AppleClang：运算符重载编译错误

Question

I implemented operator overloading as follows (see the methods bool operator== and bool operator< ):

#include "../data_types.h"

class OffsetValuePair {
private:
    unsigned_value offset;
    unsigned_value value;

public:
    OffsetValuePair(unsigned_value address, unsigned_value value) {
        this->offset = address;
        this->value = value;
    }

    bool operator==(const OffsetValuePair offsetValuePair) {
        return this->getOffset() == offsetValuePair.offset;
    }

    bool operator<(const OffsetValuePair offset_value_pair) {
        return this->getValue() < offset_value_pair.value;
    }

    unsigned_value getOffset() {
        return offset;
    }

    unsigned_value getValue() {
        return value;
    }
};

I'm using the operator for finding the lower bound using will use my overloaded operator for less than ( < ):

const auto lower_bound_offset_value_pair = OffsetValuePair(0, 1234);
const auto lower_bound = std::lower_bound(pointer_map_sorted_by_value_.begin(),
                                                  pointer_map_sorted_by_value_.end(),
                                                  lower_bound_offset_value_pair);

When I compile the code, I get the following error with AppleClang :

/Library/Developer/CommandLineTools/usr/include/c++/v1/algorithm:719:71: error: invalid operands to binary expression ('const OffsetValuePair' and 'const OffsetValuePair')
    bool operator()(const _T1& __x, const _T1& __y) const {return __x < __y;}
                                                                  ~~~ ^ ~~~
/Library/Developer/CommandLineTools/usr/include/c++/v1/algorithm:4285:13: note: in instantiation of member function 'std::__1::__less<OffsetValuePair, OffsetValuePair>::operator()' requested here
        if (__comp(*__m, __value_))
            ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/algorithm:4307:12: note: in instantiation of function template specialization 'std::__1::__lower_bound<std::__1::__less<OffsetValuePair, OffsetValuePair> &, std::__1::__wrap_iter<OffsetValuePair *>, OffsetValuePair>' requested here
    return __lower_bound<_Comp_ref>(__first, __last, __value_, __comp);
           ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/algorithm:4316:19: note: in instantiation of function template specialization 'std::__1::lower_bound<std::__1::__wrap_iter<OffsetValuePair *>, OffsetValuePair, std::__1::__less<OffsetValuePair, OffsetValuePair> >' requested here
    return _VSTD::lower_bound(__first, __last, __value_,
                  ^
/Users/bully/Desktop/PointerSearcher/src/pointer_search_objects/PointerSearcher.h:660:33: note: in instantiation of function template specialization 'std::__1::lower_bound<std::__1::__wrap_iter<OffsetValuePair *>, OffsetValuePair>' requested here
                const auto lower_bound = std::lower_bound(pointer_map_sorted_by_value_.begin(),
                                              ^
/Users/bully/Desktop/PointerSearcher/src/pointer_search_objects/OffsetValuePair.h:21:7: note: candidate function not viable: 'this' argument has type 'const OffsetValuePair', but method is not marked const
        bool operator<(const OffsetValuePair offset_value_pair) {
             ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/utility:572:1: note: candidate template ignored: could not match 'pair<type-parameter-0-0, type-parameter-0-1>' against 'const OffsetValuePair'
operator< (const pair<_T1,_T2>& __x, const pair<_T1,_T2>& __y)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/iterator:702:1: note: candidate template ignored: could not match 'reverse_iterator<type-parameter-0-0>' against 'const OffsetValuePair'
operator<(const reverse_iterator<_Iter1>& __x, const reverse_iterator<_Iter2>& __y)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/iterator:1143:1: note: candidate template ignored: could not match 'move_iterator<type-parameter-0-0>' against 'const OffsetValuePair'
operator<(const move_iterator<_Iter1>& __x, const move_iterator<_Iter2>& __y)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/iterator:1512:1: note: candidate template ignored: could not match '__wrap_iter<type-parameter-0-0>' against 'const OffsetValuePair'
operator<(const __wrap_iter<_Iter1>& __x, const __wrap_iter<_Iter2>& __y) _NOEXCEPT_DEBUG
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:1187:1: note: candidate template ignored: could not match 'tuple<type-parameter-0-0...>' against 'const OffsetValuePair'
operator<(const tuple<_Tp...>& __x, const tuple<_Up...>& __y)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:2920:1: note: candidate template ignored: could not match 'unique_ptr<type-parameter-0-0, type-parameter-0-1>' against 'const OffsetValuePair'
operator< (const unique_ptr<_T1, _D1>& __x, const unique_ptr<_T2, _D2>& __y)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:2978:1: note: candidate template ignored: could not match 'unique_ptr<type-parameter-0-0, type-parameter-0-1>' against 'const OffsetValuePair'
operator<(const unique_ptr<_T1, _D1>& __x, nullptr_t)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:2987:1: note: candidate template ignored: could not match 'unique_ptr<type-parameter-0-0, type-parameter-0-1>' against 'const OffsetValuePair'
operator<(nullptr_t, const unique_ptr<_T1, _D1>& __x)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:4758:1: note: candidate template ignored: could not match 'shared_ptr<type-parameter-0-0>' against 'const OffsetValuePair'
operator<(const shared_ptr<_Tp>& __x, const shared_ptr<_Up>& __y) _NOEXCEPT
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:4823:1: note: candidate template ignored: could not match 'shared_ptr<type-parameter-0-0>' against 'const OffsetValuePair'
operator<(const shared_ptr<_Tp>& __x, nullptr_t) _NOEXCEPT
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:4831:1: note: candidate template ignored: could not match 'shared_ptr<type-parameter-0-0>' against 'const OffsetValuePair'
operator<(nullptr_t, const shared_ptr<_Tp>& __x) _NOEXCEPT

Using gcc and MSVC the code compiles fine, I'm only getting this error on Mac OS X with the standard platform compiler ( AppleClang ).

I do not understand why AppleClang does not accept the code. What is wrong with it?

I cannot declare the methods const since they use a this referenced method call. If I convert the this object into a 2nd method argument I get another error telling me that the method signature is wrong: error: 'bool OffsetValuePair::operator<(OffsetValuePair, OffsetValuePair)' must have exactly one argument

Answer 1

Nevermind, I just found the solution:

bool operator==(const OffsetValuePair offsetValuePair) const {
    return this->offset == offsetValuePair.offset;
}

bool operator<(const OffsetValuePair offset_value_pair) const {
    return this->value < offset_value_pair.value;
}

The changes were to define both methods as const as suggested by the error message. Furthermore, when using the this reference method calls are not allowed due to the const method definition so I replaced the getter with the direct member access.