not compiling...

This is a discussion on not compiling... within the C++ Programming forums, part of the General Programming Boards category; Code: #include <iostream> #include <list> #include <algorithm> #include <set> #include <iterator> using namespace std; int main() { set<string> words; list<string> ...

  1. #1
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    not compiling...

    Code:
    #include <iostream>
    #include <list>
    #include <algorithm>
    #include <set>
    #include <iterator>
    
    using namespace std;
    
    int main()
    {     
          set<string> words;
          list<string> index;
          
          for(string new_word; cin >> new_word, new_word != "Quit"; ) {
               pair<set<string>::iterator, bool> &trace = words.insert(new_word);
               if(trace.second)
                  index.push_back(trace.first);
          }
          copy(index.begin(), index.end(), ostream_iterator<string>(cout, "\n"));
          copy(words.begin(), words.end(), ostream_iterator<string>(cout, "\n"));
          
          return 0;
    }
    In line
    pair<set<string>::iterator, bool> &trace = words.insert(new_word);

    it says

    invalid initialization of non-const reference of type 'std:air<std::_Rb_tree_const_iterator<std::string>, bool>&' from a temporary of type 'std:air<std::_Rb_tree_const_iterator<std::string>, bool>'

    I took the code from a book so i'm confused on why it won't work...

  2. #2
    spurious conceit MK27's Avatar
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    Here's my guess: notice the "non-const" -- this is a reference, think about why the const is important with that,* so:
    Code:
          for(string new_word; cin >> new_word, new_word != "Quit"; ) {
               const pair<set<string>::iterator, bool> &trace = words.insert(new_word);
               if(trace.second)
                  index.push_back(*(trace.first));
          }
    I also de-reference the *(iterator) here to yield an actual string (and not a string::iterator).

    Compiles fine!

    * C++ objects (unless created with new) are "auto-destroyed" based on their reference count (and a reference counts ); if the reference counting does not track re-assignments (only scope, etc), we cannot permit this reference to be re-assigned or the count will be wrong, resulting in leaked memory. So the compiler says this must be const (sort of like some interior pointers used in C standard lib functions). Nb, this one is destroyed by scope and flow anyway.
    Last edited by MK27; 07-03-2010 at 12:36 PM.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  3. #3
    Registered User whiteflags's Avatar
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    Set's insert does not return a reference to a pair.

  4. #4
    spurious conceit MK27's Avatar
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    Quote Originally Posted by whiteflags View Post
    Set's insert does not return a reference to a pair.
    Yeah, but that will work anyway -- you just need the const.

    Of course, it is pointless in context (using a reference).
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  5. #5
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by MK27 View Post
    * C++ objects (unless created with new) are "auto-destroyed" based on their reference count (and a reference counts ); if the reference counting does not track re-assignments (only scope, etc), we cannot permit this reference to be re-assigned or the count will be wrong, resulting in leaked memory. So the compiler says this must be const (sort of like some interior pointers used in C standard lib functions). Nb, this one is destroyed by scope and flow anyway.
    No idea what you mean by this.
    The reason you cannot assign it a non-const reference is to stop you from accidentally shooting yourself in the foot. The changes in a temporary will not be reflected in the caller (because it's a temporary).
    But the standard allows you to bind temporaries to const references, in which case the lifetime of the temporary will be extended.
    Also, C++0x also allows you to bind temporaries to r-value references.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  6. #6
    Registered User whiteflags's Avatar
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    I'm still confused. In the first place, how is a return value a "temporary"?

    Why does what C++0x let you do matter right now? Is insert's return value an r-value? No.

  7. #7
    C++まいる!Cをこわせ! Elysia's Avatar
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    Of course a return value is a temporary. It's a copy (unless it actually returns a reference) of whatever value the function deemed fit to return. Furthermore, it's an r-value, because it cannot be assigned (again, if it doesn't return a reference).
    In other words, if...
    words.insert(new_word) = something;
    ...works, then it's an l-value and can be assigned to a non-const reference.
    If it doesn't work, then it's an r-value, and that means it can only be assigned to const references.

    And can it matter? Yes. Why? Because a copy is made once you return the value and a copy is made when you assign the value.
    Unless, that is, the compiler supports named return value optimization.
    If you bind the return value to an r-value reference, you can just steal the temporaries contents without having to allocate more resources and copy everything over and later destroying the temporaries resources.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  8. #8
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    Well... the original code was this:

    Code:
    #include <iostream>
    #include <iterator>
    #include <list>
    #include <algorithm>
    #include <set>
    
    using namespace std;
    
    int main()
    {     
          typedef set<string> WordSet;
          typedef WordSet::iterator WordIter;
          typedef list<WordIter> Index;
          
          WordSet words;
          Index input_order;
          
          for(string new_word; cin >> new_word && new_word != "Quit";) {
              const pair<WordIter, bool> &ref = words.insert(new_word);
              if(ref.second) 
                 input_order.push_back(ref.first);
          }
          copy(input_order.begin(), input_order.end(), ostream_iterator<string>(cout, "\n"));
          copy(words.begin(), words.end(), ostream_iterator<string>(cout, "\n"));
          
          return 0;
    }
    And it shows some crazy long error at the bolded line... i mean why did they put this code if this doesnt work? Weird...

  9. #9
    C++まいる!Cをこわせ! Elysia's Avatar
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    That is a very good question. I have honestly no idea why they did it. The problem is that input_order stores iterators to strings. Hence the bolded copy line sends iterators to an output iterator that is expecting a string.

    I was able to get it compiling by...
    Code:
    typedef list<WordIter> Index;
    to
    typedef list<string> Index;
    
    input_order.push_back(ref.first);
    to
    input_order.push_back(*ref.first);
    Another solution might be (more efficient):
    Code:
    Replace
    copy(input_order.begin(), input_order.end(), ostream_iterator<string>(cout, "\n"));
    by
    for_each(input_order.begin(), input_order.end(), [](WordIter it) { std::cout << *it << "\n"; });
    (Requires C++0x.)
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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