C++ multi-dimensional array initialization

Question

in C++ I want to initialize a double matrix (2-dimensional double array) like I would normally do without pointers like so:

 double data[4][4] = {
 1,0,0,0,
 0,1,0,0,
 0,0,1,0,
 0,0,0,1
};

However, since I want to return and pass it to functions, I need it as a double** pointer. So, basically I need to initialize data in a nice way (as above), but then afterwards I need to save the pointer to the 2D-array without losing the data when the function exits.

Any help on this? :-)

• Lara Bailey: Why do you need it as a double** ? What's wrong with a double (*)[4] ?
• Merlyn Morgan-Graham: This is C++. Can this be a class, with this array as a member, which passes a reference back when you call a member function?
• Lara Bailey: possible duplicate of Passing two-dimensional array via pointer It's C, not C++, but it's exactly the same issue.
• Felix: None of the answers have hit bulls eye. I found out I can do this: double ** d = new double[4][4]; But, how will I initialize the value without the chore of writing code like: d[0][0] = 1; d[0][1] = 0;...
• fredoverflow: No, you can't. For example, g++ refuses to compile your snippet: error: cannot convert 'double (*)[4]' to 'double**' in initialization.

Answer 1

Unless you are particular about pointers, I would prefer a reference here

void init( double (&r)[4][4]){
 // do assignment
 r[0][0] = 1;
}
int main(){
 double data[4][4] = {
 1,0,0,0,
 0,1,0,0,
 0,0,1,0,
 0,0,0,1
 };
 init(data);
}

By the way, if you pass it to a function in this manner, you would be "assigning" rather than "initializing".

• Felix: I think this answer is getting warmer. It might be the one I'm looking for!
• Felix: By replacing "init" with a copy function, this models my problem elegantly. Thanks!

Answer 2

Are all your matrices 4x4? Then I would simply define a class with a double[4][4] member and pass objects of that class around:

class Matrix
{
 double m[4][4];
 // ...
};
void function(const Matrix& matrix)
{
 // ...
}

If you need matrices of various dimensions, but they are known at compile time, use a template:

template <size_t n>
class Matrix
{
 double m[n][n];
 // ...
};
template <size_t n>
void function(const Matrix<n,n>& matrix)
{
 // ...
}

This saves you from dealing with array-to-pointer decay and makes the code more readable IMHO.

Answer 3

First, declaration of the double dimensional array is not correct. It needs to be done as follows:

double data[4][4] = { 
 {1.0,0,0,0}, 
 {0,1,0,0}, 
 {0,0,1,0}, 
 {0,0,0,1} 
 };

Second, for passing it in a function you can do it like

show(data);

In the function declaration, you need to give the argument as an array with giving all dimensions except the first. So the declaration would look like:

void show(double arr[][4])
{
 ...
 ...
}

This passes the array as a reference wihout you needing to use a pointer.

Hope this helped.

• sleighty: Thanks! That one-line initialization was what I had been looking for.

Answer 4

double (*)[4] is very different from double **

Just sketch the layout of your doubles in the memory for both and you should understand why you can't use them interchangeably.

• Felix: since I don't know or understand "double (*)[4]", I can't sketch it. What does it mean?

Answer 5

Initialize temporary variable in this way and then copy it to the dynamically allocated memory.

Answer 6

How about this (with pointers, and does what you asked for)

#include <iostream>
using namespace std;
int refer(double (*a)[4])
{
 cout<<"First value is "<<(*a)[0];
 (*a)[0] = 37;
 cout<<"changed value is "<<(*a)[0];
}
int main()
{
 double data[4][4] = {
 1.0,0,0,
 0,1,0,0,
 0,0,1,0,
 0,0,0,1
 };
 refer(data);
 return 0;
}

Answer 7

late to the party, but.... c++ is meant to work with c++ stl. (At least according to Bjarne Stroustrup if you're using raw pointers in c++ then you're doing something wrong.)

#include <array> // on top
// .. 
std::array<std::array<int, 4>, 4> data {{
 {1,0,0,0},
 {0,1,0,0},
 {0,0,1,0},
 {0,0,0,1}
}};
// access it normally 
data[1][2] = 123;
std::cout << data[0][0]; // prints 1. (#include <iostream>)
// You can return std::array from a function as a normal return value, don't worry it is not going to be reallocated, or missing.
std::array<std::array<int, 4>, 4> give_me_data() {
 std::array<std::array<int, 4>, 4> data {{
 {1,0,0,0},
 {0,1,0,0},
 {0,0,1,0},
 {0,0,0,1}
 }};
 return data;
}

(works with Mac's XCode, also gcc supports it https://stackoverflow.com/a/12616826/1031191)

What's the benetif? It works nicely with std algorithms, knows its size, same performance, etc. ref: std::array vs array performance

• Barney Szabolcs: Also, if you don't know the size, use std::vector.