If you have large and complex objects you may want to choose a list 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.

Special list operations

The list has some special operations that are built-in to make the best use of the structure of the list. You’ve already seen reverse( ) and sort( ), and here are some of the others in use:

//: C04:ListSpecialFunctions.cpp #include "Noisy.h"

#include <list> #include <iostream> #include <algorithm> using namespace std;

ostream_iterator<Noisy> out(cout, " ");

void print(list<Noisy>& ln, char* comment = "") { cout << "\n" << comment << ":\n"; copy(ln.begin(), ln.end(), out);

cout << endl;

}

int main() {

typedef 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, "l1"); print(l2, "l2");

print(l3, "l3"); print(l4, "l4"); LN::iterator it1 = l1.begin(); it1++; it1++; it1++; l1.splice(it1, l2);

print(l1, "l1 after splice(it1, l2)"); print(l2, "l2 after splice(it1, l2)"); LN::iterator it2 = l3.begin();

it2++; it2++; it2++; l1.splice(it1, l3, it2);

print(l1, "l1 after splice(it1, l3, it2)"); LN::iterator it3 = l4.begin(), it4 = l4.end(); it3++; it4--;

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l1.splice(it1, l4, it3, it4);

print(l1, "l1 after splice(it1,l4,it3,it4)"); Noisy n;

LN l5(3, n);

generate_n(back_inserter(l5), 4, NoisyGen()); l5.push_back(n);

print(l5, "l5 before remove()"); l5.remove(l5.front());

print(l5, "l5 after remove()"); l1.sort(); l5.sort(); l5.merge(l1);

print(l5, "l5 after l5.merge(l1)"); cout << "\n Cleanup" << endl;

} ///:~

The print( ) function is used to display results. After filling four lists with Noisy objects, one list is spliced into another in three different ways. In the first, the entire list l2 is spliced into l1 at the iterator it1. Notice that after the splice, l2 is empty – splicing means removing the elements from the source list. The second splice inserts elements from l3 starting at it2 into l1 starting at it1. The third splice starts at it1 and uses elements from l4 starting at it3 and ending at it4 (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).

The output from the code that demonstrates remove( ) shows that the list does not have to be sorted in order for all the elements of a particular value to be removed.

Finally, if you merge( ) 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 moved to the destination list).

There’s also a unique( ) member function that removes all duplicates, but only if the list has been sorted first:

//: C04:UniqueList.cpp

// Testing list's unique() function #include <list>

#include <iostream> using namespace std;

int a[] = { 1, 3, 1, 4, 1, 5, 1, 6, 1 }; const int asz = sizeof a / sizeof *a;

int main() {

// For output:

ostream_iterator<int> out(cout, " "); list<int> li(a, a + asz);

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li.unique();

//Oops! No duplicates removed: copy(li.begin(), li.end(), out); cout << endl;

//Must sort it first: li.sort();

copy(li.begin(), li.end(), out); cout << endl;

//Now unique() will have an effect: li.unique();

copy(li.begin(), li.end(), out); cout << endl;

}///:~

The list 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.

If you run this program, you’ll see that unique( ) will only remove adjacent duplicate elements, and thus sorting is necessary before calling unique( ).

There are four additional list member functions that are not demonstrated here: a remove_if( ) that takes a predicate which is used to decide whether an object should be removed, a unique( ) that takes a binary predicate to perform uniqueness comparisons, a merge( ) that takes an additional argument which performs comparisons, and a sort( ) that takes a comparator (to provide a comparison or override the existing one).

list vs. set

Looking at the previous example you may note that if you want a sorted list with no duplicates, a set can give you that, right? It’s interesting to compare the performance of the two containers:

//: C04:ListVsSet.cpp

// Comparing list and set performance #include <iostream>

#include <list> #include <set> #include <algorithm> #include <ctime> #include <cstdlib> using namespace std;

class Obj {

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int a[20]; // To take up extra space int val;

public:

Obj() : val(rand() % 500) {} friend bool

operator<(const Obj& a, const Obj& b) { return a.val < b.val;

}

friend bool

operator==(const Obj& a, const Obj& b) { return a.val == b.val;

}

friend ostream&

operator<<(ostream& os, const Obj& a) { return os << a.val;

}

};

template<class Container> void print(Container& c) {

typename Container::iterator it;

for(it = c.begin(); it != c.end(); it++) cout << *it << " ";

cout << endl;

}

struct ObjGen {

Obj operator()() { return Obj(); }

};

int main() {

const int sz = 5000; srand(time(0)); list<Obj> lo;

clock_t ticks = clock(); generate_n(back_inserter(lo), sz, ObjGen()); lo.sort();

lo.unique();

cout << "list:" << clock() - ticks << endl; set<Obj> so;

ticks = clock(); generate_n(inserter(so, so.begin()),

sz, ObjGen());

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