What is the difference between resize and reserve in C++ vectors?

In C++, both the resize() and reserve() functions of a vector can be used to adjust the size of the vector, but there are some key differences between them.

1. The resize() function is used to adjust the number of elements in a vector and can specify a new size. If the new size is larger than the current size, new elements will be added and initialized; if the new size is smaller than the current size, excess elements will be deleted. The resize() function will change the size of the vector and may change the capacity of the vector.
2. The reserve() function is used to allocate space for a vector in advance without changing its size. When calling reserve(), the capacity of the vector increases while the size remains the same. This helps reduce the number of memory reallocations needed when inserting elements, improving performance.

In summary, the resize() function is typically used to adjust the size of a vector, while the reserve() function is used to optimize the performance of memory allocation.