Question

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.

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)
{
    ...
}

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;

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[] ?

Answer 1

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())