argument returning

[josh]

Hi, I'm converting (for learning purpose) )a program from Java to C++
and I've a doubt:

In Java every argument (variable and reference types) is passed and
returned in functions by-value so if I want to create inside that
function a matrix (i.e. int a[][]) and than passing the reference to it
I can simply
returning that value:

in main()
....
int z[][] = pass(); // here 'z' contain the reference to the array
object created in pass() by 'a'
....

the method
....
public static int[][] pass()
{
int a[][] = {{1,2},{3,4}};
return a;
}
....

instead in C++

in main()
....
int **p = pass();
....

the function
....
int ** pass()
{
static int z[] = {1,2,3, -1};
static int o[] = {4,5,6,7, -1};
static int j[] = {8,9,10,11,12, -1};

static int *p[3];

p[0] = z;
p[1] = o;
p[2] = j;

return p;
}
....

in the C++ if I don't use the static when I go out from that function
in the caller 'p' has not
the correct array values. In C++ the argument can be passed by-value
and by-reference so
when in the function I call 'return p' is passed its address but if I
don't use static that address is
not more available bacause 'p' is not more available....

is that the correct conversion ?

Thanks

 

[lwz]

It works for me without static...

 

[josh]

lwz ha scritto:

It works for me without static...

Sorry but it's not possible:

try this, eliminating static

int main()
{
int **p = pass();

for(int ix=0; ix < 3; ix++)
{
int *row = *(p+ix);

for(int iy=0; *(row+iy) != -1; iy++)
printf(" %d ", *(row+iy));
}
printf("\n");
}

if you try to compile the compiler show a warning:
warning: address of local variable 'p' returned cpp_test
and in fact the output shows trashing values......

 

[Boris]

Hi, I'm converting (for learning purpose) )a program from Java to C++
and I've a doubt:

In Java every argument (variable and reference types) is passed and
returned in functions by-value so if I want to create inside that
function a matrix (i.e. int a[][]) and than passing the reference to
it I can simply
returning that value:
[...]
in the C++ if I don't use the static when I go out from that function
in the caller 'p' has not
the correct array values. In C++ the argument can be passed by-value
and by-reference so
when in the function I call 'return p' is passed its address but if I
don't use static that address is
not more available bacause 'p' is not more available....

is that the correct conversion ?

Yes, that's true. In Java the garbage collector will not release the
variable as there is a reference passed around. In C++ the variable will be
released if it goes out of scope - which means for local variables when the
function returns. You must use static then (which might cause new problems
though in multithreaded programs).

Boris

 

[josh]

Boris ha scritto:

Yes, that's true. In Java the garbage collector will not release the
variable as there is a reference passed around.

not just... the object created on the heap is not released because
there is 'z'
that is pointing to but the variable 'a' when the function return is
not more available

 

[peter koch]

Hi, I'm converting (for learning purpose) )a program from Java to C++
and I've a doubt:

In Java every argument (variable and reference types) is passed and
returned in functions by-value

Well - sort of. Remember that in Java, you can not declare an object -
only a reference:

//JAVA code (sorry!)
Object obj = new Object;

Here obj is not an object but a reference to an object. This
corresponds to the following C++ code:
Object* obj = new Object;

so if I want to create inside that
function a matrix (i.e. int a[][]) and than passing the reference to it
I can simply
returning that value:

in main()
...
int z[][] = pass(); // here 'z' contain the reference to the array
object created in pass() by 'a'
...

the method
...
public static int[][] pass()
{
int a[][] = {{1,2},{3,4}};
return a;
}
...

instead in C++

in main()
...
int **p = pass();

Here you're wrong. There is a difference between a pointer and a
matrix, and in C/C++ you can not return an array (and thus not a
matrix).

What you should do is find out what you want to return and declare that
type, e.g.
typedef std::vector<std::vector<int> > matrix;

(Depending on what you do return the above might be overkill. If e.g.
you always return a 2*2 integer matrix, the above is overkill).

