构造、operator=和assign区别
最后更新于：2022-04-01 06:43:56
##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;
}
~~~
===========================================================
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: 0
Size 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: 7
Size of second: 5
Size of third: 3
~~~
这里同样需要说明：
replacing its current contents
modifying its size accordingly