Initialization of a class array member in C++

Question

Using VS2012, I have encountered an unprecedented problem, my main class have a member, which is a an array of another small class. This small class has an explicit constructor. Now the problem: How to supply the initialization list?

The following is the code:

class A { //This is the small class
private:
    int value;
public:
    explicit A(int newNumber) {
        value = newNumber;
    }
};

class B {//This is the major class
private:
    A arrayOfA[5];
public:
    B() {//Compiler complain "no default constructor for class A"
         //But I really don't know how to supply the initialization list
    }
};

Thanks a lot!

ghostofstandardspast · Accepted Answer · 2014-06-16 02:42:48Z

4

With a constructor initializer list:

B() : arrayOfA{A(1), A(2), A(3), A(4), A(5)} {}

I would also suggest changing the array to a std::array. If this is not C++11, I'm afraid you cannot initialize your member without having a default constructor for A. However, there is still the possibility of making your own (it's not hard) or using boost::array. In such a case, I am not sure whether it is possible to initialize the array directly, but it definitely is with a helper function.

answered Jun 16, 2014 at 2:42

ghostofstandardspast

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3 Comments

Praetorian Over a year ago

+1 but I'm pretty sure VS2012 will not accept this. It might work if he has the CTP installed.

ghostofstandardspast Over a year ago

@Praetorian, I believe it will only work with the CTP, but then VS2013 is a better option anyway. That's a good point, though, which I guess makes the pre-C++11 advice more relevant. I can't remember whether VS2012 has std::array, or frankly whether it would work by using ({{A(1), ...}}) or something without the brace initialization syntax.

Leon Over a year ago

Thanks a lot! I learned this syntax in a C++ book I have bought, but wonder why it never worked. Seems that It is not supported in VS2012. In the end, I used the default constructor. Maybe time to upgrade to VS2013.

R Sahu · Accepted Answer · 2014-06-16 02:43:12Z

2

One way to resolve this is to provide a default constructor in A.

class A {
private:
    int value;
public:
    A() : value(0) {}

    explicit A(int newNumber) {
        value = newNumber;
    }
};

answered Jun 16, 2014 at 2:43

R Sahu

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2 Comments

Tony Delroy Over a year ago

An alternative that avoids accidental implicit use of such a default constructor is to add a placeholder argument e.g. enum Uninitialised { uninitialised }; A(Uninitialised) { } - here I'm also avoiding the value = 0 given the caller's intent to assign to the variable later....

Tony Delroy Over a year ago

Yes... that's an orthogonal design decision, but done as and for reasons it explains it the comment. Sometimes it's useful to avoid initialisation - especially when creating a large array that you'll soon assign meaningful values to.

pipja · Accepted Answer · 2014-06-16 03:18:33Z

0

I suggest that you initialize all values of the array to 0 with a default constructor and then use a setter function that accepts 2 params to deal with manipulating the array:

A::setVal(int val)
{
    value = val;
}

B::setA(int index, int value)
{
    arrayOfA[index]->setVal(value);
}

Something like that should work.

edited Jun 16, 2014 at 3:18

answered Jun 16, 2014 at 3:01

pipja

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Comments

Deduplicator · Accepted Answer · 2014-06-16 14:13:33Z

0

This is a workaround for non-default-ctors not being useable here pre-C++11.

Just wrap your arrayOfA in an anonymous union. This means neither the constructors nor destructors will be called automatically. Use placement-new/delete. (Beware of automatically defined special member-functions though)

It would look like this:

class B {//This is the major class
private:
    union{A arrayOfA[5];};
public:
    B() {
        for(size_t i = 0; i < sizeof arrayOfA / sizeof *arrayOfA; i++)
            new((void*)&arrayOfA[i]) A(i);
    }
};

edited Jun 16, 2014 at 14:13

answered Jun 16, 2014 at 2:58

Deduplicator

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