Array Initialization from a struct

Question

I was wondering if there was a way to initialize an array out of a variable from a struct. Say you have a struct like this-

struct Test{  
    int Number;  
};

And you wanted to initialize the int Number to become an array.
I've already tried this, and it doesn't work:

Test t1;  
t1.Number = new int[3];   
t1.Number[3] = 6;

I know ISO C++ forbids resizing arrays, but if there was a way to initialize the integer to be an array, that's not really resizing(isn't it?) Also, vectors don't work inside of structs. I get a "Vector does not name a type" error.

P.S., I can't do this either:

struct Test{  
    int Number[5];  
};

Because at that time I don't know the size of the array I want.

How could an int magically become an array? It would need to be an int* at the least, but even then you'd best steer clear and look at std::vector<> instead. — ildjarn
– ildjarn, Commented Mar 31, 2012 at 3:20
@ildjarn That's right, it doesn't. That only works if you use a pointer. But I think the 'std::vector<>' approach is a better idea. Thanks — Niro56
– Niro56, Commented Mar 31, 2012 at 3:28

Sean · Accepted Answer · 2012-03-31 03:20:50Z

2

vector works just fine in structs:

#include <vector>

struct Test {
    std::vector<int> Numbers;
};

I'm not sure what you're really trying to do but I think this comes close.

answered Mar 31, 2012 at 3:20

Sean

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woodings · Accepted Answer · 2012-03-31 03:24:53Z

0

One trick to do this

struct Test {
  int Numbers[1];
};

when you initialize the struct, you need to use your own allocation function.

struct Test *NewTest(int sizeOfNumbers) {
  return (struct Test*)malloc(sizeof(struct Test) + sizeof(int)*(sizeOfNumbers - 1));
}

then, you will be able to access the numbers by using,

struct Test *test1 = NewTest(10);
test1->Numbers[0]...
test1->Numbers[1]...
test1->Numbers[9]...

answered Mar 31, 2012 at 3:24

woodings

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Comments

voltrevo · Accepted Answer · 2012-03-31 03:27:31Z

0

The return value of new int[3] is an int* not an int. To make your code work you can do:

struct Test {  
    int* Number;  
};

int main() {
    Test t1;  
    t1.Number = new int[4]; // Note this should be 4 so that index 3 isn't out of bounds
    t1.Number[3] = 6;
    delete t1.Number;

    return 0;
}

However you should really use a std::vector rather than a static array. Vectors work just fine inside structs:

#include <vector>

struct Test {  
    std::vector<int> Number;  
};

int main() {
    Test t1;  
    t1.Number.resize(4);   
    t1.Number[3] = 6;

    return 0;
}

answered Mar 31, 2012 at 3:27

voltrevo

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Comments

Ernest Friedman-Hill · Accepted Answer · 2012-03-31 03:47:03Z

0

You can use a pointer to int -- i.e.,

struct Test{  
    int *Number;  
};

Then you can assign this at any future time to point to an array of your preferred size:

t.Number = new int[5];

But as other posters have already said, std::vector, with a small "v", works fine; be sure to #include <vector> so the compiler knows what you're talking about.

edited Mar 31, 2012 at 3:47

answered Mar 31, 2012 at 3:22

Ernest Friedman-Hill

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