Array in Struct, Pointers [C++ Beginner]

Question

Coming from a Java, PHP background, I am trying to get into C++. I would like to store an array in a struct. My problem is specifying the size of the array after initialising the struct.

Here's my code for the struct:

struct SpriteAnimation {
    // ...
    int parts;                  // total number of animation-parts
    unsigned int textures[];    // array to store all animation-parts
    // ...
};

And here for the main function:

SpriteAnimation bg_anim;
bg_anim.parts = 3; 
unsigned int *myarray = new unsigned int[bg_anim.parts];
bg_anim.textures = myarray;

What do I need to change to fix this?

Kerrek SB · Accepted Answer · 2011-08-06 21:53:50Z

9

In modern C++, you would use a dynamic container for the inner "array":

struct SpriteAnimation {
  std::vector<unsigned int> textures;    // array to store all animation-parts
  size_t num_parts() const { return textures.size(); }
};

This is by far safer and more modular than anything you could try with manually allocated storage. Usage:

SpriteAnimation x;
x.textures.push_back(12);  // add an element
x.textures.push_back(18);  // add another element

SpriteAnimation y = x;     // make a copy

std::cout << "We have " << x.num_textures() << " textures." std::endl; // report

edited Aug 6, 2011 at 21:53

answered Aug 6, 2011 at 21:38

Kerrek SB

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3 Comments

Kerrek SB Over a year ago

Downvoter, care to explain your objection? I understand that this isn't the "C array way" of doing things, but the question asks for a C++ solution, and the idiomatic and moral way to do C++ is to use (exception-safe and resource-managing and single-responsibility) containers as building blocks.

Ben Over a year ago

How does this know the size of the array (or vector - could you explain the difference, please)? Does push_back() resize it by creating a new one each time it is called? That would be horribly inefficient compared to John's solution, or am I misunderstanding something?

Kerrek SB Over a year ago

@Ben: Explaining the full workings of vector is beyond this comment, but do read it up -- yes, push_back does append a new element, but this is amortized O(1) thanks to magic. In fact, most of the C++ standard library can only be described as such.

John · Accepted Answer · 2011-08-06 21:37:30Z

0

struct SpriteAnimation {
    // ...
    int parts;                  // total number of animation-parts
    unsigned int * textures;    // array to store all animation-parts
    // ...
};

You can use type name[] syntax only if you declare members inline.

answered Aug 6, 2011 at 21:37

John

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3 Comments

Ben Over a year ago

Is it correct to then use it with bg_anim.textures = new unsigned int[bg_anim.parts]; ?

Kerrek SB Over a year ago

What happens when you copy your struct? What happens when there's an exception?

Will03uk Over a year ago

@Ben yes that works correctly, in fact it wasn't correct before, as texture is a dynamically allocated array and should be shown as a pointer. You should lookup classes if you wish to use C++ as you are using c style syntax. Also remember to delete[] the array at the end

Michał Šrajer · Accepted Answer · 2011-08-06 21:40:13Z

0

Size of a struct must be known at a compilation time.

answered Aug 6, 2011 at 21:40

Michał Šrajer

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1 Comment

Will03uk Over a year ago

He was meaning to use a pointer and the type unsigned int * is a static size

Anony-mouse · Accepted Answer · 2015-06-19 13:11:19Z

0

I worked around the issue through following code.It might be having design issues so please look it up the following code works for me.

#include <iostream>
using namespace std;
struct lol {
  // ...
  int parts;                  // total number of animation-parts
  unsigned int *texture;   // array to store all animation-parts
  // ...
};

int main() {
  // your code goes here
  lol bg_anim;
  bg_anim.parts = 3; 
  unsigned int *myarray = new unsigned int[bg_anim.parts];
  bg_anim.texture = myarray;
  return 0;
 }

Forgive me for using lol instead of your specified name.Do tell me any issues.And help me if there are other issues in my code. Thank you !! :)

answered Jun 19, 2015 at 13:11

Anony-mouse

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1 Comment

Anony-mouse Over a year ago

by the way a struct is nothing but a class with everything public.A better solution would have been to use classes.

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