Resize() or Reserve()? When Should You Preallocate Memory for Vectors?

Choosing between vector::resize() and vector::reserve() for Preallocation

Preallocating memory for vectors can offer performance benefits, but choosing the appropriate method is crucial.

vector::resize()

The resize() method inserts or deletes elements to achieve the desired vector size. It alters the vector's size, allowing direct access and iteration through all elements. However, it's important to note that resize() impacts the vector's internal storage, which can lead to performance issues when resizing frequently or unpredictably.

vector::reserve()

In contrast, reserve() only allocates memory without initializing values. It reserves space for future insertions, avoiding the need for reallocation during successive insertions. This method only affects the vector's capacity, leaving its size unchanged.

Choosing the Right Method

The choice between resize() and reserve() depends on the desired behavior.

• Use resize() when you need a vector of a specific size with default values.
• Use reserve() when you anticipate adding a significant number of elements and want to minimize reallocations.

Alternative Option

In cases where initial estimates are available, it's generally more efficient to avoid manual preallocation and allow the vector to handle reallocation automatically. However, if precise estimates are readily available, reserving the estimated size can be beneficial.

Additional Notes

• If the vector reaches its reserved capacity, it will still resize to accommodate additional elements, although less efficiently.
• For situations where the vector size may vary significantly or unpredictably, consider using a linked list or other data structures that can handle dynamic growth more efficiently.

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