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tanvalley
tanvalley
Last Activity: Mar 16 '16, 02:10 PM
Joined: Jun 9 '12
Location: Toronto
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  • tanvalley
    started a topic Bash Process Substitution
    in Linux

    Bash Process Substitution

    In this sequence at the command prompt
    Code:
    $ x=()
$ { while read;  do  x+=("$REPLY"); done; } <<<"$(ls)"
$ printf 'count=%d\n' ${#x[@]}
count=432
$ wc -l <(ls)
     432 /dev/fd/63
$ 
$ 
$ { while read;  do  x+=("$REPLY"); done; } <(ls)
-bash: syntax error near unexpected token `<(ls)'
    what causes the error noted?
    ...
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    • tanvalley
      replied to static assertion about inheritance
      in C
      When the entire statement is
      static_cast<B>( *static_cast<D* >(0));
      there is no generated code and no runtime reference to location 0, so actually it is pretty.

      Furthermore, the sequence
      T* pt = 0;
      S<N>* ps = pt;
      when it does compile, as it should all of the time when the classes are used correctly, will usually produce executable code unless optimised away and possible warnings about unused...
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      • tanvalley
        replied to static assertion about inheritance
        in C
        I don't think I should have to justify my design to ask a simple question about static assertions, but here goes:

        I have a template class that a programmer (also me) uses directly in code:

        template<class T,class N> class R {
        ...
        }

        There's also a "service class" whose implementation is entirely hidden from the programmer:

        template<class N> class S;
        ...
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        • tanvalley
          replied to static assertion about inheritance
          in C
          If casts are so bad way are there hundreds of them (over 700 on my system) in the distributed standard template library?
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          • tanvalley
            replied to How to convert a structure name into a string and vice versa?
            in C
            If you were to switch to a c++ compiler (and you can do this without changing to an obect-oriented design) then using the GNU compilers, you can include
            Code:
            const char* classname(void) {return __PRETTY_FUNCTION__; }
            in every structure of interest. Then call it (parameter.clas sname()) and parse out the stucture name (just before the ::).

            Your mileage may vary, depending on the compiler you're using, and it's not just a matter of changing...
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            • tanvalley
              started a topic static assertion about inheritance
              in C

              static assertion about inheritance

              Is this the canonical way to make a static assertion that class D is derived from class B?
              Code:
              (B)(*(D*)(0));
              Or is there any reason to use static_casts instead?
              Code:
              static_cast<B>(*static_cast<D*>(0));
              Neither seems to generate any code, even at no optimisation.
              Both give the error message "error: no matching function for call to 'B::B(D&)'" when the assertion fails, and are silent otherwise....
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              • tanvalley
                replied to template friend classes
                in C
                I need a single instance of C<T1> to be accessible by B<T1,T2>::f1() and by B<T1,T3>::f1() . By naming the functions individually (in my real case there are more of them) I can allow all of the members of each B<T1,x> access to the internals of C<T1>. I don't want to change my privates.
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                • tanvalley
                  started a topic template friend classes
                  in C

                  template friend classes

                  I'd like to avoid listing all the functions in a template class as friends by declaring the whole class as a friend, but...

                  Code:
                  template<class T1> class A;
template<class T1, class T2> class B;

template<class T1> class C {

      friend void A<T1>::f1(void);
      friend void A<T1>::f2(void);
      friend void A<T1>::f3(void);
                  ...
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                  • tanvalley
                    replied to Get return type of function in template parameter
                    in C
                    This all started with:
                    (i) my expressed dislike of casts, and
                    (ii) my search for a legal syntactic replacement for "(reinterpret_c ast<PARAMETER*> (0))->function" which somehow seems inelegant but also seems like a reasonable idiomatic way to refer to a non-static member in a place where NO DEREFERENCE IS MADE AT RUN TIME.

                    I didn't expect a fundamentalist rant on the foolishness of casting or a lesson on...
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                    • tanvalley
                      replied to Get return type of function in template parameter
                      in C
                      Then, of course, in order to store the result of Get() I need a way in my template<Test> class to determine Test::Get()'s return type. Any hints?
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                      • tanvalley
                        replied to Get return type of function in template parameter
                        in C
                        I think casting a null pointer without actually dereferencing it in generated code ought to be an exception to your rule. There's no type safety issue here.

                        I'm curious to learn how to remove the cast from this:
                        Code:
                        #include <iostream>
using namespace std;

struct Test {
  int i;
};

int main() {
  cout << "sizeof i is "<<sizeof ((Test*)(0))->i
                        ...
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                        • tanvalley
                          replied to Get return type of function in template parameter
                          in C
                          The purpose of this base class is (i) to enforce the requirement that the template parameter class actually has a function named, in this example, "function" and (ii) to extract its return type for use in derived classes.

                          I think it's reasonable to forgive the cast (and a simple C-style cast compiles here, using g++-4.2.1, with the same result) because it's in code that's used for type determination only and, as with traditional...
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                          • tanvalley
                            replied to Get return type of function in template parameter
                            in C
                            Why would I introduce a data member into a base class when all I want is a typedef?
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                            • tanvalley
                              started a topic Get return type of function in template parameter
                              in C

                              Get return type of function in template parameter

                              Is this really the preferred way to get the return type, for use in a derived class, of a function defined in the template parameter?

                              Code:
                              template<class PARAMETER> class C {
         protected:
            typedef typeof (reinterpret_cast<PARAMETER*>(0))->function() returntype;
      };
                              This works just fine for me, but seems inelegant.
                              See more | Go to post
                              • tanvalley
                                replied to integer constant expression
                                in C
                                Nothing... that was just a simple way of showing, in response to the first reply, that "abcd"[1] is *not* recognised (by gcc) as a constant integer expression. All this stuff is really in a too-complex-to-post-macro.

                                Thanks for the detailed response.
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                                • tanvalley
                                  replied to integer constant expression
                                  in C
                                  when I compile
                                  Code:
                                    switch (0) {
      case 1: break;
      case  ("abcd"[1]): break;
   }
                                  I get "error: an array reference cannot appear in a constant-expression"...
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                                  • tanvalley
                                    started a topic integer constant expression
                                    in C

                                    integer constant expression

                                    why is
                                    Code:
                                     ("abcd"[1])
                                    not an integer constant expression?
                                    See more | Go to post
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