To get the size of a vector in C++, you can use the size() method. Here's an example:

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v {1, 2, 3, 4, 5};

    // get the size of the vector
    std::cout << "The size of the vector is: " << v.size() << std::endl;

    return 0;
}

Output:

The size of the vector is: 5

In the above example, we first initialize a vector v with 5 elements. We then use the size() method to get the number of elements in the vector, which is 5 in this case. The size() method returns an unsigned integer, which represents the number of elements in the vector.