`resize()` vs. `reserve()` for Dynamically Growing Vectors: When Should You Use Which?

Choosing Between vector::resize() and vector::reserve()

When working with containers in C , programmers often face the decision between using vector::resize() and vector::reserve(). Understanding the purpose and effects of both functions is crucial for optimizing performance and memory utilization.

vector::resize()

vector::resize() modifies the size of a vector by appending or removing elements. If the new size is smaller than the current size, elements are removed from the end of the vector. If it's larger, additional elements with unspecified values are appended. This function affects both the size() and capacity() of the vector and directly modifies the elements within it.

vector::reserve()

Unlike resize(), vector::reserve() doesn't alter the vector's size. Instead, it allocates memory to accommodate a specified number of elements, increasing the capacity() without affecting size(). This memory remains uninitialized, and the elements must be inserted manually. Reserve() optimises future insertions by pre-allocating memory, avoiding costly reallocation when elements are added.

Choice in the Given Scenario

In the provided scenario, where the vector's size is dynamically increased by 100 when it reaches certain thresholds, neither resize() nor reserve() is the ideal choice. Manually managing the size of the vector can lead to inefficient memory allocation and performance issues.

The recommended course of action is to avoid preallocating memory manually and rely on the vector's built-in mechanisms to handle resizing. The vector will automatically grow as needed, optimising memory usage and ensuring efficient insertions.

Exception:

If you have a precise estimate of the maximum size the vector will require, using reserve() to allocate that capacity upfront can be beneficial. This ensures that all future insertions can be performed without incurring the overhead of reallocation, improving performance.

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