`std::vector::reserve()` vs. `std::vector::resize()`: When Should You Use Which?

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

In a recent discussion, the topic of using std::vector::reserve() vs. std::vector::resize() arose. In this post, we'll delve into the differences between these two methods.

To provide context, let's consider a sample code snippet:

void MyClass::my_method() {
    my_member.reserve(n_dim);
    for (int k = 0; k < n_dim; k++) my_member[k] = k;
}

In this code, the intention is to create a vector with a specified capacity (n_dim) and then iterate through the vector, accessing and modifying its elements. However, the question arises whether using reserve() is appropriate for this purpose.

Understanding the Difference

std::vector::reserve() is designed to allocate memory in the vector to accommodate a specified number of elements. However, it does not actually modify the vector's size. The vector's logical size remains the same, which means that if you attempt to access elements that exceed the current logical size, the behavior is undefined.

On the other hand, std::vector::resize() both allocates memory and modifies the vector's size. It sets the logical size of the vector to the specified value. Any additional elements that are created as a result of resizing are initialized to their default values (e.g., 0 for ints).

Application to the Sample Code

In the sample code presented, using std::vector::reserve() instead of std::vector::resize() can lead to undefined behavior. The code assumes that the vector has a size of n_dim after calling reserve(), but this is not the case. Hence, accessing my_member[k] with k greater than or equal to the original size of the vector will result in a memory access violation or other undefined behavior.

Conclusion

In conclusion, to correctly access and modify elements in a vector, it is essential to understand the distinction between std::vector::reserve() and std::vector::resize(). reserve() is used to allocate memory without resizing, while resize() modifies both memory allocation and the vector's size, initializing any newly created elements. In the sample code provided, std::vector::resize() should be used to allocate memory and set the vector's size, enabling proper access and modification of its elements.

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