Notes from C++ Primer
Initialize container by iterator
When copy a container to another, the container type and element type must be match at the same time:
vector<int> ivec; vector<int> ivec2(ivec); // ok: ivec is vector<int> list<int> ilist(ivec); // error: ivec is not ilist<int> vector<double> dvec(ivec); // error: ivec holds int not double
Use iterator to intialize container:
// initialize slist with copy of each element of svec list<string> slist(svec.begin(), svec.end()); // find midpoint in the vector vector<string>::iterator mid = svec.begin() + svec.size() / 2; // initialize front with first half of svec: the elements up to but not including *mid deque<string> front(svec.begin(), mid); // initialize front with second half of svec: the elements *mid through end of svec deque<string> back(mid, svec.end());
Because the pointer is iterator, so we can use pointer to initialize container.
char *words[] = {"stately", "plump", "buck", "mulligan"};
// calculate how many elements in words
size_t words_size = sizeof[words] / sizeof(char *);
// use entire array to initialize words2
list<string> words2(words, words + words_size);
Initialize container by specifying size
We can use variable to specify the size of container. And it's alternative to assign the initial element or not.
const list<int>::size_type list_size = 64;
list<string> slist(list_size, "eh?"); // 64 strings, each is eh?
...
list<int> ilist(list_size); // 64 elements, each initialized to 0
// svec has as many elements as the return value from tet_word_count
extern unsigned get_word_count(const string &file_name);
vector<string> svec(get_word_count("Chimera"));
P.S.: Only the sequential container can be initialized by size.
The type of container must satisfy two base conditions:
- the element supports assignment
- element can be copied
Some operations of container only can be used when the container element supports corresponding operation. For example, if type Foo doesn't have default constructor, its constructor has one int formal parameter. Then vector<Food> can't be initialized only by assigning size:
vector<Foo> empty; // ok: no need for element default constructor vector<Foo> bad(10); // error: no default constructor for Foo vector<Foo> ok(10, 1); // ok: each element initialized to 1
As container satisfy the above two base conditions, then container can be container type. But when we define the contianer of container type, we need to notice the space between close ">":
// note spacing: use "> >" not ">>" when specifying a container element type vector< vector<string> > lines; // vector of vectors vector< vector<string> > lines; // ok: space required between close ">" vecotr< vector<string>> lines; // error: >> treated as shift operator
When we use iterator, only vector and deque supports using position of element accessing container element:
vector<int>::iterator iter = vec.begin() + vec.size() / 2; ... // copy elements from vec into ilist list<int> ilist(vec.begin(), vec.end()); ilist.begin() + ilist.size() / 2; // error: no addition on list iterators.
Adding elements in sequential container
Add elements at the end:
list<int> ilist; // add elements at the end of ilist for(size_t ix = 0; ix != 4; ++ix) ilist.push_back(ix);
Add elements at the beginning, which only can be used by list and deque:
// add elements to the start of ilist for(size_t ix = 0; ix != 4; ++ix) ilist.push_front(ix);
When we add elements to the container, system only copy the value into the container. The object copied and element in container are separate and independent.
Add element at specifying position:
- c.insert(p, t): inserting element with value $t$ in front of iterator p, return the iterator of new element in container.
vector<string> svec; list<string> slist; string spouse("Beth"); // equivalent to calling slist.push_front(pouse); slist.insert(slist.begin(), spouse); // no push_front on vector but we can insert before begin() // warning: inserting anywhere but at the end of a vector is an expensive operation svec.insert(svec.begin(), spouse); ... slist.insert(iter, spouse); // insert spouse just before iter ... list<string> lst; list<string>::iterator iter = lst.begin(); while(cin >> word) iter = lst.insert(iter, word); // same as calling push_front - c.insert(p, n, t): insert $n$ elements with same value $t$ in front of iterator p, return void:
svec.insert(svec.end(), 10, "Anna");
- c.insert(p, b, e): insert elements from iterator b to just before iterator e in front of p, return void:
string sarray[4] = {"quasi", "simba", "frollo", "scar"}; // insert all the elements in sarray and at the end of slist slist.insert(slist.end(), sarray, sarray + 4); list<string>::iterator slist_iter = slist.begin(); // insert last two elements of sarray before slist_iter slist.insert(slist_iter, sarray + 2, sarray + 4);
The insert and push operation both may make the iterator invalid especially in a loop. It's better to ensure the update of iterators after every steps of a loop.
vector<int>::iterator first = v.begin(), last = v.end(); // cache end iterator
// disaster: behavior of this loop is undefined
while(first != last)
{
// do some processing
// insert new value and reassign first, which otherwise would be invalid
first = v.insert(first, 42);
++first; // advance first just pass the element we added
}
So, in order to avoid cache end iterator, we need to keep updating:
// safer: recalculate end on each trip whenever the loop adds/erases elements
while(first != v.end())
{
// do some processing
first = v.insert(first, 42); // insert new value
++first; // advance first just pass the element we added
++first; // advance the element we just operated
}
Container size operation
There're two ways to resize container. If the current size of container is larger than the new container, the tailing elements will be deleted.
- c.resize(n): make size of the container to be $n$.
