#include 
  
   
using namespace std;

int main()
{
    vector
   
    vec; for(int i = 0 ;i < 100 ; ++i) vec.push_back(i); cout << vec.size() << endl; //100 cout << vec.capacity() << endl; //128 vec.erase(vec.begin()+10,vec.end()); //改变了size，但是并未改变capccity cout << vec.size() << endl; //10 cout << vec.capacity() << endl; //128 vector
    
     (vec).swap(vec); cout << vec.size() << endl; //10 cout << vec.capacity() << endl; //10 vec.clear(); //clear并未真正释放空间！！！ cout << vec.size() << endl; //0 cout << vec.capacity() << endl; //10 vector
     
       (vec).swap(vec); //这才真正释放了空间！！ cout << vec.size() << endl; //0 cout << vec.capacity() << endl; //0 return 0; } 
     
    
   
  

PS：C++11中已经实现了shink_to_fit函数。实现上述功能。