What kind of conversion is this code doing?

Question

I ran across this code, where they are trying to convert from float to int

int val[5];
union {
    int i;
    float f;
} conv;

... 
val is updated with some value
...
case OUT_FORMAT_FLOAT:
for (i = 0; i < count; i++) {
   conv.f = val[i];
   val[i] = conv.i;
}

But I am just not able to understand how this would work. The val[i] is assigned to conv.f and then the conv.i is used store back the value into val[i]. conv is a union type since we are using f, i will not have a valid value right?

Am I missing something here?

refer http://www.cplusplus.com/doc/tutorial/other_data_types/#unions — hdost, Nov 30 '15 at 18:06

score 5 · Accepted Answer · edited May 23 '17 at 11:59

5

It's doing something called type punning.

The thing to remember here is that floating point values are often stored in a very different format than integers (most commonly IEEE floating point format), and the use of the union is to get the raw floating point format.

To be more specific, this is what happens:

The assignment conv.f = val[i]. This converts the integer in val[i] to a floating point value, and stores it in conv.f.
The assignment val[i] = conv.i. This gets the raw floating-point bit-pattern stored in the union, and assigns it to val[i].

This works because a union is not like a structure with separate members. In a union all members share the same memory. Modifying one member of the union will modify all members.

A note on why a union is used: This conversion could be made in other ways as well, but then that would break the strict aliasing rule, however using unions for type punning is allowed.

edited May 23 '17 at 11:59

Community

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answered Nov 06 '15 at 06:09

Some programmer dude

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I think, you should also mention that virtually all type punning is undefined behavior under strict aliasing rules. Afaik, the only safe way to get to the bit pattern of a `float` is to use the `memcpy()` function. – cmaster - reinstate monica Nov 06 '15 at 06:17
What could be the practical purpose of this particular punning? I guess, inspecting the float-representing bits in the integer? – Blagovest Buyukliev Nov 06 '15 at 06:20
@BlagovestBuyukliev Without seeing more context from the OP it's impossible for anyone else to say. – Some programmer dude Nov 06 '15 at 06:26
1

@cmaster union punning is not subject to the strict aliasing rule. The only potential for UB is if a trap representation is generated in the type being read. – M.M Nov 06 '15 at 08:29

score 0 · Answer 2 · answered Nov 06 '15 at 06:09

0

union allows different data types to be stored at the same location. Space is allocated only for the member having sizeof(member) maximum.

Once member f is initialized, i can be accessed from that location.

answered Nov 06 '15 at 06:09

haccks

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What kind of conversion is this code doing?

2 Answers2