Why is std::vector::operator[] 5 to 10 times faster than std::vector::at()?

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The reason is that an unchecked access can probably be done with a single processor instruction. A checked access will also have to load the size from memory, compare it with the index, and (assuming it’s in range) skip over a conditional branch to the error handler. There may be more faffing around to handle the possibility of throwing an exception. This will be many times slower, and this is precisely why you have both options.

If you can prove that the index is within range without a runtime check then use operator[]. Otherwise, use at(), or add your own check before access. operator[] should be more or less as fast as possible, but will explode messily if the index is invalid.

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