Cast void* to array of struct

Question

I have a vector of RGB data.

vector<int> image({1,1,1, 2,2,2, 3,3,3 , 4,4,4});

I want to flip this image horizontally (about the center column).

to perform flip operation, I have created a function

flip(void* image, int rows, int cols)

where i would like to perform flip, but keep individual triplets of RGB intact

So I created a

struct color{
    char r;
    char g;
    char b;
};

and calling the function as

int main(){
vector<int> image({1,1,1, 2,2,2, 3,3,3 , 4,4,4});
flip(&image[0],rows,cols);
}

but casting void* to struct color[] raises compilation error. How can I proceed around this ?

If you really really really really have to go the `void *` way, see [this question](http://stackoverflow.com/questions/310451/should-i-use-static-cast-or-reinterpret-cast-when-casting-a-void-to-whatever). — Ami Tavory, Mar 26 '16 at 21:06
vector is a class, not a C char array. Also in c++ you need to make an explicit cast — xvan, Mar 26 '16 at 21:08

score 1 · Accepted Answer · answered Mar 26 '16 at 21:37

You can't cast to an array in C++. You'll have to use a pointer.

struct color* image = (struct color*)arr;

And for your purposes, this won't work right away. You'll have to start using a vector of char not int. This is because and int is usually 4 bytes and a char is 1 byte. Trying to assign an int to your struct (which is 3 bytes) wouldn't give you the result you expect.

Then you can access the pointer as an array. For example image[0].r would return the first value in the vector, and image[1].r would return the fourth. Just be careful to not go beyond the size of the vector.

score 0 · Answer 2 · answered Mar 27 '16 at 05:20

0

i changed the struct as

struct color{
int rgb[3];
};

and

struct color* image = (struct color*)arr;

provides an array of struct color which can be flipped keeping the rgb data intact.

answered Mar 27 '16 at 05:20

Akshay Mathur

560
1
4
14

Cast void* to array of struct

2 Answers2