- looking for validation/rebuttal on this claim - 3 Updates
- Confused by C++ Primer's discusion of const_cast - 4 Updates
| Daniel Davidson <phyton.dbd@gmail.com>: Apr 25 08:22AM -0700 On Thursday, April 23, 2015 at 4:58:44 PM UTC-5, Alain Ketterlin wrote: curs in the one case and not in the other? > view on the question). But this does not mean that it is easy to find > something valuable to add... > -- Alain. This is great! Thank you Alain |
| Jorgen Grahn <grahn+nntp@snipabacken.se>: Apr 25 07:06PM On Thu, 2015-04-23, Daniel Davidson wrote: > I should not have said hand-written. This question theoretical - but > the setting is actually code generation versus templates. Generated > code is easier to understand/debug, etc. Isn't one of the main problems with generated code that it's /hard/ to understand and debug? If you have a buggy code generator (compiler which generates C++, really) you're going to have major problems. > But I want to find if and > when templates can outperform. It's an interesting question, but I don't have any answers. /Jorgen -- // Jorgen Grahn <grahn@ Oo o. . . \X/ snipabacken.se> O o . |
| Daniel Davidson <phyton.dbd@gmail.com>: Apr 25 01:47PM -0700 On Saturday, April 25, 2015 at 2:06:51 PM UTC-5, Jorgen Grahn wrote: > Isn't one of the main problems with generated code that it's /hard/ to > understand and debug? If you have a buggy code generator (compiler > which generates C++, really) you're going to have major problems. Well, you are asking for an opinion. I'm using tools that make the code fairly straightforward - https://pub.dartlang.org/packages/ebisu_cpp I just want to know case where I may be giving something up. |
| Paul <pepstein5@gmail.com>: Apr 25 11:28AM -0700 Code from C++ Primer is: const char* pc; char * p = const_cast<char*>(pc); // Ok but writing through p is undefined. Why is writing through p undefined? I thought this was an archetypal use of const_cast -- create a non-const object from a low-level const object. Why is this use of const_cast wrong? Paul |
| Victor Bazarov <v.bazarov@comcast.invalid>: Apr 25 02:44PM -0400 On 4/25/2015 2:28 PM, Paul wrote: > const char* pc; > char * p = const_cast<char*>(pc); // Ok but writing through p is undefined. > Why is writing through p undefined? I thought this was an archetypal use of const_cast -- create a non-const object from a low-level const object. > Why is this use of const_cast wrong? The "Code from C++ Primer" is incomplete to be right or wrong. The pointer named 'pc' is uninitialized. Writing through 'p' or even reading through 'pc' yields undefined behavior. V -- I do not respond to top-posted replies, please don't ask |
| Jorgen Grahn <grahn+nntp@snipabacken.se>: Apr 25 06:56PM On Sat, 2015-04-25, Victor Bazarov wrote: > The "Code from C++ Primer" is incomplete to be right or wrong. The > pointer named 'pc' is uninitialized. Writing through 'p' or even > reading through 'pc' yields undefined behavior. Yes. Such an example needs to have an actual object to be useful: char foo = 'a'; const char* pc = &foo; char* p = const_cast<char*>(pc); // Ok const char foo = 'a'; const char* pc = &foo; char* p = const_cast<char*>(pc); // "Ok but" according to the book /Jorgen -- // Jorgen Grahn <grahn@ Oo o. . . \X/ snipabacken.se> O o . |
| Dombo <dombo@disposable.invalid>: Apr 25 10:05PM +0200 Op 25-Apr-15 20:28, Paul schreef: > Why is writing through p undefined? I thought this was an archetypal > use of const_cast -- create a non-const object from a low-level const > object. Nope, const_cast does not create a new non-const object, it only tells the compiler: "you may think this is a pointer to a const object, but take my word for it: it really points to a non-const object". You'd better be right, or the compiler will have its revenge. > Why is this use of const_cast wrong? pc is not initialized in the example; the effects of reading or writing through an uninitialized pointer is undefined. But even if it were initialized writing to it is still undefined if pc is pointing to a const object (this is probably the point C++ Primer is trying to make). const objects may have been been placed in read-only memory, making it impossible to modify them. const_cast<> should only be used to cast away constness if you are absolutely sure that the object being referred to really is non-const. For example: int main() { char[1] data; // non-const object foo(data); return 0; } void foo(const char* pc) { bar(pc); } void bar(const char* pc) { char* p= const_cast<char*>(pc); p[0] = '!'; // Ok, because pc is really pointing to a non-const object } Normally you should never have to use const_cast unless there is some code you cannot change that passes pointers it got from your code back to your code. If you need to cast it often indicates there a design issue. |
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