C++ Primer, Fifth Edition

capacity and size

It is important to understand the difference between capacity and size. The size of a container is the number of elements it already holds; its capacity is how many elements it can hold before more space must be allocated.

The following code illustrates the interaction between size and capacity:

vector<int> ivec;// size should be zero; capacity is implementation definedcout << "ivec: size: " << ivec.size()     << " capacity: "  << ivec.capacity() << endl;// give ivec 24 elementsfor (vector<int>::size_type ix = 0; ix != 24; ++ix)     ivec.push_back(ix);// size should be 24; capacity will be >= 24 and is implementation definedcout << "ivec: size: " << ivec.size()     << " capacity: "  << ivec.capacity() << endl;

When run on our ...