size vs capacity of a vector?

Question

I am a bit confused about this both of these look same to me. Although it may happen that capacity and size may differ on different compilers. how it may differ. Its also sa

Answer 1

Size is number of elements present in a vector

Capacity is the amount of space that the vector is currently using.

Let's understand it with a very simple example:

using namespace std;

int main(){
  vector vec;
  vec.push_back(1); 
  vec.push_back(1); 
  vec.push_back(1); 
  cout<<"size of vector"<

currently size is 3 and capacity is 4.

Now if we push back one more element,

using namespace std;
  int main(){
  vector vec;
  vec.push_back(1); 
  vec.push_back(1); 
  vec.push_back(1); 
  vec.push_back(1);
  cout<<"size of vector"<

now size is: 4 capacity is 4

now if we try to insert one more element in vector then size will become 5 but capacity will become 8.

it happens based on the datatype of vector, as here in this case vector in of type int, as we know size of int is 4 bytes so compiler will allocate 4 block of memory ..and when we try to add 5th element , vector::capacity() is doubled what we have currently.

same keep on..for example : if we try to insert 9th element then size of vector will be 9 and capacity will b 16..