What is the difference between string::at and string::operator[]?

Question

I was taught string::at in school, but by exploring the string library I saw string::operator[], which I was never shown before.

I'm now using operator[] and haven't used at since, but what is the difference? Here is some sample code:

std::string foo = "my redundant string has some text";
std::cout << foo[5];
std::cout << foo.at(5);

They are essentially the same in terms of output, but are there some subtle differences I'm not aware of?

Answer 1

Yes, there is one major difference: using .at() does a range check on index passed and throws an exception if it's over the end of the string while operator[] just brings undefined behavior in that situation.

Answer 2

at does bounds checking, exception of type std::out_of_range will be thrown on invalid access.

operator[] does NOT check bounds and thus can be dangerous if you try to access beyond the bounds of the string. It is a little faster than at though.