How does the realloc function work in a vector?

std::vector does not have a realloc function. It uses its own memory management mechanisms, which typically involve:

When increasing size (adding elements when current capacity is exhausted), the vector allocates a new block of memory larger than needed, copies or moves elements from the old block to the new one, and frees the old block. This new size usually exceeds the minimum required to avoid frequent reallocations. This behavior is governed by the allocation policy, which depends on the specific implementation of the standard library.

When decreasing size (removing elements), the vector usually does not immediately free memory. The capacity (used memory) remains the same until a method that reduces capacity is explicitly called, such as shrink_to_fit().

Thus, realloc from C is not used directly in std::vector.