union members accessed with another member (whose data type requires less space)
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From: Francis Glassborow on 9 Nov 2009 03:01 krishna wrote: > int main() > { > union my_union{ > int i; > float f; > }; > my_union u; > u.f = 2.0; > std::cout << u.i; > } > > why is int casted value of u.f not given? (the start address is the > same for all the members). > > -Krishna. > FOFL That is not how unions work. They are a way of recycling memory and at any one time the bit pattern stored in one represents exactly one of the fields. Trying to read a union through a different field than the one by which it was written is strictly undefined behaviour (there are some special cases which are exceptions) What you did was to initialise u as a float and then try to read it as an int. Officially the program can do anything, in practice (in this case) it will probably just reinterpret the stored bit pattern (as many bits as it needs) as an int. If the result happens to be a trap value then your program may stop. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: George Neuner on 9 Nov 2009 04:56
On Mon, 9 Nov 2009 14:01:56 CST, Francis Glassborow <francis.glassborow(a)btinternet.com> wrote: >krishna wrote: >> int main() >> { >> union my_union{ >> int i; >> float f; >> }; >> my_union u; >> u.f = 2.0; >> std::cout << u.i; >> } >> >> why is int casted value of u.f not given? (the start address is the >> same for all the members). >> >> -Krishna. >> > >FOFL >That is not how unions work. They are a way of recycling memory and at >any one time the bit pattern stored in one represents exactly one of the >fields. Trying to read a union through a different field than the one by >which it was written is strictly undefined behaviour (there are some >special cases which are exceptions) What you did was to initialise u as >a float and then try to read it as an int. Officially the program can do >anything, in practice (in this case) it will probably just reinterpret >the stored bit pattern (as many bits as it needs) as an int. If the >result happens to be a trap value then your program may stop. It should be noted that, while C++ does not define the result, unions are used in this way to pass binary floating point data between CPUs with opposite endianess. But pretty much any usage other than such deliberate data conversion is asking for trouble. George -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |