C++编程思想 第2卷 第7章 通用容器 基本序列容器:vector list deque 被检查的随机访问

vector和deque都提供了两个随机访问函数
进行索引操作符 operator[]
at() 检测正被索引的容器的边界
如果超出了边界抛出一个异常

//: C07:IndexingVsAt.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Comparing "at()" to operator[].
#include <ctime>
#include <deque>
#include <iostream>
#include <vector>
#include "../require.h"
using namespace std;

int main(int argc, char* argv[]) {
  long count = 1000;
  int sz = 1000;
  if(argc >= 2) count = atoi(argv[1]);
  if(argc >= 3) sz = atoi(argv[2]);
  vector<int> vi(sz);
  clock_t ticks = clock();
  for(int i1 = 0; i1 < count; i1++)
    for(int j = 0; j < sz; j++)
      vi[j];
  cout << "vector[] " << clock() - ticks << endl;
  ticks = clock();
  for(int i2 = 0; i2 < count; i2++)
    for(int j = 0; j < sz; j++)
      vi.at(j);
  cout << "vector::at() " << clock()-ticks <<endl;
  deque<int> di(sz);
  ticks = clock();
  for(int i3 = 0; i3 < count; i3++)
    for(int j = 0; j < sz; j++)
      di[j];
  cout << "deque[] " << clock() - ticks << endl;
  ticks = clock();
  for(int i4 = 0; i4 < count; i4++)
    for(int j = 0; j < sz; j++)
      di.at(j);
  cout << "deque::at() " << clock()-ticks <<endl;
  // Demonstrate at() when you go out of bounds:
  try {
    di.at(vi.size() + 1);
  } catch(...) {
    cerr << "Exception thrown" << endl;
  }
  getchar();
} ///:~

输出
vector[] 39
vector::at() 39
deque[] 979
deque::at() 969
Exception thrown

不同的系统采用不同的方法来处理未捕获的异常
使用at()时，可以通过多个途径知道程序的某一部分发生了错误
可是在使用operator[]时却可能一无所知