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

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

When working with dynamic arrays in C , it's important to understand the difference between std::vector::reserve() and std::vector::resize(). Both serve specific purposes and can impact performance significantly.

std::vector::reserve()

As its name suggests, reserve() reserves memory for a specified number of elements without altering the vector's size. This is useful for optimizing memory allocation if you know the maximum number of elements you'll need. It avoids unnecessary reallocations that occur when the vector grows beyond its current capacity.

std::vector::resize()

On the other hand, resize() both reserves memory and resizes the vector to the specified size. Unlike reserve(), if the new size is larger than the current size, resize() adds empty (or default-initialized) elements to the vector. If the new size is smaller, elements beyond the new size are truncated.

Correct Usage in the Example

In your example code, you're attempting to write elements to the vector after calling reserve(). This will result in errors in debug builds because reserve() does not resize the vector. To correctly write elements to the vector, you need to use resize():

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

Conclusion

Both std::vector::reserve() and std::vector::resize() serve distinct purposes. reserve() optimizes memory allocation, while resize() resizes the vector while maintaining internal data validity. By understanding their differences, you can use them effectively to enhance the performance of your C code.

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