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Full Discussion: Ambiguity in operator []
Top Forums Programming Ambiguity in operator [] Post 302530190 by 14341 on Monday 13th of June 2011 09:39:30 AM
Old 06-13-2011
Ambiguity in operator []
Hi All,

When i try to compile the following code for 64-bit it works, whereas for 32-bit version, Compiler barfs:

Code:
#include <iostream>
#include <string>

class String {
  public:
    String() { }
    String(const char* src) : myStr(src) { }
    String(const std::string& src) : myStr(src) { }
    String(const String& src) : myStr(src.myStr) { }

    operator const char*() const         { return myStr.c_str(); }
    char& operator[](size_t index)       { return myStr[index]; }
    char operator[] (size_t index) const { return myStr[index]; }

    size_t length() const { return myStr.length(); }
  private:
    std::string myStr;
};

int main() {
    String s("abcd");

    char c = s[1];

    std::cout << c << std::endl;

    return 0;
}

Code:
$ g++ -m32 -Wall -g ambiguous.cc
ambiguous.cc: In function ‘int main()’:
ambiguous.cc:23: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
ambiguous.cc:12: note: candidate 1: char& String::operator[](size_t)
ambiguous.cc:23: note: candidate 2: operator[](const char*, int) <built-in>

$ g++ -Wall -g ambiguous.cc
$./a.out
b

1) I cannot modify all the instances like, c = s[1]. There are hundreds of similar things in code.
2) operator const char* is required.

Could someone suggest any work-around, considering the above 2 points.

Thanks,
14341

---------- Post updated at 08:39 AM ---------- Previous update was at 08:38 AM ----------

Hi All,

Overloading the operator[], for all integer types (unsigned/signed) (int; long; long long) solved the problem.

Cheers,
14341
 

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LEARN ABOUT DEBIAN

__gnu_cxx::_pointer_adapter

__gnu_cxx::_Pointer_adapter< _Storage_policy >(3cxx)					      __gnu_cxx::_Pointer_adapter< _Storage_policy >(3cxx)

NAME

       __gnu_cxx::_Pointer_adapter< _Storage_policy > -

SYNOPSIS

       Inherits _Storage_policy.

   Public Types
       typedef std::ptrdiff_t difference_type
       typedef
	   _Storage_policy::element_type element_type"
       typedef
	   std::random_access_iterator_tag iterator_category"
       typedef _Pointer_adapter pointer
       typedef _Reference_type
	   < element_type >::reference reference"
       typedef _Unqualified_type
	   < element_type >::type value_type"

   Public Member Functions
       _Pointer_adapter (element_type *__arg=0)
       _Pointer_adapter (const _Pointer_adapter &__arg)
       template<typename _Up > _Pointer_adapter (_Up *__arg)
       template<typename _Up > _Pointer_adapter (const _Pointer_adapter< _Up > &__arg)
       operator __unspecified_bool_type () const
       bool operator! () const
       reference operator* () const
       _Pointer_adapter & operator++ ()
       _Pointer_adapter operator++ (int)
       _Pointer_adapter & operator+= (short __offset)
       _Pointer_adapter & operator+= (unsigned short __offset)
       _Pointer_adapter & operator+= (int __offset)
       _Pointer_adapter & operator+= (unsigned int __offset)
       _Pointer_adapter & operator+= (long __offset)
       _Pointer_adapter & operator+= (unsigned long __offset)
       template<typename _Up > std::ptrdiff_t operator- (const _Pointer_adapter< _Up > &__rhs) const
       _Pointer_adapter & operator-- ()
       _Pointer_adapter operator-- (int)
       _Pointer_adapter & operator-= (short __offset)
       _Pointer_adapter & operator-= (unsigned short __offset)
       _Pointer_adapter & operator-= (int __offset)
       _Pointer_adapter & operator-= (unsigned int __offset)
       _Pointer_adapter & operator-= (long __offset)
       _Pointer_adapter & operator-= (unsigned long __offset)
       element_type * operator-> () const
       _Pointer_adapter & operator= (const _Pointer_adapter &__arg)
       template<typename _Up > _Pointer_adapter & operator= (const _Pointer_adapter< _Up > &__arg)
       template<typename _Up > _Pointer_adapter & operator= (_Up *__arg)
       reference operator[] (std::ptrdiff_t __index) const

