Type cast problem with VC++ 2005 Express Edition
in [Visual C]
|
From: aslan on 3 Dec 2009 10:41 "David Wilkinson" <no-reply(a)effisols.com>, iletisinde şunu yazdı, news:%23UmhHzCdKHA.2160(a)TK2MSFTNGP02.phx.gbl... > aslan wrote: >> OK I tried something else. >> >> struct bool_struct >> { >> bool a[32]; >> }; >> int main(int argc, char**argv) >> { >> bool_struct* p=new bool_struct; >> return 0; >> } >> >> so "new bool_struct" ends up by calling the following cb=32. >> >> void * operator new( unsigned int cb ) >> { >> void *res = _nh_malloc( cb, 1 ); >> >> return res; >> } >> >> So 32 byte is allocated for "bool a[32];" >> >> >> Also the following quote from MSDN help installed with VC++ 6 which >> confirms the size of 1 byte for VC++ 6; >> >> Microsoft Specific >> >> In Visual C++4.2, the Standard C++ header files contained a typedef that >> equated bool with int. In Visual C++ 5.0 and later, bool is implemented >> as a built-in type with a size of 1 byte. That means that for Visual C++ >> 4.2, a call of sizeof(bool) yields 4, while in Visual C++ 5.0 and later, >> the same call yields 1. This can cause memory corruption problems if you >> have defined structure members of type bool in Visual C++ 4.2 and are >> mixing object files (OBJ) and/or DLLs built with the 4.2 and 5.0 or later >> compilers. >> >> END Microsoft Specific > > All this is about the bool type. In VC6 and beyond, sieof(bool) is 1. OK. Tim Roberts was talking about MS STL implementation whereas I use SGI STL implementation so I didn't notice it then. > > The complication is that vector<bool> is not always implemented in the > straightforward way that it is for other types. Though it should be, IMHO. > > -- > David Wilkinson > Visual C++ MVP
From: Tim Roberts on 6 Dec 2009 00:27 "aslan" <aslanski2002(a)yahoo.com> wrote: >> >OK I tried something else. > >struct bool_struct >{ > bool a[32]; >}; That is ENTIRELY different. Seriously, are you really unable to see that? That is a standard C++ array. That is NOT an STL vector. ENTIRELY different. >So 32 byte is allocated for "bool a[32];" Absolutely. And 128 bytes is allocated for an "int a[32]". Neither point is relevent to the discussion. The POINT we were talking about is this: std::vector<bool> a; a.resize(32); THAT allocates 4 bytes, not 32 bytes. >Also the following quote from MSDN help installed with VC++ 6 which confirms >the size of 1 byte for VC++ 6; Yes, but you are conveniently and repeatedly ignoring the most important point. "std::vector<bool> a" is NOT the same as "bool a[32]". There is an special optimization allowed for std::vectors of bool, which allows them to be packed 8 to a byte. That CANNOT be done for "bool a[32]". For example, this is legal: std::vector<int> vi; vi.resize(32); int * pi = &vi[0]; After this, pi will point to a 128-byte piece of memory, with room for 32 ints. But when you do this: std::vector<bool> vb; vb.resize(32); bool * pb = &vb[0]; you get a compile-time errror, to prevent you from accidentally overwriting memory. You were trying to the same kind of thing using vb.begin(), but the iterator for a std::vector<bool> is not just a pointer. It have to maintain the the byte offset and the bit number of the current spot. It's NOT just a pointer to bool. -- Tim Roberts, timr(a)probo.com Providenza & Boekelheide, Inc.
From: Tim Roberts on 6 Dec 2009 00:31 "aslan" <aslanski2002(a)yahoo.com> wrote: > >OK. Tim Roberts was talking about MS STL implementation whereas I use SGI >STL implementation so I didn't notice it then. I'm not convinced. The STL implementation in VC++6 was written by PJ Plauger and contains HP's copyright. HP and SGI cooperated on STL. Here's a way to tell. Try to compile this: std::vector<bool> vb; vb.resize(32); bool * pb = &vb[0]; If that compiles, then your implementation of STL does not have this optimization, and the vector probably occupies 32 bytes. If that gets a compile-time error, then your STL has the same optimization as Microsoft's, and the vector occupies 4 bytes. -- Tim Roberts, timr(a)probo.com Providenza & Boekelheide, Inc.
