`std::vector::reserve()` vs. `std::vector::resize()`: When to Use Which for Efficient Memory Management?

std::vector::resize() vs. std::vector::reserve()

In C , std::vector is a commonly used container for storing elements of the same type. It offers two methods, std::vector::reserve() and std::vector::resize(), that play distinct roles in memory management.

std::vector::reserve()

• Allocates memory for a specified number of elements without modifying the logical size of the vector.
• Ensures that future push_backs or inserts can be accommodated without reallocating memory.
• Does not resize the vector or modify its elements.

std::vector::resize()

• Resizes the vector to a specified size, adding or removing elements as needed.
• Initializes newly added elements with their default values.
• Can cause the vector's memory to be reallocated, potentially invalidating existing references, iterators, or pointers.

Application in the Provided Code

In the given sample code, the member vector my_member is initially allocated memory for n_dim elements using std::vector::reserve(). However, no elements are initialized, and accessing elements beyond the current logical size (0) would result in undefined behavior or errors.

To write elements to the vector, std::vector::resize() should be used. It would resize the vector to n_dim and initialize all elements to zero in this case.

VS2010 SP1 Behavior

The test code provided fails in debug builds with VS2010 SP1 because it attempts to access element 5 of a vector with only a logical size of 0. This is considered undefined behavior and results in a crash in debug mode.

Conclusion

Based on the provided information, it is correct to use std::vector::resize() when elements need to be written to the vector. std::vector::reserve() should be used when it is known that future insertions will occur and memory should be reserved for them in advance, but element values do not need to be initialized upfront.

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