c++ class constructor for array

Question

I am writing a Matrix2D class. At the beginning I was using constructor as folows,

My code:

Matrix2D(float a,float b, float c,float d)
{
 a_=a;
 ....
} 

However, I have just realized that it would be a lot better if I could use multi dimensional array [2][2]. That's where the problem lies, How do I write constructor for array ?

class Matrix
{
 float matrix[2][2];
 public:
 Matrix2D(float a,float b,float c, float d)
 {
 matrix[2][2]={a,b,c,d} // not valid
 }
}

Just to let you know, I don't ask for a complete code. I just need someone to put me on a right track.

• Ben Voigt: BTW, the class name and constructor name need to match.
• Tomas: thx, its just typo, they match in a source code

Answer 1

For C++11 you can do:

Matrix(float a,float b,float c, float d) :
 matrix{{a,b},{c,d}}
{
}

There's no clean alternative for C++03.

• Jesse Good: You need to take off the parentheses, i.e. matrix{{a,b},{c,d}} because it's an array.
• Jesse Good: LWS uses gcc 4.7.2
• Tomas: ok, I have tried it and apparently vs 2010 doesnt support this feature, so I will have to go with " matrix[0][0]=a; " and so on.. Thx for support

Answer 2

matrix[0][0] = a; // initialize one element

and so on.

• Tomas: Thx for the answer. That's the way I'll do it

Answer 3

matrix[0][0] = value you want to matrix [n][n] = value you want but count up in a loop so the matrix can be dynamic in size or you can reuse your code later.

for(int ii(0); ii < first dimension size; ++ii)
{
 for(int ll(0); ii < second dimension size; ++ll)
 {
 matrix[ii][ll] = value you want;
 }
}

this will make your code more extensible and more useful outside of this application and maybe it's not useful or maybe it is.

Answer 4

If it will be a matrix 2X2, then you can pass a float array and then loop through it.

for example

for(int x = 0;x<4;x++)
{
 matrix[0][x] = myarray[x];
}

• Bartlomiej Lewandowski: @LuchianGrigore how is that invalid?
• Code-Apprentice: @coolbartek If matrix is declared as float matrix[2][2] then matrix[0][3] is obviously invalid.
• Bartlomiej Lewandowski: it looks fine for me, an two-dimensional array is actually a normal array, where you can access the elements by [0][x]. An array[x][y] is the same as an array[0][x*y]
• Ben Voigt: @coolbartek: Don't you mean that array[y][x] is the same as array[0][x+y*width]?
• Ben Voigt: @Luchian, are you forgetting that a[i] is the same as *(a + i) for built-in types?

Answer 5

Luchian's version is best if you have a C++11 compiler. Here's one that works for all versions of C++:

struct matrix_holder { float matrix[2][2]; };
class Matrix : matrix_holder
{
 static matrix_holder pack(float a,float b,float c, float d)
 {
 matrix_holder h = { {{a, b}, {c, d}} };
 return h;
 }
public:
 Matrix(float a,float b,float c, float d) : matrix_holder(pack(a,b,c,d))
 {
 }
};

The optimizer will inline away the helper.