Why Does C `std::vector`'s `[]` Operator Not Throw Errors for Out-of-Bounds Access?

Vector Accessing Out of Bounds Without Errors

In C , std::vector is a built-in container that holds a collection of elements of the same type. It provides dynamic memory allocation, allowing for efficient resizing and element addition. However, when accessing elements beyond the vector's size, unexpected behavior may occur.

Unlike the at() member function, which throws an error when accessing out-of-bounds elements, the square bracket operator ([]) behaves differently. When using [], attempting to access an index beyond the vector's size does not result in an error. Instead, it returns uninitialized or garbage values as if the element existed.

Consider the following example:

struct Element {
    std::vector<double> face;
};

int main() {
    Element elm;

    // Add six elements to elm.face
    ...

    std::cout << elm.face.size() << std::endl; // Outputs 6
    std::cout << elm.face[6] << std::endl; // Outputs an unexpected number
}

In this code, the vector face within the Element struct contains six elements. When accessing face[6], which is out of bounds, the vector does not throw an error. Instead, it returns a random value present in the memory location pointed by the index.

This behavior can lead to unpredictable and erroneous results, especially when relying on the vector's size to ensure proper indexing. It is important to be cautious when using the square bracket operator, and to check the bounds before accessing elements to avoid undefined results.

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