From: aslan on 7 Dec 2009 03:05 "Tim Roberts" <timr(a)probo.com>, iletisinde sunu yazdi, news:fbfmh5p0d0bjnujo55h0s9g2tj1hi9jrup(a)4ax.com... > "aslan" <aslanski2002(a)yahoo.com> wrote: >>> >>OK I tried something else. >> >>struct bool_struct >>{ >> bool a[32]; >>}; > > That is ENTIRELY different. Seriously, are you really unable to see that? > That is a standard C++ array. That is NOT an STL vector. ENTIRELY > different. > Yes, Sir. >>So 32 byte is allocated for "bool a[32];" > > Absolutely. And 128 bytes is allocated for an "int a[32]". Neither point > is relevent to the discussion. > > The POINT we were talking about is this: > > std::vector<bool> a; > a.resize(32); > > THAT allocates 4 bytes, not 32 bytes. > >>Also the following quote from MSDN help installed with VC++ 6 which >>confirms >>the size of 1 byte for VC++ 6; > > Yes, but you are conveniently and repeatedly ignoring the most important > point. "std::vector<bool> a" is NOT the same as "bool a[32]". There is > an > special optimization allowed for std::vectors of bool, which allows them > to > be packed 8 to a byte. That CANNOT be done for "bool a[32]". For > example, > this is legal: > > std::vector<int> vi; > vi.resize(32); > int * pi = &vi[0]; > > After this, pi will point to a 128-byte piece of memory, with room for 32 > ints. But when you do this: > > std::vector<bool> vb; > vb.resize(32); > bool * pb = &vb[0]; > No Sir, I have tried it and I don't get any compile or run-time error with SGI STL on my machine. The following is fine for me: void f() { std::vector<bool> vb; vb.resize(32); bool * pb = &vb[0]; pb[31]=true; } > you get a compile-time errror, to prevent you from accidentally > overwriting > memory. You were trying to the same kind of thing using vb.begin(), but > the iterator for a std::vector<bool> is not just a pointer. It have to > maintain the the byte offset and the bit number of the current spot. It's > NOT just a pointer to bool. > -- > Tim Roberts, timr(a)probo.com > Providenza & Boekelheide, Inc.
From: Ulrich Eckhardt on 7 Dec 2009 03:43
aslan wrote: > "Tim Roberts" <timr(a)probo.com>, iletisinde sunu yazdi, >> Yes, but you are conveniently and repeatedly ignoring the most important >> point. "std::vector<bool> a" is NOT the same as "bool a[32]". There is >> an special optimization allowed for std::vectors of bool, which allows >> them to be packed 8 to a byte. That CANNOT be done for "bool a[32]". >> For example, this is legal: >> >> std::vector<int> vi; >> vi.resize(32); >> int * pi = &vi[0]; >> >> After this, pi will point to a 128-byte piece of memory, with room for 32 >> ints. But when you do this: >> >> std::vector<bool> vb; >> vb.resize(32); >> bool * pb = &vb[0]; >> > No Sir, > I have tried it and I don't get any compile or run-time error with SGI STL > on my machine. The STL is not standard C++. You can install it, but if you then rely on things where it differs from the C++ standard library, you get exactly what you got. Let me repeat that: The STL, as far as the C++ standard is concerned is just a library. Since it also puts symbols into the 'std' namespace, it could even be called a bad library, as that namespace is reserved. Aslan, why are you using the STL anyway? Development of it was abandoned more than half a decade ago. Most parts of it have been incorporated into the C++ standard (which is probably the reason many people confuse the two), so they are available in a standard-conformant way there. I see no reason to use the STL in modern code. Uli -- C++ FAQ: http://parashift.com/c++-faq-lite Sator Laser GmbH Geschäftsführer: Thorsten Föcking, Amtsgericht Hamburg HR B62 932 |