C++ - 排序算法看不到用户定义类型的重载"<"运算符。
C++ - sort algorithm doesnt see my overloaded "<" operator for user-defined type.
好的，我有一个名为fraction的用户定义类型，它表示一个具有分子和分母的普通分数。这是代码：
class Fraction
{
private:
int numerator;
int denominator;
public:
Fraction(int numer,int denom)
:numerator(numer),denominator(denom){}
int get_denom(){return denominator;}
int get_numer(){return numerator;}
};
正如您所看到的，它是在一个头文件中完全实现的。
所以我想用排序算法对分数向量进行排序。这是重载的"<"运算符的代码（注意：它和Fraction类放在同一个文件中，但在类之外）：
bool operator<(Fraction& first,Fraction& second)
{
if(first.get_denom() == second.get_denom())
{
return first.get_numer()<second.get_numer()?true:false;
}
int first_num=first.get_denom();
int second_num=second.get_denom();
int lcm=(first_num*second_num)/gcd(first_num,second_num);
int first_new_numerator=(lcm/first_num)*first.get_numer();
int second_new_numerator=(lcm/second_num)*second.get_numer();
return first_new_numerator<second_new_numerator?true:false;
}
它所做的是检查分母是否相等，如果它们相等，那么它测试分子的值。如果它们不相等，则使用最小公倍数和gcd来均衡它们。
当我想在main中使用排序算法（编译时错误）时，真正的问题就开始了
Fraction parse_fraction(string& input)
{
stringstream fraction_string(input);
int numer;
fraction_string>>numer;
char seperator;
fraction_string>>seperator;
int denom;
fraction_string>>denom;
return Fraction(numer,denom);
}
int main()
{
vector<Fraction> fractions;
string input;
while(cin>>input)
{
if(input=="|") break;
fractions.push_back(parse_fraction(input));
}
sort(fractions.begin(),fractions.end);
for(int i=0;i<fractions.size();i++)
{
cout<<fractions[i];
}
return 0;
}
我得到的错误是（注意：我已经为分数适当地重载了"<<"运算符，但不想在这里造成混乱）：
/usr/include/c++/4.6/bits/stl_algo.h||In function ‘_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, const _Tp&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Fraction*, std::vector<Fraction> >, _Tp = Fraction]’:|
/usr/include/c++/4.6/bits/stl_algo.h:2253|70|instantiated from ‘_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Fraction*, std::vector<Fraction> >]’|
/usr/include/c++/4.6/bits/stl_algo.h:2284|54|instantiated from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Fraction*, std::vector<Fraction> >, _Size = long int]’|
/usr/include/c++/4.6/bits/stl_algo.h:5330|4|instantiated from ‘void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<Fraction*, std::vector<Fraction> >]’|
/home/vanio/Desktop/workspace/C++/Ordner/main.cpp:35|43|instantiated from here|
/usr/include/c++/4.6/bits/stl_algo.h|2212|error: no match for ‘operator<’ in ‘__first.__gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* [with _Iterator = Fraction*, _Container = std::vector<Fraction>, __gnu_cxx::__normal_iterator<_Iterator, _Container>::reference = Fraction&]() < __pivot’|
/usr/include/c++/4.6/bits/stl_algo.h|2212|note: candidates are:|
/usr/include/c++/4.6/bits/stl_pair.h|207|note: template<class _T1, class _T2> bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)|
/usr/include/c++/4.6/bits/stl_iterator.h|291|note: template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)|
/usr/include/c++/4.6/bits/stl_iterator.h|341|note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_IteratorL>&, const std::reverse_iterator<_IteratorR>&)|
/usr/include/c++/4.6/bits/basic_string.h|2510|note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const std::basic_string<_CharT, _Traits, _Alloc>&)|
/usr/include/c++/4.6/bits/basic_string.h|2522|note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)|
/usr/include/c++/4.6/bits/basic_string.h|2534|note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const std::basic_string<_CharT, _Traits, _Alloc>&)|
/usr/include/c++/4.6/bits/stl_vector.h|1290|note: template<class _Tp, class _Alloc> bool std::operator<(const std::vector<_Tp, _Alloc>&, const std::vector<_Tp, _Alloc>&)|
/home/vanio/Desktop/workspace/C++/Ordner/Fraction.h|39|note: bool operator<(Fraction&, Fraction&)|
/home/vanio/Desktop/workspace/C++/Ordner/Fraction.h|39|note: no known conversion for argument 2 from ‘const Fraction’ to ‘Fraction&’|
/usr/include/c++/4.6/bits/stl_algo.h|2215|error: no match for ‘operator<’ in ‘__pivot < __last.__gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* [with _Iterator = Fraction*, _Container = std::vector<Fraction>, __gnu_cxx::__normal_iterator<_Iterator, _Container>::reference = Fraction&]()’|
/usr/include/c++/4.6/bits/stl_algo.h|2215|note: candidates are:|
/usr/include/c++/4.6/bits/stl_pair.h|207|note: template<class _T1, class _T2> bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)|
/usr/include/c++/4.6/bits/stl_iterator.h|291|note: template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)|
/usr/include/c++/4.6/bits/stl_iterator.h|341|note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_IteratorL>&, const std::reverse_iterator<_IteratorR>&)|
/usr/include/c++/4.6/bits/basic_string.h|2510|note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const std::basic_string<_CharT, _Traits, _Alloc>&)|
/usr/include/c++/4.6/bits/basic_string.h|2522|note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)|
/usr/include/c++/4.6/bits/basic_string.h|2534|note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const std::basic_string<_CharT, _Traits, _Alloc>&)|
/usr/include/c++/4.6/bits/stl_vector.h|1290|note: template<class _Tp, class _Alloc> bool std::operator<(const std::vector<_Tp, _Alloc>&, const std::vector<_Tp, _Alloc>&)|
/home/vanio/Desktop/workspace/C++/Ordner/Fraction.h|39|note: bool operator<(Fraction&, Fraction&)|
/home/vanio/Desktop/workspace/C++/Ordner/Fraction.h|39|note: no known conversion for argument 1 from ‘const Fraction’ to ‘Fraction&’|
||=== Build finished: 22 errors, 0 warnings ===|
bool operator<(Fraction& first,Fraction& second)
问题是，operator<的重载只接受非常数参数，但算法试图将对Fraction的常量引用与容器中的元素进行比较。
由于operator<不修改被比较对象的内容，它应该通过常量引用来获取它们
bool operator<(Fraction const & first,Fraction const & second)
考虑以下代码：
Fraction a, b;
bool res = (a < b);
第二条线路本质上是对的呼叫
bool res = a.operator<(b);
- 正如上面所建议的那样，从长远来看，保持常量正确会对你很有帮助
- 定义一个成员
bool operator<(const Fraction&) const以使排序实现愉快，或者只使用上面的第一个建议-使用bool operator<(const Fraction& a, const Fraction& b)保持常量正确
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