实战c++中的vector系列--构造、operator=和assign区别

vector也许是实际过程中使用最多的stl容器，看似简单，其实有很多技巧和陷阱。

着重看一看vector的构造，暂时按照C++11：

default (1) explicit vector (const allocator_type& alloc = allocator_type());fill (2)    explicit vector (size_type n);         vector (size_type n, const value_type& val,                 const allocator_type& alloc = allocator_type());range (3)   template <class InputIterator>  vector (InputIterator first, InputIterator last,          const allocator_type& alloc = allocator_type());copy (4)    vector (const vector& x);vector (const vector& x, const allocator_type& alloc);move (5)    vector (vector&& x);vector (vector&& x, const allocator_type& alloc);initializer list (6)    vector (initializer_list<value_type> il,       const allocator_type& alloc = allocator_type());

直接看看下面的代码，就知道如何构造一个vector了：

#include <iostream>#include <vector>int main (){  std::vector<int> first;                                // default (1)   std::vector<int> second (4,100);                       // fill (2)  std::vector<int> third (second.begin(),second.end());  // range (3)  std::vector<int> fourth (third);                       // a copy of third  // the iterator constructor can also be used to construct from arrays:  int myints[] = {16,2,77,29};  std::vector<int> fifth (myints, myints + sizeof(myints) / sizeof(int) );  std::cout << "The contents of fifth are:";  for (std::vector<int>::iterator it = fifth.begin(); it != fifth.end(); ++it)    std::cout << ' ' << *it;  std::cout << '\n';  return 0;}

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vector重载了=运算符，也有一个叫assign的方法，而且有什么区别吗？

std::vector::operator=
直接代码：

#include <iostream>#include <vector>int main (){  std::vector<int> foo (3,0);  std::vector<int> bar (5,0);  bar = foo;  foo = std::vector<int>();  std::cout << "Size of foo: " << int(foo.size()) << '\n';  std::cout << "Size of bar: " << int(bar.size()) << '\n';  return 0;}//结果：Size of foo: 0Size of bar: 3

这里需要说明的是：
replacing its current contents
modifying its size accordingly

std::vector::assign
同样直接代码：

#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;}//输出：Size of first: 7Size of second: 5Size of third: 3

这里同样需要说明：
replacing its current contents
modifying its size accordingly