CODEWITHSUNDEEP
c++structurememcpy
Alex
Alex

Reputation: 4180

Copy values to and from a member of a structure in C++

I have a structure as shown below, which I use for the purpose of sorting a vector while keeping track of the indices. 

struct val_order{
        int order;
        double value;
};

(1). Currently what I do is use a loop to copy values as shown below. So my first question is if there is a faster way to copy values to a member of a structure (not copying an entire structure to another structure)

int n=x.size();
std::vector<val_order> index(n);
for(int i=0;i<x.size();i++){        //<------So using a loop to copy values.
    index[i].value=x[i]; 
    index[i].order=i;
}

(2). My second question has to do with copying one member of a structure to an array. I found a post here that discusses using memcpy to accomplish that. But I was unable to make it work (code below). The error message I got was class std::vector<val_order, std::allocator<val_order> > has no member named ‘order’. But I was able to access the values in index.order by iterating over it. So I wonder what is wrong with my code.

int *buf=malloc(sizeof(index[0].order)*n);
int *output=buf;
memcpy(output, index.order, sizeof(index.order));

Upvotes: 2

Views: 1288

Answers (1)

D&#233;j&#224; vu
D&#233;j&#224; vu

Reputation: 28850

Question 1

Since you are initializing your n vectors from two different sources (array x and variable i), it would be difficult to avoid a loop. (you could initialize the vectors from index.assign if you had an array of val_order already filled with values, see this link)

Question 2

You want to copy all n order values into a int array, and memcpy seems convenient for that. Unfortunately,

  • each element of a vector is a val_order structure, so even though you could copy via memcpy that would not only copy the int *order* value but also the double *value* value
  • furthermore, you are dealing with vector, which internal structure is not a simple array (vector allows operations that are not possible with a regular array) and thus you cannot copy a bunch of vector to a int array by simply giving the address of, say, the first vector element to memcpy.
  • also, memcpy wouldn't work like you want anyway, but it expects an address - thus you would have to give, e.g., &index[0] ... but again, this is not what you want given the points above

So you would have to make another loop instead, like

int *buf = (int *)malloc(sizeof(int)*n);
int *output = buf;
for (int i=0 ; i<n ; i++) {
   output[i] = index[i].order;
}

Upvotes: 1

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