A Presentation of the STL Vector Container

原文地址 http://www.codeproject.com/Articles/5378/A-Presentation-of-the-STL-Vector-Container

Download Console Demo - 6.19 Kb

Download MFC Demo - 14.6 Kb

Introduction

This article aims to introduce the 

std::vector

 as well as cover some of the most common 

vector

 member functions and how to use them properly. The article will also discuss predicates and function pointers as used in common iterator
algorithms such as 

remove_if()

 and 

for_each()

. After reading this article, the reader should be able to use the 

vector

 container effectively and should find no use for dynamic C-style arrays again.

Vector Overview

The 

vector

 is part of the C++ Standard Template Library (STL); an amalgamation of general-purpose, templatized classes and functions that implement a plethora of common data structures and algorithms. The 

vector

 is considered a container class.
Like containers in real-life, these containers are objects that are designed to hold other objects. In short, a 

vector

 is a dynamic array designed to hold objects of any type, and capable of growing and shrinking as needed.

In order to use 

vector

, you need to include the following header file:

#include <vector>

The vector is part of the 

std

 namespace, so you need to qualify the name. This can be accomplished as shown here:

using std::vector;
vector<int> vInts;

or you can fully qualify the name like this:

std::vector<int> vInts;

I do however suggest that you refrain from declaring global namespaces such as:

using namespace std;

This pollutes the global namespace and may cause problems in later implementations. As for the interface to the

vector

 container, I have listed the member functions and operators of 

vector

 in the tables below.

Vector Member Functions 1

Function	Description
assign	Erases a  vector  and copies the specified elements to the empty vector.
at	Returns a reference to the element at a specified locati 4000 on in the  vector .
back	Returns a reference to the last element of the  vector .
begin	Returns a random-access iterator to the first element in the container.
capacity	Returns the number of elements that the  vector  could contain without allocating more storage.
clear	Erases the elements of the  vector .
empty	Tests if the  vector  container is empty.
end	Returns a random-access iterator that points just beyond the end of the vector .
erase	Removes an element or a range of elements in a  vector  from specified positions.
front	Returns a reference to the first element in a  vector .
get_allocator	Returns an object to the allocator class used by a  vector .
insert	Inserts an element or a number of elements into the  vector  at a specified position.
max_size	Returns the maximum length of the  vector .
pop_back	Deletes the element at the end of the  vector .
push_back	Adds an element to the end of the  vector .
rbegin	Returns an iterator to the first element in a reversed  vector .
rend	Returns an iterator to the end of a reversed  vector .
resize	Specifies a new size for a  vector .
reserve	Reserves a minimum length of storage for a  vector  object.
size	Returns the number of elements in the  vector .
swap	Exchanges the elements of two vectors.
vector	Constructs a  vector  of a specific size or with elements of a specific value or with a specific allocator or as a copy of some other  vector .

Vector Operators 1

Operator	Description
operator[]	Returns a reference to the  vector  element at a specified position.

Constructing a Vector

There are several constructors provided for the 

vector

 container. Here are some of the most common.

Construct an empty vector to hold objects of type Widget:

vector<Widget> vWidgets;
//     ------
//      |
//      |- Since vector is a container, its member functions
//         operate on iterators and the container itself so
//         it can hold objects of any type.

Construct a vector to hold 500 Widgets:

vector<Widget> vWidgets(500);

Construct a vector to hold 500 Widgets initialized to 0:

vector<Widget> vWidgets(500, Widget(0));

Construct a vector of Widgets from another vector of Widgets:

vector<Widget> vWidgetsFromAnother(vWidgets);

Adding Elements to a Vector

The default method of adding elements to a 

vector

 is by using 

push_back()

. The 

push_back()

 member function is responsible for adding elements to the end of a 

vector

 and allocating memory as needed. For
example, to add 10 

Widget

s to a 

vector<Widget>

, you would use the following code:

for(int i= 0;i<10; i++)
vWidgets.push_back(Widget(i));

Getting the Number of Elements in a Vector

Many times it is necessary to know how many elements are in your 

vector

 (if there are any at all). Remember, the

vector

 is dynamic, and the number of allocations and 

push_back()

s are usually determined by a user file
or some other data source. To test if your 

vector

 contains anything at all, you call 

empty()

. To get the number of elements, you call 

size()

. For example, if you wanted to initialize a variable to -1 if your 

vector

, 

v

,
is empty or to the number of elements in 

v

 if it's not, you would use the following code:

int nSize = v.empty() ? -1 : static_cast<int>(v.size());

Accessing Elements in a Vector

There are two ways to access objects stored in a 

vector

.

vector::at()

vector::operator[]

The 

operator[]

 is supported mainly for compatibility with a legacy C code base. It operates the same way a standard C-style array 

operator[]

 works. It is always preferred
to use the 

at()

 member function, however, as

at()

 is bounds-checked, and will throw an exception if we attempt to access a location beyond the 

vector

 limits. However, 

operator[]

 could
care less and can cause some serious bugs as we will now demonstrate.

Consider the following code:

vector<int> v;
v.reserve(10);

for(int i=0; i<7; i++)
v.push_back(i);

try
{
int iVal1 = v[7];  // not bounds checked - will not throw
int iVal2 = v.at(7); // bounds checked - will throw if out of range
}
catch(const exception& e)
{
cout << e.what();
}

Now, just because we reserved space for 10 

int

s doesn't mean they are initialized or even defined. This scenario is best illustrated in the figure below.



You can try this code on your own and observe the results or see a similar situation in the demo. The bottom line: use 

at()

 whenever you can.

Removing Elements from a Vector

Getting things into a 

vector

 is fairly easy, but getting them out can be subtly tricky. First of all, the only member functions a 

vector

 has, to get rid of elements are 

erase()

, 

pop_back()

, and 

clear()

.
Now these are sufficient if you know: which elements you need to remove; if you only need to remove the last element; or if you want to remove all of the elements, respectively. Now before you go off writing some crazy loop to flag which elements you want
to erase and brute-force yourself an application-specific implementation, sit back, take a deep breath, and think to yourself: "S...T...L...".

The remove_if() Algorithm

Now, this is where we get into some fun stuff. To use 

remove_if()

, you will need to include the following file:

#include <algorithm>

remove_if()

 takes 3 parameters:

iterator _First

: An iterator pointing to the first element in the range on which to operate.

iterator _Last

: An iterator pointing to the last element in the range on which to operate.

predicate _Pred

: A predicate to apply to the iterator being evaluated.

Predicates

Predicates are basically pointers to a function or an actual function object that returns a yes/no answer to a user defined evaluation of that object. The function object is required to supply a function call operator, 

operator()()

.
In the case of 

remove_if()

, it is often useful to supply a function object derived from 

unary_function

 to allow the user to pass data to the predicate for evaluation.

For example, consider a scenario in which you want to remove elements from a 

vector<CString>

 based on certain matching criteria, i.e. if the string contains a value, starts with a value, ends with a value or is a
value. First you would set up a data structure that would hold the relevant data, similar to the following code:

#include <functional>
enum findmodes
{
FM_INVALID = 0,
FM_IS,
FM_STARTSWITH,
FM_ENDSWITH,
FM_CONTAINS
};
typedef struct tagFindStr
{
UINT iMode;
CString szMatchStr;
} FindStr;
typedef FindStr* LPFINDSTR;

You could then proceed to implement the predicate as shown:

class FindMatchingString
: public std::unary_function<CString, bool>
{

public:
FindMatchingString(const LPFINDSTR lpFS) : m_lpFS(lpFS) {}

bool operator()(CString& szStringToCompare) const
{
bool retVal = false;

switch(m_lpFS->iMode)
{
case FM_IS:
{
retVal = (szStringToCompare == m_lpFDD->szMatchStr);
break;
}
case FM_STARTSWITH:
{
retVal = (szStringToCompare.Left(m_lpFDD->szMatchStr.GetLength())
== m_lpFDD->szWindowTitle);
break;
}
case FM_ENDSWITH:
{
retVal = (szStringToCompare.Right(m_lpFDD->szMatchStr.GetLength())
== m_lpFDD->szMatchStr);
break;
}
case FM_CONTAINS:
{
retVal = (szStringToCompare.Find(m_lpFDD->szMatchStr) != -1);
break;
}
}

return retVal;
}

private:
LPFINDSTR m_lpFS;
};

With this implementation, you could effectively remove strings from a 

vector

 as shown below:

// remove all strings containing the value of
// szRemove from vector<CString> vs.