- c.resize(n, t): make size of the container to be $n$, and all adding new elements will be $t$.
list<int> ilist(10, 42); // 10 ints: each has value 42 ilist.resize(15); // adds 5 elements of value 0 to back of ilist ilist.resize(25, -1); // adds 10 elements of value -1 to back of ilist ilist.resize(5); // erases 20 elements from the back of ilist
Access element
If the container is not empty, then we can use front() and back() to access first and last elements:
// check that there are elements before dereferencing an iterator
// or calling front or back
if(!ilist.empty())
{
// val and val2 refer to the same element
list<int>::reference val = *ilist.begin();
list<int>::reference val2 = ilist.front();
// last and last2 refer to the same element
list<int>::reference last = *--ilist.end();
list<int>::reference last2 = ilist.back();
}
Before calling front() and back(),we must check if the container is empty. The of dereferencing empty container's iterator and calling front() and back() are undefined.
In order to be safer, we can call at() to access element of container:
vector<string> svec; // empty vector cout << svec[0]; // run-time error: There are no elements in svec! cout << svec.at(0); // throw out_of_range exception
Delete elements
We can use pop_font() and pop_back() to erase the first and last elements. But pop_front() only can be used by list and deque containers. They all return void. So if we want to get the deleted element, we need to call front() or back() before calling pop_font() and pop_back().
while(ilist.empty())
{
process(ilist.front()); // do something with the current top of ilist
ilist.pop_front(); // done: remove first element
}
The way to delete a piece of elements is using an iterator or a couple of iterators. They both return an iterator indicating the element just after the deleted element or elements. Before using erase, we must check if the element we want to delete is existing.
#include<algorithm>
...
string searchValue("Quasimodo");
list<string>::iterator iter = find(slist.begin(), slist.end(), searchValue);
if(iter != slist.end())
slist.erase(iter);
If we want to erase all the elements in the container, we can call clear() function or pass begin and end iterator to erase function.
slist.clear(); // delete all the elements within the container slist.erase(slist.begin(), slist.end()); // equivalent
More over, erase function supports delete a piece of elements as mentioned above:
// delete range of elements between two values list<string>::iterator elem1, elem2; // elem1 refers to val1 elem1 = find(slist.begin(), slist.end(), val1); // elem2 refers to val2 elem2 = find(slist.begin(), slist.end(), val2); // erase range from val1 to val2 but not including val2 slist.erase(elem1, elem2);
Assignment and swap
Assignment operator(=) of container is equivalent to erase all the elements of left operand and then insert all the elements of right operand into left operand:
c1 = c2; // replace contents of c1 with a copy of elements in c2 // equivalent operation using erase and insert c1.erase(c1.begin(), c1.end()); // delete all the elements in c1 c1.insert(c2.begin(), c2.end()); // insert c2
Assign operation( assign() ) is also equivalent to delete all the elements in the container and then inserts new elements as specified by the assignments. But they do have DIFFERENCE:
Assignment operator(=) is the same as copy constructor only can be used when the two operands have same container and elements type. So, if we want to assign elements of a different but compatible element type and/or from a different container type, then we MUST use the assign operation: assign().
The code is almost the same as assignment operator:
// equivalent to slist1 = slist2 slist1.assign(slist2.begin(), slist.end()); ... // equivalent to: slist1.clear() // follow by slist1.insert(slist1.begin(), 10, "Hiya!"); slist1.assign(10, "Hiya!"); // 10 elements: each one is Hiya!
Operation swap executes much faster than assign operation. More important it can guarantee the iterators validate, because no elements are removed.
vector<string> svec1(10); // vector with 10 elements vector<string> svec2(24); // vector with 24 elements svec1.swap(svec2);
Capacity and reserve member variable
vector class offers two member capacity and reserve to implement partial memory allocation. size is the number of current elements in the container. But the capacity indicates the total number of elements can be stored in container before reallocating more memory space:
vector<int> ivec; // size should be zero; capacity is implementation defined cout << "ivec: size: " << ivec.size() << " capacity: " << ivec.capacity() << endl; // give ivec 25 elements for(vector<int>::size_type ix = 0; ix != 24; ++ix) ivec.push_back(ix); // size should be 24; capacity will be >= 24 and is implementation defined cout << "ivec: size: " << ivec.size() << " capacity: " << ivec.capacity() << endl;
After the execution, we get the answer:
ivec: size: 0 capacity: 0 ivec: size: 24 capacity: 32
Now, we can use reserve() funtion to change the size of capacity:
ivec.reserve(50); // sets capacity to at least 50; might be more // size should be 24; capacity will be >= 50 and is implementation defined cout << "ivec: size: " << ivec.size() << " capacity: " << ivec.capacity() << endl;
then the result will be:
ivec: size: 24 capacity: 50
Now, let's use an example to show the situation when we use up the reserved capacity:
// add elements to use up the excess capacity while(ivec.size() != ivec.capacity()) ivec.push_back(0); // size should be 50; capacity should be unchanged cout << "ivec: size: " << ivec.size() << " capacity: " << ivec.capacity() << endl;
the result is:
ivec: size: 50 capacity: 50
If we continue to add element, then the vector has to reallocate its memory:
ivec.push_back(42); // add one more element // size should be 51; capacity will be >= 51 and is implementation defined cout << "ivec: size: " << ivec.size() << " capacity: " << ivec.capacity() << endl;
the result becomes:
ivec: size: 51 capacity: 100