Why Doesn't C Vector's `[]` Operator Throw an Error on Out-of-Bounds Access?

Why Doesn't C Vector Error on Out-of-Bounds Access with [] Operator?

When working with C vectors, you can face a curious behavior: attempting to access an element beyond its size using the square bracket operator ([ ]) doesn't trigger an error. This differs from the at() member function, which performs bounds checking.

Example:

Consider the following code:

std::vector<double> face;

face.push_back(2.3); // Insert 1st element
face.push_back(4.5); // Insert 2nd element
face.push_back(6.7); // Insert 3rd element
face.push_back(8.9); // Insert 4th element
face.push_back(11.1); // Insert 5th element
face.push_back(13.3); // Insert 6th element

std::cout << face.size() << std::endl; // Prints 6
std::cout << face[6] << std::endl; // Prints some number without an error

Explanation:

The std::vector class provides the at() member function for safe access to elements within the vector's bounds. If an index outside the valid range is used, at() throws a std::out_of_range exception.

In contrast, the [] operator doesn't perform any bounds checking. If you attempt to access an element beyond the vector's size using [], it will produce undefined results. In practice, this typically results in accessing memory that doesn't belong to the vector, which can lead to unpredictable behavior or crashes.

Recommendation:

To avoid such issues, it's strongly recommended to use the at() member function for element access within the vector's boundaries. This ensures that an error is thrown when an attempt is made to access an invalid index.

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