Как узнать длину строки у двумерного динамического массива

    int n=11;
    int **mas = new int *[n];

for (k = n - 1; k >= 0; k--)
{
   gcd=1 + rand() % 8 ;
    mas[k] = new __int64[gcd];
}

mas[k] = new __int64[gcd];
mas[k][0] = gcd;

sizeof(int) != sizeof(int64_t)

std::vector<std::vector<int64_t>> mas(n);
//...
mas[k] = std::vector<int64_t>(random_size);

mas[k].resize(random_size);

#include <ctime>
#include <vector>
#include <iostream>
#include <random>
 
typedef std::vector<int> Row;
typedef std::vector<Row> Matrix;
 
const size_t N = 10;
const int FROM_NUM = 1;
const int TO_NUM = 9;
 
std::mt19937 mt( ( unsigned )time( nullptr ) );
std::uniform_int_distribution<int> dist( FROM_NUM, TO_NUM );
 
void Init( Matrix & matrix )
{
    Row row( N );
    for ( size_t i = 0; i < N; ++i )
    {
        for (size_t j = 0; j < N; ++j )
            row[j] = dist( mt );
 
        matrix.push_back( row );
    }
}
 
void Display( const Matrix & matrix )
{
    for ( const Row & row : matrix )
    {
        for ( const int elem : row )
        {
            std::cout << elem << " ";
        }
        std::cout << "\n";
    }
    std::cout << "\n";
}
 
size_t LengthOfRow(const Matrix & matrix, size_t indexOfRow)
{
    return matrix[indexOfRow].size();
}
 
int main()
{
    Matrix matrix;
 
    Init( matrix );
    Display( matrix );
 
    std::cout << "Length of 7th row: " << LengthOfRow(matrix, 7);
 
    return 0;
}

size_t LengthOfRow(const Matrix & matrix, size_t indexOfRow)
{
    return matrix.at(indexOfRow).size();
}