I wanted to design a composition using C as shown below:

#define NUMBER (4)

class wheel {
    int radius;
public:
    wheel();
    wheel(int);
    //copy constructors prototype
    //getters and setters prototypes
};

wheel::wheel() : radius(1) {}
wheel::wheel(int r) : radius(r) {}
//wheel class copy constructor definition
//wheel class setter and getter definitions

class car {
    wheel fourwheels[NUMBER];
public:
    car();
};

car::car() {
    fourwheels[NUMBER] = {wheel(1), wheel(1), wheel(1), wheel(1)};            //error code
    //wheel fourwheels[NUMBER] = {wheel(1), wheel(1), wheel(1), wheel(1)};    //non-error code
}

int main() {
    car mycar;
    return 0;
}

While compiling the code, I am getting the following error:

error: no match for 'operator=' (operand types are 'wheel' and '<brace-enclosed initializer list>')

Queries:

1. Why does this error occur ??
2. When I comment the error code line and uncomment the non-error code line, it works fine. Why do we have to add the type wheel for the array definition?

CodePudding user response：

You are attempting to assign to an array element. And one that's out of range at that.

Using the constructor's initializer list, this will compile, though you should consider using STL containers rather than a raw array.

car::car() : fourwheels{wheel(1), wheel(1), wheel(1), wheel(1)}
{
}

The code you had commented out "worked" because it declared and initialized a new array of four wheels.

Since the default constructor for wheel provides a radius of 1, you could also write:

car::car() : fourwheels{wheel(), wheel(), wheel(), wheel()}
{
}

But, if we used std::array to hold our wheels, we can simplify this further, as the elements of fourwheels will be initialized using the wheel type's default constructor, which we don't have to write.

class car {
    std::array<wheel, NUMBER> fourwheels;
};

CodePudding user response：

Why does this error occur ??

Raw arrays are not copy-assignable. That is, the following will not work:

int nums[4] = {};
nums = {1, 2, 3, 4};

That is essentially what you're trying to do in your car constructor. Instead, you need to initialize the fourwheels member in the member initialization list:

car::car()
  : fourwheels{wheel(1), wheel(1), wheel(1), wheel(1)}
{}

When I comment the error code line and uncomment the non-error code line, it works fine. Why do we have to add the type wheel for the array definition ??

Because you're not initializing your car class's member there. You're declaring and initializing a different array, also named fourwheels that is local to the constructor body. The class member remains default-initialized.