[CPP - STL] swap技巧

最近在看《Effective STL》，【条款17：使用“交换技巧”修整过剩容量】中提到容器的成函数void swap(container& from)，即实现容器对象与from对象的交换。

另外，对于连续内存容器vector和string，还有shrink to fit（收缩到合适，根据容器的实际size设置capacity，减少实际内存分配）的功能。

以vector为例，首先说下，size和capacity、resize和reserve的含义及区别。

1. size和capacity、resize和reserve

size：对象的元素个数，除reserve外的几乎所有vector操作都可以影响size；

capacity：容器预留容量，即占用真实内存量，capacity >= size。push_back、insert、resize、reserve操作可以影响capacity，其特点为只增不减（即使erase和clear操作）；

resize：可以改变size，如果resize的元素个数大于capacity，则同时改变capacity，增加对象内存空间；

reserve：不改变size，如果reserve的元素个数大于capacity，则改变capacity，增加对象内存空间，反之，则不影响capacity；

代码示例如下：

 #include <iostream>
 #include <vector>

 using namespace std;

 int main()
 {
     vector<int> v;
     cout << "initial: "                         << endl;
     cout << "size of v is     " << v.size()     << endl;
     cout << "capacity of v is " << v.capacity() << endl << endl;

     v.insert(v.end(), , );
     cout << "after insert:  "                   << endl;
     cout << "size of v is     " << v.size()     << endl;
     cout << "capacity of v is " << v.capacity() << endl << endl;

     v.erase(v.end()-, v.end());
     cout << "after erase:  "                    << endl;
     cout << "size of v is     " << v.size()     << endl;
     cout << "capacity of v is " << v.capacity() << endl << endl;

     v.reserve();
     cout << "after reserve:  "                  << endl;
     cout << "size of v is     " << v.size()     << endl;
     cout << "capacity of v is " << v.capacity() << endl << endl;

     v.resize();
     cout << "after resize:  "                   << endl;
     cout << "size of v is     " << v.size()     << endl;
     cout << "capacity of v is " << v.capacity() << endl << endl;

     v.reserve();
     cout << "after reserve:  "                  << endl;
     cout << "size of v is     " << v.size()     << endl;
     cout << "capacity of v is " << v.capacity() << endl << endl;

     v.clear();
     cout << "after clear:  "                    << endl;
     cout << "size of v is     " << v.size()     << endl;
     cout << "capacity of v is " << v.capacity() << endl << endl;

     return ;
 }

输出结果为：

initial:
size of v is
capacity of v is 

after insert:
size of v is
capacity of v is 

after erase:
size of v is
capacity of v is 

after reserve:
size of v is
capacity of v is 

after resize:
size of v is
capacity of v is 

after reserve:
size of v is
capacity of v is 

after clear:
size of v is
capacity of v is 

根据上面几个概念及Demo，可以看出来，vector对象的实际内存（即capacity）在多次insert和erase操作后大于实际元素个数（size），即使在clear操作后，capacity依然不会减小为零。

所以，有必要对这种基于内存池的容器做内存收缩。

2. swap：shrink to fit

修改vector对象多余内存空间的方法：

vector<int>(v).swap(v); // v为一个vector对象

vector<int>(v)基于v拷贝构造了一个临时vector，其分配了v.size()个元素的内存空间，即capacity为v.size()，然后vector<int>(v)与v交换数据，交换后，v的capacity也减少为v.size()，而临时vector对象vector<int>(v)被销毁。

另外，vector<int>().swap(v); // 可以清除v的内存空间

代码示例如下：

 #include <iostream>
 #include <vector>

 using namespace std;

 int main()
 {
     vector<int> v(, );
     cout << "initial: "                         << endl;
     cout << "size of v is     " << v.size()     << endl;
     cout << "capacity of v is " << v.capacity() << endl << endl;

     v.erase(v.end()-, v.end());
     cout << "after erase:  "                    << endl;
     cout << "size of v is     " << v.size()     << endl;
     cout << "capacity of v is " << v.capacity() << endl << endl;

     vector<int>(v).swap(v);
     cout << "after swap (shrink to fit):  "     << endl;
     cout << "size of v is     " << v.size()     << endl;
     cout << "capacity of v is " << v.capacity() << endl << endl;

     vector<int>().swap(v);
     cout << "after swap (clear):  "             << endl;
     cout << "size of v is     " << v.size()     << endl;
     cout << "capacity of v is " << v.capacity() << endl << endl;

     return ;
 }

输出结果为：

initial:
size of v is
capacity of v is 

after erase:
size of v is
capacity of v is 

after swap (shrink to fit):
size of v is
capacity of v is 

after swap (clear):
size of v is
capacity of v is 

如果使用C++11及更新标准的话，可以直接使用shrink_to_fit函数。