实战c++中的vector系列--C++11对vector成员函数的扩展(cbegin()、cend()、crbegin()、crend()、emplace()、data())

前面提到的emplace_back就是C++11所增加的内容。

所以这篇博客就是想罗列一下C++11对vector容器的扩充。

std::vector::cbegin和std::vector::cend

这两个方法是与std::vector::begin和std::vector::end相对应的，从字面就能看出来，多了一个’c’，顾名思义就是const的意思。

所以：

std::vector::cbegin： Returns a const_iterator pointing to the first element in the container.

std::vector::cend： Returns a const_iterator pointing to the past-the-end element in the container.

#include <iostream>
#include <vector>

int main ()
{
  std::vector<int> myvector = {10,20,30,40,50};

  std::cout << "myvector contains:";

  for (auto it = myvector.cbegin(); it != myvector.cend(); ++it)
    std::cout << ‘ ‘ << *it;
  std::cout << ‘\n‘;

  return 0;
}
Output:
myvector contains: 10 20 30 40 50

std::vector::crbegin和std::vector::crend

这两个方法就不解释了，与上面的相比就是多了个’r’， reverse的缩写，反转迭代器，代码就省略了。

std::vector::emplace

之前已经对emplace_back进行了讨论，其实还有一个方法叫emplace。

我想说的就是，emplace之于emplace_back就像insert之于push_back。

看英文描述就直观：

emplace：Construct and insert element

emplace_back：Construct and insert element at the end

如何使用：

#include <iostream>
#include <vector>

int main ()
{
  std::vector<int> myvector = {10,20,30};

  auto it = myvector.emplace ( myvector.begin()+1, 100 );
  myvector.emplace ( it, 200 );
  myvector.emplace ( myvector.end(), 300 );

  std::cout << "myvector contains:";
  for (auto& x: myvector)
    std::cout << ‘ ‘ << x;
  std::cout << ‘\n‘;

  return 0;
}
Output:
myvector contains: 10 200 100 20 30 300

std::vector::data

Returns a direct pointer to the memory array used internally by the vector to store its owned elements.

#include <iostream>
#include <vector>

int main ()
{
  std::vector<int> myvector (5);
  int* p = myvector.data();
  *p = 10;
  ++p;
  *p = 20;
  p[2] = 100;
  std::cout << "myvector contains:";
  for (unsigned i=0; i<myvector.size(); ++i)
    std::cout << ‘ ‘ << myvector[i];
  std::cout << ‘\n‘;

  return 0;
}
Output:
myvector contains: 10 20 0 100 0

std::vector::shrink_to_fit

Requests the container to reduce its capacity to fit its size.

就是减少空间

#include <iostream>
#include <vector>
int main ()
{
  std::vector<int> myvector (100);
  std::cout << "1. capacity of myvector: " << myvector.capacity() << ‘\n‘;
  std::cout << "1. size of myvector: " << myvector.size() << ‘\n‘;

  myvector.resize(10);
  std::cout << "2. capacity of myvector: " << myvector.capacity() << ‘\n‘;
  std::cout << "2. size of myvector: " << myvector.size() << ‘\n‘;

  myvector.shrink_to_fit();
  std::cout << "3. capacity of myvector: " << myvector.capacity() << ‘\n‘;
 std::cout << "3. size of myvector: " << myvector.size() << ‘\n‘;
  return 0;
}
//输出
1. capacity of myvector: 100
1. size of myvector: 100
2. capacity of myvector: 100
2. size of myvector: 10
3. capacity of myvector: 10
3. size of myvector: 10

此时，就是要明白size和capacity的区别，也就会更加理解resize和reserve的区别了！