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

Question

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!

If both `n` and `_nodes` are pointers to `Graph::node`, and both are pointing into the same memory area, then you don't have to do any casting. — Some programmer dude, Aug 26 '14 at 03:20
You're casting a 64 bit pointer to an `unsigned int` which is most likely 32 bits. — Jonathan Potter, Aug 26 '14 at 03:20
On (most) 64-bit Linuxes, pointers are 64-bits long and unsigned int is 32 bits long. So the error message is exactly correct. — Tom, Aug 26 '14 at 04:09
Problem solved. Many thanks to 0x499602D2, Joachim, Jonathan and Tom for your comments. — f10w, Aug 26 '14 at 10:11

score 2 · Accepted Answer · answered Aug 26 '14 at 03:21

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.

score 1 · Answer 2 · edited May 23 '17 at 12:23

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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);

edited May 23 '17 at 12:23

Community

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answered Aug 26 '14 at 03:22

R Sahu

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Hi Sahu. Thanks for the answer. This solution works. However for my case I think @Alok's solution is more elegant. +1 – f10w Aug 26 '14 at 10:09

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

2 Answers2