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Is Assuming Contiguity in STL Vectors Safe After Resizing?

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Nov 16, 2024 am 08:58 AM

Is Assuming Contiguity in STL Vectors Safe After Resizing?

Understanding STL Vector Storage: Is Assumption of Contiguity Safe?

In C , the Standard Template Library (STL) provides powerful data structures, including the vector class. While resizing a vector is a common operation, concerns arise about the safety of assuming memory contiguity.

The question posed is: "If an STL vector has been resized, can we assume that addressing element 0 provides access to consecutive memory locations containing the remaining vector elements?"

According to the C 03 standard (23.2.4.1), the answer is resoundingly Yes. The standard states that vector elements are stored contiguously, ensuring that &v[n] == &v[0] n for all 0 ≤ n

However, it's crucial to note the caveat that this assumption holds true only until the vector is reallocated. Adding elements to the vector can trigger reallocation, invalidating any pointers or iterators that were derived from the original memory block.

Therefore, while it is safe to assume contiguity in the immediate aftermath of resizing, it becomes unsafe if elements are subsequently added or removed, potentially causing the vector to reallocate.

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