   Friends
       _Pointer_adapter operator+ (const _Pointer_adapter &__lhs, short __offset)
       _Pointer_adapter operator+ (short __offset, const _Pointer_adapter &__rhs)
       _Pointer_adapter operator+ (const _Pointer_adapter &__lhs, unsigned short __offset)
       _Pointer_adapter operator+ (unsigned short __offset, const _Pointer_adapter &__rhs)
       _Pointer_adapter operator+ (const _Pointer_adapter &__lhs, int __offset)
       _Pointer_adapter operator+ (int __offset, const _Pointer_adapter &__rhs)
       _Pointer_adapter operator+ (const _Pointer_adapter &__lhs, unsigned int __offset)
       _Pointer_adapter operator+ (unsigned int __offset, const _Pointer_adapter &__rhs)
       _Pointer_adapter operator+ (const _Pointer_adapter &__lhs, long __offset)
       _Pointer_adapter operator+ (long __offset, const _Pointer_adapter &__rhs)
       _Pointer_adapter operator+ (unsigned long __offset, const _Pointer_adapter &__rhs)
       _Pointer_adapter operator+ (const _Pointer_adapter &__lhs, unsigned long __offset)
       std::ptrdiff_t operator- (const _Pointer_adapter &__lhs, element_type *__rhs)
       std::ptrdiff_t operator- (element_type *__lhs, const _Pointer_adapter &__rhs)
       template<typename _Up > std::ptrdiff_t operator- (const _Pointer_adapter &__lhs, _Up *__rhs)
       template<typename _Up > std::ptrdiff_t operator- (_Up *__lhs, const _Pointer_adapter &__rhs)
       _Pointer_adapter operator- (const _Pointer_adapter &__lhs, short __offset)
       _Pointer_adapter operator- (const _Pointer_adapter &__lhs, unsigned short __offset)
       _Pointer_adapter operator- (const _Pointer_adapter &__lhs, int __offset)
       _Pointer_adapter operator- (const _Pointer_adapter &__lhs, unsigned int __offset)
       _Pointer_adapter operator- (const _Pointer_adapter &__lhs, long __offset)
       _Pointer_adapter operator- (const _Pointer_adapter &__lhs, unsigned long __offset)

Detailed Description
   template<typename _Storage_policy>class __gnu_cxx::_Pointer_adapter< _Storage_policy >
       The following provides an 'alternative pointer' that works with the containers when specified as the pointer typedef of the allocator.

       The pointer type used with the containers doesn't have to be this class, but it must support the implicit conversions, pointer arithmetic,
       comparison operators, etc. that are supported by this class, and avoid raising compile-time ambiguities. Because creating a working pointer
       can be challenging, this pointer template was designed to wrapper an easier storage policy type, so that it becomes reusable for creating
       other pointer types.

       A key point of this class is also that it allows container writers to 'assume' Alocator::pointer is a typedef for a normal pointer. This
       class supports most of the conventions of a true pointer, and can, for instance handle implicit conversion to const and base class pointer
       types. The only impositions on container writers to support extended pointers are: 1) use the Allocator::pointer typedef appropriately for
       pointer types. 2) if you need pointer casting, use the __pointer_cast<> functions from ext/cast.h. This allows pointer cast operations to
       be overloaded is necessary by custom pointers.

       Note: The const qualifier works with this pointer adapter as follows:

       _Tp* == _Pointer_adapter<_Std_pointer_impl<_Tp> >; const _Tp* == _Pointer_adapter<_Std_pointer_impl<const _Tp> >; _Tp* const == const
       _Pointer_adapter<_Std_pointer_impl<_Tp> >; const _Tp* const == const _Pointer_adapter<_Std_pointer_impl<const _Tp> >;

       Definition at line 285 of file pointer.h.

Author
       Generated automatically by Doxygen for libstdc++ from the source code.

libstdc++							  Tue Nov 27 2012	      __gnu_cxx::_Pointer_adapter< _Storage_policy >(3cxx)
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