`Vector Pre-Allocation: When to Use resize() vs. reserve()`

Vector Pre-Allocation: Resize() vs. Reserve()

Pre-allocating memory for vector members is a common optimization technique to improve performance. However, determining the correct method for pre-allocation between vector::resize() and vector::reserve() can be puzzling.

Understanding vector::resize() and vector::reserve()

vector::resize()

• Resizes the vector to the specified size.
• Inserts or deletes elements as needed.
• Modifies the vector's size().

vector::reserve()

• Allocates memory for the specified size.
• Leaves the memory uninitialized.
• Modifies the vector's capacity() but not its size().

Choosing the Right Method

Use vector::resize() when:

• You want to create a vector of a specific size with default values.

Use vector::reserve() when:

• You anticipate inserting a known number of elements and want to avoid multiple memory reallocations.

Scenarios for Pre-Allocation

Given the scenario of pre-allocating a vector of names (t_Names) with an initial size of 1000 and a potential growth, the most efficient approach is not to pre-allocate manually.

Vector is designed to reallocate as needed in a more efficient manner than manual pre-allocation. However, if you still want to pre-allocate, here are some guidelines:

• Initial Estimate Available: If you have a reasonably precise estimate of the total size needed, use vector::reserve() to pre-allocate that size.
• No Initial Estimate: Avoid pre-allocating and let the vector reallocate as needed.

Conclusion

Understanding the differences between vector::resize() and vector::reserve() is crucial for choosing the right method for pre-allocating memory. For cases where manual pre-allocation is not necessary, the vector's self-reallocation mechanism provides optimal performance.

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