Why Does Out-of-Bounds Access in C 's `std::vector` Using the `[]` Operator Not Throw an Error?

Unexpected Behavior with std::vector Out-of-Bounds Access

In C , the std::vector container is widely used for managing dynamic arrays. While its efficiency is highly valued, a peculiar behavior arises when accessing elements beyond its defined range using the [] operator.

Consider the following code snippet:

std::vector<double> vec;
for (int i = 0; i < 6; ++i) {
  vec.push_back(i);
}
std::cout << vec.size() << std::endl; // Output: 6
std::cout << vec[6] << std::endl; // No error, but unexpected output

Surprisingly, when attempting to access vec[6], which is out of bounds, no error is thrown. Instead, an unexpected number is displayed as output.

Explanation

Unlike the at() member function of std::vector, which performs bounds checking, the [] operator does not. When accessing elements beyond its range, the [] operator triggers undefined behavior. This could lead to unpredictable results, including memory corruption and crashes.

The specific behavior in this case depends on the implementation and operating system. It is possible that the out-of-bounds access wraps around to the start of the vector, or it could access an uninitialized memory location, resulting in garbage values.

Consequences

This behavior can introduce errors that are difficult to detect and debug. As a vector grows, its end point can shift, making it challenging to predict the consequences of out-of-bounds access.

Best Practices

To maintain code reliability and avoid undefined behavior, it is essential to avoid accessing std::vector elements that are out of bounds. Instead, use the at() member function for bounds checking or ensure that indices are within the valid range before using the [] operator.

The above is the detailed content of Why Does Out-of-Bounds Access in C 's `std::vector` Using the `[]` Operator Not Throw an Error?. For more information, please follow other related articles on the PHP Chinese website!