std::vector（续）

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• Capacity（容量/元素个数）

size

返回容器中元素个数 (public member function )

max_size

返回该容器最多能存储的元素个数 (public member function )

resize

该表容器大小 (public member function )

capacity

返回已经为容器分配的内存的容量 (public member function )

empty

判别容器是否为空 (public member function )

reserve

重新设定容器内存的容量 (public member function )

shrink_to_fit 

Shrink to fit (public member function )

示例如下：

• resize

void resize (size_type n, value_type val = value_type());参数 n:     代表新设置的容器可包含的元素数量参数 val ： 容器在变大的过程中，默认填充的数据

注意:

在修改容器大小过程中根据传入的n的不同分为如下三种处理情况：

（1）当要设置的n比当前的容器的数据元素个数小时，容器将会保存前n个元素，其他的元素将会被移除

（2）当要设置的n比当前的容器的数据元素个数大时，容器会用val指定的值去填充新增加的未赋值的元素，如 果没有指定val值，容器会使用默认值进行填充。

（3） 当要设置的n比当前的容器的已分配的内存大时，容器就会重新分配内存。

// resizing vector#include <iostream>#include <vector>int main (){  std::vector<int> myvector;  // set some initial content:  for (int i=1;i<10;i++) myvector.push_back(i);  myvector.resize(5);  myvector.resize(8,100);  myvector.resize(12);  std::cout << "myvector contains:";  for (int i=0;i<myvector.size();i++)    std::cout << ' ' << myvector[i];  std::cout << '\n';  return 0;}

Output:

myvector contains: 1 2 3 4 5 100 100 100 0 0 0 0

• reserve

void reserve (size_type n);

注意：

当n比当前的容器容量大时，容器从新分配内存，当n比当前容器容量小时，不进行操作

// vector::reserve#include <iostream>#include <vector>int main (){  std::vector<int>::size_type sz;  std::vector<int> foo;  sz = foo.capacity();  std::cout << "making foo grow:\n";  for (int i=0; i<100; ++i) {    foo.push_back(i);    if (sz!=foo.capacity()) {      sz = foo.capacity();      std::cout << "capacity changed: " << sz << '\n';    }  }  std::vector<int> bar;  sz = bar.capacity();  bar.reserve(100);   // this is the only difference with foo above  std::cout << "making bar grow:\n";  for (int i=0; i<100; ++i) {    bar.push_back(i);    if (sz!=bar.capacity()) {      sz = bar.capacity();      std::cout << "capacity changed: " << sz << '\n';    }  }  return 0;}

Possible output:making foo grow:capacity changed: 1capacity changed: 2capacity changed: 4capacity changed: 8capacity changed: 16capacity changed: 32capacity changed: 64capacity changed: 128making bar grow:capacity changed: 100

• Element access(元素访问)

operator[]

下标访问元素 (public member function )

at

访问元素(public member function )

front

访问第一个元素 (public member function )

back

访问第二个元素(public member function )

data 

Access data (public member function )

示例如下：

// vector::at#include <iostream>#include <vector>int main (){  std::vector<int> myvector (10);   // 10 zero-initialized ints  // assign some values:  for (unsigned i=0; i<myvector.size(); i++)    myvector.at(i)=i;  std::cout << "myvector contains:";  for (unsigned i=0; i<myvector.size(); i++)    std::cout << ' ' << myvector.at(i);  std::cout << '\n';  return 0;}

Output:

myvector contains: 0 1 2 3 4 5 6 7 8 9

• Modifiers(修改元素)

assign

重新分配容器中内容(public member function )

push_back

在vector尾部加入元素 (public member function )

pop_back

在vector尾部删除元素 (public member function )

insert

 在指定位置插入元素(public member function )

erase

删除指定位置元素 (public member function )

swap

交换两个容器中的元素 (public member function )

clear

清空容器中的元素 (public member function )

emplace 

Construct and insert element (public member function )

emplace_back 

Construct and insert element at the end (public member function )

示例如下：

• assign

// vector assign#include <iostream>#include <vector>int main (){  std::vector<int> first;  std::vector<int> second;  std::vector<int> third;  first.assign (7,100);             // 7 ints with a value of 100  std::vector<int>::iterator it;  it=first.begin()+1;  second.assign (it,first.end()-1); // the 5 central values of first  int myints[] = {1776,7,4};  third.assign (myints,myints+3);   // assigning from array.  std::cout << "Size of first: " << int (first.size()) << '\n';  std::cout << "Size of second: " << int (second.size()) << '\n';  std::cout << "Size of third: " << int (third.size()) << '\n';  return 0;}

Output:

Size of first: 7Size of second: 5Size of third: 3

• insert（插入元素）

 (1)单个元素iterator insert (iterator position, const value_type& val);    void insert (iterator position, size_type n, const value_type& val);(3)多个元素template <class InputIterator>    void insert (iterator position, InputIterator first, InputIterator last);

// inserting into a vector#include <iostream>#include <vector>int main (){  std::vector<int> myvector (3,100);  std::vector<int>::iterator it;  it = myvector.begin();  it = myvector.insert ( it , 200 );  myvector.insert (it,2,300);  // "it" no longer valid, get a new one:  it = myvector.begin();  std::vector<int> anothervector (2,400);  myvector.insert (it+2,anothervector.begin(),anothervector.end());  int myarray [] = { 501,502,503 };  myvector.insert (myvector.begin(), myarray, myarray+3);  std::cout << "myvector contains:";  for (it=myvector.begin(); it<myvector.end(); it++)    std::cout << ' ' << *it;  std::cout << '\n';  return 0;}

Output:

myvector contains: 501 502 503 300 300 400 400 200 100 100 100

• erase（删除元素）

(1)删除一个元素iterator erase (iterator position);(2)删除一个区间元素iterator erase (iterator first, iterator last);

// erasing from vector#include <iostream>#include <vector>int main (){  std::vector<int> myvector;  // set some values (from 1 to 10)  for (int i=1; i<=10; i++) myvector.push_back(i);  // erase the 6th element  myvector.erase (myvector.begin()+5);  // erase the first 3 elements:  myvector.erase (myvector.begin(),myvector.begin()+3);  std::cout << "myvector contains:";  for (unsigned i=0; i<myvector.size(); ++i)    std::cout << ' ' << myvector[i];  std::cout << '\n';  return 0;}

Output:

myvector contains: 4 5 7 8 9 10

• swap（交换两个vector元素）

// swap vectors#include <iostream>#include <vector>int main (){  std::vector<int> foo (3,100);   // three ints with a value of 100  std::vector<int> bar (5,200);   // five ints with a value of 200  foo.swap(bar);  std::cout << "foo contains:";  for (unsigned i=0; i<foo.size(); i++)    std::cout << ' ' << foo[i];  std::cout << '\n';  std::cout << "bar contains:";  for (unsigned i=0; i<bar.size(); i++)    std::cout << ' ' << bar[i];  std::cout << '\n';  return 0;}

Output:

foo contains: 200 200 200 200 200 bar contains: 100 100 100