You’ve already seen <B>reverse( )</B> and
<B>sort( )</B>, and here are some of the others in use:</FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: C07:ListSpecialFunctions.cpp</font>
<font color=#009900>//{L} ../TestSuite/Test</font>
#include <font color=#004488>"Noisy.h"</font>
#include <list>
#include <iostream>
#include <algorithm>
<font color=#0000ff>using</font> <font color=#0000ff>namespace</font> std;
ostream_iterator<Noisy> out(cout, <font color=#004488>" "</font>);
<font color=#0000ff>void</font> print(list<Noisy>& ln, <font color=#0000ff>char</font>* comment = <font color=#004488>""</font>) {
cout << <font color=#004488>"\n"</font> << comment << <font color=#004488>":\n"</font>;
copy(ln.begin(), ln.end(), out);
cout << endl;
}
<font color=#0000ff>int</font> main() {
<font color=#0000ff>typedef</font> list<Noisy> LN;
LN l1, l2, l3, l4;
generate_n(back_inserter(l1), 6, NoisyGen());
generate_n(back_inserter(l2), 6, NoisyGen());
generate_n(back_inserter(l3), 6, NoisyGen());
generate_n(back_inserter(l4), 6, NoisyGen());
print(l1, <font color=#004488>"l1"</font>); print(l2, <font color=#004488>"l2"</font>);
print(l3, <font color=#004488>"l3"</font>); print(l4, <font color=#004488>"l4"</font>);
LN::iterator it1 = l1.begin();
it1++; it1++; it1++;
l1.splice(it1, l2);
print(l1, <font color=#004488>"l1 after splice(it1, l2)"</font>);
print(l2, <font color=#004488>"l2 after splice(it1, l2)"</font>);
LN::iterator it2 = l3.begin();
it2++; it2++; it2++;
l1.splice(it1, l3, it2);
print(l1, <font color=#004488>"l1 after splice(it1, l3, it2)"</font>);
LN::iterator it3 = l4.begin(), it4 = l4.end();
it3++; it4--;
l1.splice(it1, l4, it3, it4);
print(l1, <font color=#004488>"l1 after splice(it1,l4,it3,it4)"</font>);
Noisy n;
LN l5(3, n);
generate_n(back_inserter(l5), 4, NoisyGen());
l5.push_back(n);
print(l5, <font color=#004488>"l5 before remove()"</font>);
l5.remove(l5.front());
print(l5, <font color=#004488>"l5 after remove()"</font>);
l1.sort(); l5.sort();
l5.merge(l1);
print(l5, <font color=#004488>"l5 after l5.merge(l1)"</font>);
cout << <font color=#004488>"\n Cleanup"</font> << endl;
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The <B>print( )</B> function is used
to display results. After filling four <B>list</B>s with <B>Noisy</B> objects,
one list is spliced into another in three different ways. In the first, the
entire list <B>l2</B> is spliced into <B>l1</B> at the iterator <B>it1</B>.
Notice that after the splice, <B>l2</B> is empty – splicing means removing
the elements from the source list. The second splice inserts elements from <B>l3
</B>starting at <B>it2</B> into <B>l1 </B>starting at <B>it1</B>. The third
splice starts at <B>it1</B> and uses elements from <B>l4</B> starting at
<B>it3</B> and ending at <B>it4</B> (the seemingly-redundant mention of the
source list is because the elements must be erased from the source list as part
of the transfer to the destination list)<B>.</B></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The output from the code that
demonstrates <B>remove( )</B> shows that the list does not have to be
sorted in order for all the elements of a particular value to be
removed.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Finally, if you <B>merge( )</B> one
list with another, the merge only works sensibly if the lists have been sorted.
