Final answer:
In C++, an index out of range error occurs when a program accesses an array or container element with an index outside the bounds of the container, often leading to runtime errors or crashes.
Step-by-step explanation:
Index out of range means that the code is trying to access a matrix or vector outside of its bounds. C++ does not perform bounds checking on array access, therefore, when you attempt to access an element that does not exist (for example, accessing the eleventh element of a ten-element array), it results in undefined behavior, which often leads to runtime errors or program crashes.
For instance, if an array has a size of 5, valid indices to access elements would be 0 through 4. Trying to access array[5] or array[-1] would be an index out of range since these indices do not exist within the valid range of the array.
It is crucial for programmers to ensure they stay within the bounds of arrays and other data structures to avoid these kinds of errors. This may involve using control structures like loops with appropriate conditions, or by employing size checks using the size or length properties of the container (when available).