vector :: at vs. vector :: operator [] [英] vector::at vs. vector::operator[]

问题描述

我知道 at（）比 [] 慢，因为它的边界检查在类似问题（例如 C ++向量at / []运算符速度）或 :: std :: vector :: at（）vs operator []<惊人的结果！ 5到10倍更慢/更快！。我只是不明白 at（）方法是否有用。

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.

如果我有一个简单的向量像这样： std :: vector< int> v（10）; 并且我决定通过使用在（）而不是 [] 在我有一个索引 i 的情况下，我不知道如果它的向量边界，它强迫我包装它与try-catch ：：

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（）并检查我的索引自己，这对我来说更容易和更方便：

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：

a href =http://www.cplusplus.com/reference/stl/vector/at/> vector :: at vector :: operator [] ？

我应该何时使用 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

code> throws并不是真正意图被立即包围的代码捕获。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，你最好使用 if 语句。所以总的来说，设计你的代码，意图 vector :: at（）永远不会抛出异常，所以如果它，并且你的程序中止，它是一个符号的bug。 （就像 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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