C++: multidimensional array initialization in constructor

I want to use a two dimensional array of constant size as a class member in C++. I have problems initializing it in the constructor though.

Here are my non-working tries:

1.)

class A { public: int a[2][2]; A(); }; A::A() { a = {{1,2},{2,4}}; } 

yields: error: assigning to an array from an initializer list

2.)

class A { public: int a[2][2]; A(); }; A::A() { int b[2][2] = {{1,2},{2,4}}; a = b; } 

yields: invalid array assignment

3.)

class A { public: int **a; A(); }; A::A() { int b[2][2] = {{1,2},{2,4}}; a = b; } 

yields: cannot convert ‘int [2][2]’ to ‘int**’ in assignment

I come from C background. I know that I could use std::vector and I am aware of the disadvantages this approach has but since this is an exercise for me I would like to know how to get it working with plain arrays. I should add that I want to work on this array later on. I want to change the stored values but not the size. Maybe that matters as well (I figured a const at the right place could help somehow?).

5 Answers

If you have C++11, you can use this syntax in the constructor definition:

A() : a{{1,2}, {3, 4}} {} 

If you don't have C++11, you will need to stick to the wicked old ways:

A() { a[0][0] = 1; // etc } 

The first example also uses the constructor init-list, which should always be used to initialize members instead of intializing them in the constructor body.

various multidimensional array in constructor by example:

// int array1[1]; A() : array1{0} {} // int array2[2][2]; A() : array2{{0}} {} // int array3[3][3][3]; A() : array3{{{0}}} {} 

Try this, it works for bidimensional array (in standard C++):

 class A { public: int a[2][2]; A(); }; typedef struct{ int a[4]; } array_t; A::A() { int at[2][2] = {{1,2},{2,4}}; *(array_t*)a = *(array_t*)at; } 

Ciao Angelo

Your first variant is extremely close to the right C++11 syntax:

A::A() : a{{1,2},{2,4}} { } 

To complement the previous answers (you guys are so fast):

What you were trying to do in case 1 and 2 is array assignment, not permitted, as compiler says ;

But I would like to draw your attention to your third case, that's a grave misconception, specially coming from C as you say.

Assigning to a a pointer to a local variable?