2D Array of Object pointers in C++

Question

How do you allocate a 2D array of pointers to objects?

Currently I have:

file.h

extern ClassName **c;

file.cpp

ClassName **c;
int main(){
    // some stuff
    c = new ClassName*[sizex];
    for(int i = 0; i < sizex; ++i){
        c[i] = new ClassName[sizey];
    }

    for(int i = 0; i < sizex; ++i){
        for(int j = 0; j < sizey; ++j){
            c[i][j] = new ClassName();
        }
    }

Which fails to compile with error's stating that there is no match for operator= using ClassName and ClassName*, which looking at the error makes sense. But if I were to change the assignment of c[i][j] to

ClassName cn();
c[i][j] = cn;

It gives a plethora of other errors. The array's size cannot be known until runtime (read from stdin) and it must also be extern. What is the proper way to declare an array in this case?

If array size can't be known until runtime, you are way better off with std::vector<Classname**>. You're actually almost always better off with std::vector — kiss-o-matic
– kiss-o-matic, Commented Dec 11, 2014 at 19:10
While I totally second your comment, it does not actually answer the question. — Codor
– Codor, Commented Dec 11, 2014 at 19:11
@kiss-o-matic It would probably be std::vector<ClassName*> as the double-pointer is only necessary in a C-style array of arrays. — tadman
– tadman, Commented Dec 11, 2014 at 19:27

mauris · Accepted Answer · 2017-11-27 19:21:12Z

4

You have to declare the pointer like

extern ClassName ***c;

The allocation will look like

c = new ClassName**[sizex];
for(int i = 0; i < sizex; ++i){
    c[i] = new ClassName*[sizey];

    for(int j = 0; j < sizey; ++j){
        c[i][j] = new ClassName();
    }
}

You could define the 2D array correctly yourself if you would declare the array of some abstract type T. Then all you need is to change T to ClassName *

As for this declaration

ClassName cn();

then it declares a function that has return type ClassName and has no parameters.

edited Nov 27, 2017 at 19:21

mauris

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answered Dec 11, 2014 at 19:11

Vlad from Moscow

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8 Comments

Joseph Mansfield Over a year ago

@LightnessRacesinOrbit He's answering the question. Sure, it could do with a paragraph or two about why this is not a good way to do this, but answering the question comes first.

Vlad from Moscow Over a year ago

@Lightness Races in Orbit I am showing correct declarations instead of wrong declarations.

Lightness Races in Orbit Over a year ago

@Joseph Answering the question comes second, after a HUGE warning that none of this should ever make its way into production code. Ever. Just because it compiles doesn't mean it's advisable in any way, shape or form.

Daniel Over a year ago

@LightnessRacesinOrbit This is a question and answer site. Not a professional advice site.

Fantastic Mr Fox Over a year ago

@Daniel There's vomit on his sweater already, mom's spaghetti... </eminem>, also Lightness is right, this should encourage better programming as well as answer questions.

|

R Sahu · Accepted Answer · 2014-12-11 19:24:46Z

4

ClassName *p1;

p1 can point to one ClassName or an array of ClassNames.

ClassName **p2;

p2 can point to one ClassName* or an array of ClassName*s.

*p2 can point to one ClassName or an array of ClassNames.

When you use:

   c[i] = new ClassName[sizey];

you are allocating memory so that c[i][j] can hold a ClassName but not a ClassName*.

If c[i][j] = ClassName(); is failing, and you want to use c[i][j] = new ClassName();, then c has to be declared as:

 ClassName*** c;

However, in stead of doing that, I strongly suggest use of std::vector and a smart pointer.

 std::vector<std::vector<std::unique_ptr<ClassName>>> c;

answered Dec 11, 2014 at 19:24

R Sahu

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1 Comment

Fantastic Mr Fox Over a year ago

Add 1 for vector suggestion, wish i could +2 for smart pointer.

ClockworkV · Accepted Answer · 2014-12-11 20:29:45Z

4

The previous posters have answered correctly about using the tripple pointer, but for your sanity, and the clarity of your code, you can make use of a couple of simple typedefs:

typedef ClassName* Row;
typedef Row* Matrix;

Matrix *M;  //equivalent to : ClassName ***M, 
            //check by substiting Matrix with Row* and Row with ClassName*
int main()
{
   M = new Matrix[numRows];

   for(int row = 0; row < numRows; ++row)
   {
        M[row] = new Row[numCols];

        for(int col = 0; col < numCols; ++j)
        {
            M[row][col] = new ClassName();
        }
    }
}

this communicates your intent better, and is easier to reason about.

answered Dec 11, 2014 at 20:29

ClockworkV

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Comments

George Houpis · Accepted Answer · 2014-12-11 19:14:52Z

0

You need change your type to: ClassName ***c; as Vlad from Moscow mentioned.

ClassName **c;
int main(){
    // some stuff
    c = new ClassName**[sizex];
    for(int i = 0; i < sizex; ++i){
        c[i] = new ClassName*[sizey]
    }

    for(int i = 0; i < sizex; ++i){
        for(int j = 0; j < sizey; ++j){
            c[i][j] = new ClassName();
        }
    }

Your usage has to change too:

ClassName cn;
c[i][j] = new ClassName( cn );  // <-- this copy constructor would cause a memory leak (old pointer not deleted)
*(c[i][j]) = cn;                // <-- this would use the assignment operator.  May be weak performance.
ClassName & cn = *(c[i][]);     // <-- Allows you to work directly on the cell.

answered Dec 11, 2014 at 19:14

George Houpis

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2 Comments

George Houpis Over a year ago

Although I can agree that triple pointer is an inferior approach, it is not prohibited nor incorrect. The poor guy is obviously trying to learn about new[] and make a double dimension array of pointers. Personally, I would use a single array or vector of unique_ptr encapsulated within a class with an accessor taking an (x,y) parameter to dereference the pointer, but that was not what was asked. Why don't you give him the solution instead of criticizing everyone else's attempt to help him along his code?

Lightness Races in Orbit Over a year ago

Peer review is a crucial part of any scientific discourse. I suggest getting used to receiving it. :)

flakes · Accepted Answer · 2014-12-11 19:22:27Z

0

ClassName **c is a 2D array ie c[n][m] of ClassName objects.

You have attempted to assign c[n][m] of type ClassName as new ClassName() of type ClassName*

for(int i = 0; i < sizex; ++i){
    for(int j = 0; j < sizey; ++j){
        c[i][j] = *(new ClassName);
    }
}

edited Dec 11, 2014 at 19:22

answered Dec 11, 2014 at 19:16

flakes

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