initilize a struct array in a class constructor in c++

Question

I have a struct in my header file

 struct Foo{
     timeval t;
     struct Bar b;
 }

and i also have another one for Bar in the same header file, and I have declared a class in the header file as well.

 class Layer{
      public:
              ...
              ...
      private:
              struct Foo myarray[];
 }

I want to declare an array of Foo in my header file and initialize it in my class constructor

 Layer::Layer(unsigned int size)
 {
       myarray = new Foo[size];
 }

but that returns an error when compiling. error: incompatible types in assignment of ‘Foo*’ to ‘Foo [0]’

any idea how to fix this?

Qaz · Accepted Answer · 2012-10-15 05:55:25Z

2

That's not initializing the array, it's assigning it. To initialize it, you need a member initializer:

Layer::Layer(unsigned int size) : myarray(new Foo[size]){}

However, that won't compile, since you have an array, which needs to be initialized with one size known at compile-time (via an initializer list if the dimension isn't specified), and that size can't change. Use a vector instead:

std::vector<Foo> myarray;

Layer::Layer(unsigned int size) : myarray(size){}

If the vector isn't an option, you at least need a pointer in order to new it, but then stuff with the rule of three/five starts applying and it all becomes a mess. Stick to the vector if you can.

answered Oct 15, 2012 at 5:55

Qaz

62.2k14 gold badges149 silver badges210 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

WhozCraig Over a year ago

+1 on the kick toward std::vector<>. with any luck he'll heed that sooner rather than later.

M4rc · Accepted Answer · 2012-10-15 05:53:23Z

1

struct Foo myarray[]; should be struct Foo *myarray; if you're going to go with it in a dynamic approach. Otherwise, if you want it to be an unspecified static I think it has to have an initialization list.

answered Oct 15, 2012 at 5:53

M4rc

5032 silver badges15 bronze badges

Comments

Doc Bok · Accepted Answer · 2012-10-16 09:30:06Z

Change the declaration of myArray to read:

struct Foo * myArray;

You can access the array as normal using the index operator ( e.g myarray[index] ). You must make sure you don't access it out of bounds, i.e. index must not be less than 0 or greater than size:

myarray = new Foo[size];
myarray[0];  // Fine
myarray[-1];  // Crash!

myarray[size - 1];  // Also fine
myarray[size];  //  Another crash!

You must also ensure when you delete it you use the delete[] operator instead of the normal delete operator:

delete[] Foo;

The reason your code fails is because the compiler needs to know the size of the array at compile time, and you are trying to allocate the size at runtime. Changing it to a pointer tells the compiler you intend to point at a memory address. When you create an array using:

myarray = new Foo[size];

A new array is allocated in memory, and myarray now points to the address of that array.

Collectives™ on Stack Overflow

initilize a struct array in a class constructor in c++

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related