FindStr fs;
fs.iMode = FM_CONTAINS;
fs.szMatchStr = szRemove;

vs.erase(std::remove_if(vs.begin(), vs.end(), FindMatchingString(&fs)), vs.end());

What remove_if() Really Does

You may be wondering why I have a call to 

erase()

 when I am calling 

remove_if()

 in the above example. The reason for this is quite subtle and not intuitive for people not familiar with iterators or the STL algorithms. Since

remove()

, 

remove_if()

,
and all the remove cousins operate only on a range of iterators, they cannot operate on the container's internals. The call to 

remove_if()

 actually shuffles around the elements of the container being operated on. Consider the above example if:

szRemove

 = "o".

vs

 = the diagram shown below.



After observing the results, one can see that 

remove_if()

 actually returns a forward iterator addressing the new end position of the modified range, one past the final element of the remnant sequence, free of the specified value. The remaining
elements may or may not have their original value, to be safe, always assume they are unknown.

The call to 

erase()

 then is responsible for actually disposing off the "removed" elements. Notice in the above example that we pass the range from the result of 

remove_if()

 through 

vs.end()

 to 

erase()

.

Shrinking a Bloated Vector

Many times, after lots of data removal or a liberal call to 

reserve()

, you can end up with a 

vector 

whose internals are far larger than they need to be. In this case, it is often desirable to "shrink" the 

vector 

to
fit the size of the data. However, the 

resize() 

member function is only capable of increasing the capacity of the 

vector

. The only member capable of changing the size of the internal buffer is 

clear()

, however this has
some significant drawbacks like destroying everything in the 

vector

. So how can one solve this problem? Well, let's attempt a first pass.

Remember that we can construct a 

vector 

from another 

vector

. Let's see what happens. Consider we have a

vector

, 

v

, with a capacity of 1000. We then make a call to 

size() 

and realize we only
have 7 elements in the

vector

. Wow, we're wasting lots of memory! Let's try and make a new 

vector 

from our current one.

std::vector<CString> vNew(v);
cout << vNew.capacity();

It turns out 

vNew.capacity() 

returns 7. The new 

vector

 only allocates enough space for what it gets from 

v

. Now, I don't want
c0bf
to get rid of 

v

, because I may be using it all over the place. What if I try
and 

swap()

 the internals of 

v

 and 

vNew

?

vNew.swap(v);
cout << vNew.capacity();
cout << v.capacity();

Interesting: 

vNew.capacity() 

is now 1000, and 

v.capacity() 

is 7. Sweet, it worked.

Yes, it may have worked, but that's not an elegant solution, now is it? How can we make this swap trick have some flair? Do we really need to name our temporary variable? Lets try another approach. Consider this code:

std::vector<CString>(v).swap(v);

Can you see what we did there? We have created a temporary unnamed variable instead of a named one and did the 

swap() 

all in one step. Much more elegant. Now when the temporary variable goes out of scope, it will take its bloated internal buffer
with it, and then we're left with a nice trim 

v

.

Putting it All Together

In the sample applications, you can see how to implement the topics discussed here. The console application is a step-by-step illustration of these examples in action, and the MFC application is mainly a demonstration of using STL, algorithms, and predicates
with MFC.

Conclusion

I hope this article serves as a valuable reference and guideline for developers using the STL 

vector

 container. I would also hope that those who were skeptical about using the 

vector

 before reading this article are now undaunted
and ready to throw away their C-style arrays and try it out.

References

Plauger, P.J. Standard C++ Library Reference. February, 2003. MSDN.
Schildt, Herbert. C++ from the Ground Up, Second Edition. Berkeley: 1998.
Sutter, Herb. More Exceptional C++. Indianapolis: 2002.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here