...

the function
...
int ** pass()

now becomes
matrix pass()
{
matrix result;

....
return result;
}

{
static int z[] = {1,2,3, -1};
static int o[] = {4,5,6,7, -1};
static int j[] = {8,9,10,11,12, -1};

static int *p[3];

p[0] = z;
p[1] = o;
p[2] = j;

return p;
}
...

in the C++ if I don't use the static when I go out from that function
in the caller 'p' has not
the correct array values. In C++ the argument can be passed by-value
and by-reference so
when in the function I call 'return p' is passed its address but if I
don't use static that address is
not more available bacause 'p' is not more available....

is that the correct conversion ?

The real problem is that what you point to has been deallocated at
return - it does not exist anymore. You "resolve" this by using static
memory, but other problems come up - e.g. that you can not use pass()
when multithreading and each call to pass destroys any previously
returned values.

/Peter

 

[josh]

peter koch ha scritto:

Here you're wrong. There is a difference between a pointer and a
matrix,

yes there is a difference in fact a matrix is not a pointer but a
pointer to an array
i.e. if I have int a[2][2] I can pass it to a function writing:
1) foo(int a[][2]) or
2) foo(int (*a)[2])

matrix).

no I can return an array:
int * foo()
{
static int a[3] = {1,2,3};
return a;
}
and so I can return a matrix initializating it as a triangular array ad
so like an array of pointers
and so the function:
int ** foo()
{
static int z[] = {1,2,3, -1};
static int o[] = {4,5,6,7, -1};
static int j[] = {8,9,10,11,12, -1};

static int *p[3];

p[0] = z;
p[1] = o;
p[2] = j;

return p;
}
is legal!

 

[peter koch]

peter koch ha scritto:

Here you're wrong. There is a difference between a pointer and a
matrix,

yes there is a difference in fact a matrix is not a pointer but a
pointer to an array
i.e. if I have int a[2][2] I can pass it to a function writing:
1) foo(int a[][2]) or
2) foo(int (*a)[2])

No you can not. You are not passing the matrix to a function - what you
are passing is a pointer to the first element. I believe it must be
your Java background that confuses you.

no I can return an array:
int * foo()

This is not a array! It is a pointer to an integer - for Christs sake.

{
static int a[3] = {1,2,3};
return a;
}
and so I can return a matrix initializating it as a triangular array ad

You are not returning a matrix. What you are returning is a pointer to
the first element. If you can not understand that, I must strongly
recommend you to stop programming in C (or C++ if thats what you call
it).

so like an array of pointers
and so the function:
int ** foo()
{
static int z[] = {1,2,3, -1};
static int o[] = {4,5,6,7, -1};
static int j[] = {8,9,10,11,12, -1};

static int *p[3];

p[0] = z;
p[1] = o;
p[2] = j;

return p;
}
is legal!

It has nothing to do with legality.

/Peter

 

[josh]

peter koch ha scritto:

No you can not. You are not passing the matrix to a function - what you
are passing is a pointer to the first element. I believe it must be
your Java background that confuses you.

sorry my first language is not english so for me is difficult to
explain my concepts.
I did not want to say that I can pass an array or a matrix because a
pointer is
different from that objects. I wanted only say that as in C/C++ I can't
pass an
array or a matrix with the sintax i.e. int [] or int [][] I can receive
them using
pointer notation.

Also in Java we don't pass array or matrix directly but we pass a
reference of
the array object created.

 

[peter koch]

peter koch ha scritto:

No you can not. You are not passing the matrix to a function - what you
are passing is a pointer to the first element. I believe it must be
your Java background that confuses you.

sorry my first language is not english so for me is difficult to
explain my concepts.
I did not want to say that I can pass an array or a matrix because a
pointer is
different from that objects. I wanted only say that as in C/C++ I can't
pass an
array or a matrix with the sintax i.e. int [] or int [][] I can receive
them using
pointer notation.

You can receive their adress by returning the pointer. But this is a
very bad idea so change it to std::vector (unless you want to spend
long hours debugging the situation).

Also in Java we don't pass array or matrix directly but we pass a
reference of
the array object created.

Correct, but the situation is not the same here: in Java you create the
initial object with new, so the object has dynamic allocation from the
beginning. Unless you install a garbage-collector with C++ you can not
do the same thing - your program will leak.

/Peter