Resize and reserve in Vector

void reserve (size_type n);

The reserver function is used to give the VectorPre-allocationThe size of the storage area, that is, the value of capacity, but the memory is not initialized. The reserve parameter n is the size of the pre-allocated memory recommended. The actual allocation may be equal to or greater than this value, that is, the value of N is greater than capacity, the reallocate memory capacity value is greater than or equal to n. In this way, when the push_back function is called to make the size exceed the original default capacity value, the overhead of memory redistribution is avoided.

Note that the memory space allocated by the reserve function only indicates that the vector can use the memory, but the vector cannot effectively access the memory space, during access, the program crashes.

void resize (size_type n);

void resize (size_type n, value_type val);

Resize FunctionReallocationTo change the container size and create an object.

When N is less than the current size () value, the vector first reduces the size () value to save the first n elements, and then deletes the elements that exceed n (remove and destroy)

When N is greater than the current size () value, the vector inserts a corresponding number of elements so that the size () value reaches N and initializes these elements, if the second resize function is called and Val is specified, the vector uses Val to initialize these newly inserted elements.

When N is greater than the capacity () value, the memory storage space is automatically allocated.

Example:

# Include <algorithm> # include <vector> # include <list> # include <iostream> # include <string> # include <numeric> # include <iterator> using namespace STD; int main () {vector <double> V1 = {34,342, 34}; List <string> v2 = {"A", "", "DFD", "FD", "a"}; int count1 = count (v1.begin (), v1.end (), 34); cout <count1 <Endl; int count2 = count (v2.begin (), v2.end (), "A"); cout <count2 <Endl; cout <accumulate (v1.begin (), v1.end (), 0) <Endl; vector <int> VEC; // at this time, VEC is empty // insert with reserved space allocatedVEC. Reserve (10); cout <VEC [0] <Endl; cout <VEC [9] <Endl; fill_n (VEC. Begin (), 10, 0 );// Insert not allocated reserved space // fill_n (back_inserter (VEC), 10, 0); Return 0 ;}