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int bFlag : 1

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afaik defining bit flags be also be used like:
class a {
protected:
   int     bFlag1       : 1;
   int     bFlag2       : 1;
};

and compiler will pack them. now i'm wondering if this works for inheritance also? like:
class b: public a {
protected:
   int     bFlag3       : 1;
   int     bFlag4       : 1;
};

will these flags share "same" memory or not?

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Depends on the compiler. AFAIK, the standard says they don't need to (Since they're not in the same class/struct), but I can't see any reason why a good compiler couldn't pack them.

Easy way to test: What's sizeof(b) in the above case?

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#include <iostream>
struct A
{
int bFlag1 : 1;
int bFlag2 : 1;
};

struct B : A
{
int bFlag3 : 1;
int bFlag4 : 1;
};

int main()
{
std::cout << sizeof(B);
}



This little program gives an output of 8 of VC6 and VS2003.Net. I don't have access to any other compilers ATM.

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So it looks like most compilers don't perform this optimization. Can't say I'm suprised - bit fields aren't used that much, and it could be faked with bitwise operations if it were really important.

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This topic is 4022 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

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