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up vote 3 down vote favorite

I'm writing a class for the Arduino. It's been going well so far, but I'm sort of stuck now...

I have declared an int array in my class 

class myClass
{
  public: MyClass(int size);
  private:
    int _intArray[];
};

When I initialize the class MyClass myClass1(5) I need the array to look like this {0,0,0,0,0}.

My question: what do I need to do so that the array contains 'size' amount of zeros?

MyClass::MyClass(int size)
{
    //what goes here to dynamically initialize the array
    for(int i=0; i < size; i++) _intArray[i] = 0;
}
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4 Answers

up vote 2 down vote accepted

Your code as I'm writing this:

class myClass
{
  public: MyClass(int size);
  private:
    int _intArray[];
};

The declaration of _intArray is not valid C++: a raw array needs to have a size specified at compile time.

You can instead instead use a std::vector:

class myClass
{
public:
    MyClass( int size )
        : intArray_( size )    // Vector of given size with zero-valued elements.
    {}

private:
    std::vector<int> intArray_;
};

Note 1: some compilers may allow your original code as a language extension, in order to support the "struct hack" (that's a C technique that's not necessary in C++).

Note 2: I've changed the name of your member. Generally underscores at the start of names can be problematic because they may conflict with names from the C++ implementation.

Cheers & hth.,

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1  
I get an error saying "ISO C++ forbids declaration of 'vector' with no type" ledLib.h:40: error: ISO C++ forbids declaration of 'vector' with no type MyClass.h:40: error: invalid use of '::' MyClass.h:40: error: expected ';' before '<' token MyClass.h: In constructor 'MyClass::MyClass(int)': MyClass.h:36: error: class 'MyClass' does not have any field named 'intArray_' –  JNK Dec 5 '10 at 13:44
 
@JNK: have you included <vector> header? –  Cheers and hth. - Alf Dec 5 '10 at 13:49
1  
I included it (now) but it doesn't change anything...:( –  JNK Dec 5 '10 at 13:56
 
I found a work-around by simply passing an array to the class and using the pointer later on... not perfect but it works –  JNK Dec 5 '10 at 14:11
 
@JNK: using a raw array has many problems for the novice. if you want to pursue that line then read up on the "rule of three". the simplest is to get the std::vector working -- it should not be a problem (it works for everyone else). –  Cheers and hth. - Alf Dec 5 '10 at 14:20
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up vote 2 down vote

You should use a std::vector.

class myCLass {
public:
    myClass(int size)
        : intarray(size) {
    for(int i = 0; i < size; i++) intarray[i] = 0;
    }
private:
    std::vector<int> intarray;
};
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why not ": intarray(size,0) {" ? –  Matthieu N. Dec 5 '10 at 13:39
 
@Oxsnarder: Meh, not really that versed in vector's constructors. –  DeadMG Dec 5 '10 at 13:53
1  
The zeroing loop is not ncessary: std::vector guarantees to zero those elements. –  Cheers and hth. - Alf Jun 14 '11 at 18:37
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up vote 2 down vote

I'll try the following:

class myClass
{
  public: 
    MyClass(int size);
    ~MyClass();
  private:
    int* _intArray;
};

MyClass::MyClass(int size) {
  _intArray = new int[size];
  for (int i=0; i<size; ++i) _intArray[i] =0; // or use memset ...
}

MyClass::~MyClass() {
  delete[] _intArray;
}

Or, even better, use a STL vector instead ...

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up vote 1 down vote

You should really use vectors as others have suggested. A work-around could be as shown (in case you do not want to use memcpy or a loop).

This would be useful if you have a really huge array. Note that it would add a level of indirection to access the array.

class myClass 
{ 
public: 
   myClass(){
      mt = T();    // value initialize.
   }
private:
   struct T{
      int _intArray[10]; 
   } mt;
};

int main(){
   myClass m;
}
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