What you end up with in that case is a sorted list containing all the elements
from both lists (the source list is erased – that is, the elements are
<I>moved</I> to the destination list).</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">There’s also a
<B>unique( )</B> member function that removes all duplicates, but only if
the <B>list</B> has been sorted first:</FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: C07:UniqueList.cpp</font>
<font color=#009900>// Testing list's unique() function</font>
<font color=#009900>//{L} ../TestSuite/Test</font>
#include <list>
#include <iostream>
<font color=#0000ff>using</font> <font color=#0000ff>namespace</font> std;
<font color=#0000ff>int</font> a[] = { 1, 3, 1, 4, 1, 5, 1, 6, 1 };
<font color=#0000ff>const</font> <font color=#0000ff>int</font> asz = <font color=#0000ff>sizeof</font> a / <font color=#0000ff>sizeof</font> *a;
<font color=#0000ff>int</font> main() {
<font color=#009900>// For output:</font>
ostream_iterator<<font color=#0000ff>int</font>> out(cout, <font color=#004488>" "</font>);
list<<font color=#0000ff>int</font>> li(a, a + asz);
li.unique();
<font color=#009900>// Oops! No duplicates removed:</font>
copy(li.begin(), li.end(), out);
cout << endl;
<font color=#009900>// Must sort it first:</font>
li.sort();
copy(li.begin(), li.end(), out);
cout << endl;
<font color=#009900>// Now unique() will have an effect:</font>
li.unique();
copy(li.begin(), li.end(), out);
cout << endl;
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The <B>list</B> constructor used here
takes the starting and past-the-end iterator from another container, and it
copies all the elements from that container into itself (a similar constructor
is available for all the containers). Here, the “container” is just
an array, and the “iterators” are pointers into that array, but
because of the design of the STL it works with arrays just as easily as any
other container.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">If you run this program, you’ll see
that <B>unique( )</B> will only remove <I>adjacent</I> duplicate elements,
and thus sorting is necessary before calling
<B>unique( )</B>.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">There are four additional <B>list
</B>member functions that are not demonstrated here: a <B>remove_if( )</B>
that takes a predicate which is used to decide whether an object should be
removed, a <B>unique( )</B> that takes a binary predicate to perform
uniqueness comparisons, a <B>merge( )</B> that takes an additional argument
which performs comparisons, and a <B>sort( )</B> that takes a comparator
(to provide a comparison or override the existing one).</FONT><BR></P></DIV>
<A NAME="Heading218"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H4 ALIGN="LEFT">
list vs. set</H4></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Looking at the previous example you may
note that if you want a sorted list with no duplicates, a <B>set</B> can give
you that, right? <B> </B>It’s interesting to compare the performance of
the two containers:</FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: C07:ListVsSet.cpp</font>
<font color=#009900>// Comparing list and set performance</font>
<font color=#009900>//{L} ../TestSuite/Test</font>
#include <iostream>
#include <list>
#include <set>
#include <algorithm>
#include <ctime>
#include <cstdlib>
<font color=#0000ff>using</font> <font color=#0000ff>namespace</font> std;
<font color=#0000ff>class</font> Obj {
<font color=#0000ff>int</font> a[20]; <font color=#009900>// To take up extra space</font>
<font color=#0000ff>int</font> val;
<font color=#0000ff>public</font>:
Obj() : val(rand() % 500) {}
<font color=#0000ff>friend</font> <font color=#0000ff>bool</font>
<font color=#0000ff>operator</font><(<font color=#0000ff>const</font> Obj& a, <font color=#0000ff>const</font> Obj& b) {
<font color=#0000ff>return</font> a.val < b.val;
}
<font color=#0000ff>friend</font> <font color=#0000ff>bool</font>
<font color=#0000ff>operator</font>==(<font color=#0000ff>const</font> Obj& a, <font color=#0000ff>const</font> Obj& b) {
<font color=#0000ff>return</font> a.val == b.val;
}
<font color=#0000ff>friend</font> ostream&
<font color=#0000ff>operator</font><<(ostream& os, <font color=#0000ff>const</font> Obj& a) {
<font color=#0000ff>return</font> os << a.val;
}
};
<font color=#0000ff>template</font><<font color=#0000ff>class</font> Container>
<font color=#0000ff>void</font> print(Container& c) {
<font color=#0000ff>typename</font> Container::iterator it;
<font color=#0000ff>for</font>(it = c.begin(); it != c.end(); it++)
cout << *it << <font color=#004488>" "</font>;
cout << endl;
}
<font color=#0000ff>struct</font> ObjGen {
Obj <font color=#0000ff>operator</font>()() { <font color=#0000ff>return</font> Obj(); }
};
<font color=#0000ff>int</font> main() {
<font ⌨️ 快捷键说明
复制代码Ctrl + C
搜索代码Ctrl + F
全屏模式F11
增大字号Ctrl + =
减小字号Ctrl + -
显示快捷键?