c++ transfer matrix to procedure ,memory leaks

i am wandering. i can't believe my eyes! if i allocate matrix in procedure input 开发者_JS百科, after exit from procedure, matrix had been deleted!

void input(int **x,int& m,int &n)
{
    int i,j;
    x=new int*[mx];

    for(i=0;i<mx;i++)
        x[i]=new int[nx];

    for(i=0;i<m;i++)
        for(j=0;j<n;j++)
            scanf("%d",&x[i][j]);   

    for(i=0;i<mx;i++)
        for(j=0;j<nx;j++)
            printf("%d",x[i][j]); - i see normal matrix
}

int main()
{
    int **x,**y,mx,my,nx,ny;
    int i,j;

    input(x,mx,nx);   
    for(i=0;i<mx;i++)
        for(j=0;j<nx;j++)
            printf("%d",x[i][j]); - i receive a lot of stuff

    return 0;
}

why matrix was deleted? as i understand, i transfer **x , it points to part of memory , there matrix lies. but when i have returned to main, i lost matrix, i have not deleted it. why have i lost matrix?

if i do this in main function

x=new int*[mx];

for(i=0;i<mx;i++)
    x[i]=new int[nx];

all is right

x is passed by value to input; whatever modifications input does to x within itself, they won't be seen outside input. (Hint: try to printf("%p\n", x) inside and outside input). So, if you want to solve it with the least modifications to your code, you have to use a triple indirection here (i.e., int ***x) and pass &x to input. But this is a very C-ish way of doing things; in C++, you are most likely better off with defining your own Matrix class and work with that.

Use

void input(int & **x,int& m,int &n)

instead of

void input(int **x,int& m,int &n)

i.e. pass x by reference instead of by value.

void input(int **x,int& m,int &n)
{ 
  int i,j;
  x=new int*[mx];
  ...

Where are mx and nx coming from? From what I can tell, they are undefined.

Change int** x to int**& x in function input.