(C++) std::vector

June 10, 2019 · View on GitHub

STL container for storing instances of any data type.

Example: create a std::vector

//Create an empty std::vector
std::vector<int> v;

//Create a std::vector with size 10, all elements zero
std::vector<int> v(10);

//Create a std::vector with size 10, all elements 42
std::vector<int> v(10, 42);

//Create a std::vector with one element: 10
std::vector<int> v{10};
std::vector<int> v = {10};

//Create a std::vector with two elements: 10 and 42
std::vector<int> v{10, 42};
std::vector<int> v = {10, 42};

Example: create a std::vector for another STL container

//Create a copy of a std::vector
std::vector<int> v(some_other_vector);
std::vector<int> v = some_other_vector; //Calls copy constructor, like above

//Create a std::vector with same elements as an STL container
std::vector<int> v(std::begin(some_container), std::end(some_container));

Example: convert `argv` to `std::vector<std::string>`

Because you can create a std::vector from two iterators, you can use argc and argv to get the command-line arguments in a std::vector<std::string>:

#include <cassert>
#include <string>
#include <vector>

int main(int argc, char* argv[])
{
  const std::vector<std::string> args(argv, argv + argc);
  assert(argc == static_cast<int>(args.size()));
  assert(argv[0] == args[0]);
}

Example: sorting a std::vector

#include <algorithm>
#include <cassert>
#include <vector>

int main()
{
  std::vector<int> v = {4, 2, 3, 1};
  std::sort(std::begin(v), std::end(v));
  const std::vector<int> expected = {1, 2, 3, 4};
  assert(v == expected);
}

std::vector versus plain array

Advantages of a std::vector over an array are:

std::vector allocates memory from the free space when increasing in size
std::vector is not a pointer in disguise [3]
std::vector can increase/decrease in size run-time
std::vector can do range checking (using at())

There is an urban myth that arrays are faster. See this benchmark of array versus std::vector to see they are equally fast.

Calling std::remove to remove a certain value from a std::vector does not change a std::vector its size * std::remove does return an iterator to where the removed elements are put. This iterator can be used to call std::vector its 'erase' member function. These two operations are called the erase-remove idiom.

Use the erase-remove idiom the really remove a value from a std::vector.

std::vector code snippets

Note that among these are also more general container code snippets.

Advice

Prefer using a std::vector over an array [1-4]
Use std::vector as your default container [5]
Don't use iterators into a resized std::vector or std::deque [6]
Don't assume performance benefits from reserve() without measurements [7]
When necessary, use reserve() to make performance predictable [8]
Do not assume that operator[] range checks [9]
Use the 'at' member function when you need guaranteed range checks [10]
Use emplace() for notational convenience [11]
Use emplace() to avoid having to pre-initialize variables [12]
Consider using emplace_back by default [13]

External links

References

[1] Herb Sutter, Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. 2005. ISBN: 0-32-111358-6. Chapter 76: 'Use vector by default. Otherwise, choose an appropriate container'.
[2] Herb Sutter, Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. 2005. ISBN: 0-32-111358-6. Chapter 77: 'Use vector and string instead of arrays'.
[3] Marshall Cline, Greg Lomow and Mike Girou. C++ FAQs. ISBN: 0-201-3098301. FAQ 28.02: 'Are arrays good or evil?' (Answer: 'Arrays are evil').
[4] Bjarne Stroustrup. The C++ Programming Language (3rd edition). 1997. ISBN: 0-201-88954-4. Chapter C.14.11 'Prefer vector over array'.
[5] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 31.6. Advice. page 924: '[2] Use vector as your default container'
[6] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 31.6. Advice. page 924: '[21] Don't use iterators into a resized vector or deque'
[7] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 31.6. Advice. page 924: '[19] Don't assume performance benefits from reserve() without measurements'
[8] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 31.6. Advice. page 924: '[22] When necessary, use reserve() to make performance predictable'
[9] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 31.6. Advice. page 924: '[23] Do not assume that [] range checks'
[10] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 31.6. Advice. page 924: '[24] Use at() when you need guaranteed range checks'
[11] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 31.6. Advice. page 924: '[25] Use emplace() for notational convenience'
[12] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 31.6. Advice. page 925: '[27] Use emplace() to avoid having to pre-initialize variables'
[13] Jason Turner * C++ Weekly - Ep 108 - Understanding emplace_back