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...
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?
Marcus Vance is a cybersecurity auditor and technology writer dedicated to educating the public about online safety, data privacy regulations, enterprise security, and emerging cyber threats.
