Can't write range based for with non-member begin and end functions

Question

I'm writing a code that uses bitboards. since iterating on all bits of a bitboard is pretty common action, I decided to write some iterator class, and to use c++0x's range based loops. However, g++ (version 4.6.3) tell me that there is not matching function for begin or end.

My code:

#include <iostream>
#include <cinttypes>

class bitscan {
    uint64_t mask;
public:
    bitscan(uint64_t m) : mask(m) {}
    bool operator!=(bitscan it) const {return mask!=it.mask;}
    bitscan &operator++() {mask&=(mask-1);return *this;}
    int operator*() {return __builtin_ctzl(mask);}
};

bitscan begin(uint64_t m) {return m;}
bitscan end(uint64_t m) {return 0;}

int main() {
    uint64_t num=49;
    for (int i : num) std::cout<<i<<std::endl;
}

The error:

err.cpp: In function ‘int main()’:
err.cpp:18:15: error: no matching function for call to ‘begin(long unsigned int&)’
err.cpp:18:15: note: candidates are:
/usr/include/c++/4.6/initializer_list:86:5: note: template<class _Tp> constexpr const _Tp* std::begin(std::initializer_list<_Tp>)
/usr/include/c++/4.6/bits/range_access.h:87:5: note: template<class _Tp, long unsigned int _Nm> _Tp* std::begin(_Tp (&)[_Nm])
/usr/include/c++/4.6/bits/range_access.h:58:5: note: template<class _Container> decltype (__cont.begin()) std::begin(const _Container&)
/usr/include/c++/4.6/bits/range_access.h:48:5: note: template<class _Container> decltype (__cont.begin()) std::begin(_Container&)
err.cpp:18:15: error: no matching function for call to ‘end(long unsigned int&)’
err.cpp:18:15: note: candidates are:
/usr/include/c++/4.6/initializer_list:96:5: note: template<class _Tp> constexpr const _Tp* std::end(std::initializer_list<_Tp>)
/usr/include/c++/4.6/bits/range_access.h:97:5: note: template<class _Tp, long unsigned int _Nm> _Tp* std::end(_Tp (&)[_Nm])
/usr/include/c++/4.6/bits/range_access.h:78:5: note: template<class _Container> decltype (__cont.end()) std::end(const _Container&)
/usr/include/c++/4.6/bits/range_access.h:68:5: note: template<class _Container> decltype (__cont.end()) std::end(_Container&)

If I replace the loop by the line: for (auto it=begin(num);it!=end(num);++it) std::cout<<*it<<std::endl; It works fine.

What is my mistake?

It's bitboard. for example, 49 is {0,4,5} (since 49==(1<<0)+(1<<4)+(1<<5) ) — asaelr, Jun 27 '12 at 21:30
@chris : `bitscan` is the iterator type, `int` is the result of dereferencing that iterator. — ildjarn, Jun 27 '12 at 21:30
@ildjarn, My bad. I thought the `begin` and `end` were taking `bitscan` arguments for some reason. — chris, Jun 27 '12 at 21:31
@chris : They need to in order to fix the OP's problem, so you're not that far off. :-] — ildjarn, Jun 27 '12 at 21:32

score 9 · Accepted Answer · answered Jun 27 '12 at 21:30

9

In the range-based for statement, the begin and end nonmember functions are only looked up via argument-dependent lookup^(*) (C++11 §6.5.4/1). This means that they will only be found in a namespace associated with the argument type (C++11 §3.4.2/2).

A fundamental type, like uint64_t, has no associated namespaces, so no begin or end will ever be found for it. You'll need to create a class type to wrap the uint64_t so that nonmember begin and end functions may be found via argument-dependent lookup.

(*) Note: There are actually three ways that begin and end may be found, but this is the only way that applies to your scenario. There are also special cases for arrays and for classes with member functions named begin and end.

answered Jun 27 '12 at 21:30

James McNellis

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So I have to wrap it in class? Is there any simple solution? – asaelr Jun 27 '12 at 21:32
2

@asaelr That's the simple solution. Also, that's the *non-evil* solution. – R. Martinho Fernandes Jun 27 '12 at 21:32
@asaelr : `bitscan` already does that, so why would that be a problem? – ildjarn Jun 27 '12 at 21:33
@ildjarn bitscan doesn't know about bitwise operation, which I really need. maybe the simple solution is `for (int i:bitscan(num))` – asaelr Jun 27 '12 at 21:35
1

@asaelr : And change `begin` and `end` to take `bitscan`, yes. – ildjarn Jun 27 '12 at 21:36
@ildjarn or (maybe) even better - I can make them member functions. – asaelr Jun 27 '12 at 21:44
Do you know why the Standard does not prescribe unqualified lookup followed by ADL for non-array/non-class types? That would also solve the OP's question. – TemplateRex May 12 '13 at 14:09

Can't write range based for with non-member begin and end functions

1 Answers1