vector::at 与 vector::operator[]

2022-01-07 00:00:00 c++ stdvector stl

我知道 at() 由于其边界检查而比 [] 慢，这也在类似的问题中讨论，例如 C++ Vector at/[] operator speed 或 ::std::vector::at() 与 operator[] <<结果出人意料！！慢/快 5 到 10 倍！.我只是不明白 at() 方法有什么用.

I know that at() is slower than [] because of its boundary checking, which is also discussed in similar questions like C++ Vector at/[] operator speed or ::std::vector::at() vs operator[] << surprising results!! 5 to 10 times slower/faster!. I just don't understand what the at() method is good for.

如果我有一个像这样的简单向量:std::vectorv(10); 并且我决定在我有索引 i 的情况下使用 at() 而不是 [] 来访问它的元素 并且我不确定它是否在向量范围内，它迫使我用 try-catch 块包装它:

If I have a simple vector like this one: std::vector<int> v(10); and I decide to access its elements by using at() instead of [] in situation when I have a index i and I'm not sure if its in vectors bounds, it forces me to wrap it with try-catch block:

try
{
    v.at(i) = 2;
}
catch (std::out_of_range& oor)
{
    ...
}

虽然我可以通过使用 size() 并自己检查索引来获得相同的行为，这对我来说似乎更容易和方便:

although I'm able to do the get the same behaviour by using size() and checking the index on my own, which seems easier and much convenient for me:

if (i < v.size())
    v[i] = 2;

所以我的问题是:
使用 vector::at 比 vector::operator[] ?
我什么时候应该使用 vector::at 而不是 vector::size + vector::operator[] ?

So my question is:
What are advantages of using vector::at over vector::operator[] ?
When should I use vector::at rather than vector::size + vector::operator[] ?

推荐答案

我想说的是 vector::at() 抛出的异常并不是真的要被周围的对象捕获代码.它们主要用于捕获代码中的错误.如果您需要在运行时进行边界检查，因为例如索引来自用户输入，您确实最好使用 if 语句.所以总而言之，设计你的代码时要保证 vector::at() 永远不会抛出异常，所以如果它抛出异常，并且你的程序中止，那就是一个错误的迹象.(就像一个 assert())

I'd say the exceptions that vector::at() throws aren't really intended to be caught by the immediately surrounding code. They are mainly useful for catching bugs in your code. If you need to bounds-check at runtime because e.g. the index comes from user input, you're indeed best off with an if statement. So in summary, design your code with the intention that vector::at() will never throw an exception, so that if it does, and your program aborts, it's a sign of a bug. (just like an assert())

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