关于二级指针为函数参数的探讨

#include <iostream>

using namespace std;

void test( int col,double **a)
{
    int i,j;
    for ( i=0; i<col; i++)
    {
        for ( j=0; j<col; j++)
        {
            a[i][j] = 2.0;
        }
    }
}

void test_1( int col,double **a)
{
    int i,j;
    for ( i=0; i<col; i++)
    {
        for ( j=0; j<col; j++)
        {
            *((double *)a+i*col+j) = 2.0;
        }
    }
}

void main()
{
    int i,j,col ;

    cout <<"Please input a number must less than 5"<<endl;
    cin >>col;
    double ** a;
    a = new double* [col];

    double a_1[5][5];

    for ( i=0; i<col; i++ )
    {
        a[i] = new double[col];
    }
    for ( i=0; i<col; i++)
    {
        for ( j=0; j<col; j++)
        {
            a[i][j] = 1.0;
             a_1[i][j] = 10.0;
        }
    }

    test(col,a);        // 1. pass

    test(5,a_1);    // 2. error

    test_1(col,a);    // 3. error

    test_1(5,a_1);    // 4. pass


    delete [] a[0];
    delete [] a[1];
    delete [] a;
    cout <<"Done!" <<endl;
}