CODEWITHSUNDEEP
c++casting
f10w
f10w

Reputation: 1586

error: cast from ‘...’ to ‘unsigned int’ loses precision [-fpermissive]

In my code Graph is a class having a member node, which is a structure. When I do

unsigned int id  = ((unsigned int)n - (unsigned int)_nodes) / sizeof(Graph::node);

I get the following error (compiled on 64-bit Linux):

error: cast from ‘Graph::node* {aka Graph::node_st*}’ to ‘unsigned int’ loses precision [-fpermissive]

Googled and found a similar question but it does not seem to me that the answer is applicable here (note that I want to get the size of the object but not itself).

Thank you in advance for any suggestions!

Upvotes: 1

Views: 5515

Answers (2)

R Sahu
R Sahu

Reputation: 206717

The first answer in the SO post you have a link to provides an answer that should work for you.

Use

intptr_t id  = ((intptr_t)n - (intptr_t)_nodes) / sizeof(Graph::node);

Upvotes: 1

Alok Singhal
Alok Singhal

Reputation: 96241

If n and _nodes point to Graph::node, i.e., they are of type Graph::node * (which seems to be the case from the error message), and if you wan to calculate the "distance" between the two in terms of the number of Graph::node elements, you can do:

unsigned int id = n - _nodes;

In C and C++, pointer arithmetic will result in the difference being the number of elements (instead of number of bytes).

For this to work portably, both n and _nodes must point to a contiguous block of Graph::node values, and n should be "after" _nodes. If you can get negative differences, you can use ptrdiff_t type instead of unsigned int.

Upvotes: 2

Related Questions