To overload operator +, I first implement that as friend function + normal function
class Cents
{
private:
int m_cents;
public:
Cents(int);
int getCents() const;
friend Cents operator+(const Cents&, const Cents&);
};
Cents::Cents(int cents)
: m_cents{cents}
{}
int Cents::getCents() const
{
return m_cents;
}
Cents operator+(const Cents &c1, const Cents &c)
{
return Cents(c1.getCents() + c.getCents());
}
Cents operator+(const Cents &c, int v)
{
return Cents(c.getCents()+v);
}
Cents operator+(int v, const Cents &c)
{
return Cents(v+c.getCents());
}
int main()
{
Cents c1{1};
Cents c2{2};
Cents c{c1+c2};
Cents c3{c+3};
Cents c4{4+c3};
std::cout << c.getCents() << '\n';
std::cout << c3.getCents() << '\n';
std::cout << c4.getCents() << '\n';
return 0;
}
>>>
3
6
10
It works well. But when I use a member function to replace the friend function, error raises
class Cents
{
private:
int m_cents;
public:
Cents(int);
int getCents() const;
Cents operator+(const Cents&);
};
Cents::Cents(int cents)
: m_cents{cents}
{}
int Cents::getCents() const
{
return m_cents;
}
Cents Cents::operator+(const Cents &c)
{
return Cents(m_cents + c.getCents());
}
Cents operator+(const Cents& c, int v)
{
return Cents(c.getCents()+v);
}
Cents operator+(int v, const Cents& c)
{
return Cents(v+c.getCents());
}
int main()
{
Cents c1{1};
Cents c2{2};
Cents c{c1+c2};
Cents c3{c+3};
Cents c4{4+c3};
std::cout << c.getCents() << '\n';
std::cout << c3.getCents() << '\n';
std::cout << c4.getCents() << '\n';
return 0;
}
>>>
error: invalid operands to binary expression ('int' and 'Cents')
Cents c4{4+c3};
If Cents c4{4+c3}; and std::cout << c4.getCents() << '\n'; are removed, the rest code works fine
int main()
{
Cents c1{1};
Cents c2{2};
Cents c{c1+c2};
Cents c3{c+3};
// Cents c4{4+c3};
std::cout << c.getCents() << '\n';
std::cout << c3.getCents() << '\n';
// std::cout << c4.getCents() << '\n';
return 0;
}
>>>
3
6
I guess it's because the compiler tries to resolve 4+c3 with the member function Cents operator+(const Cents&); and it can't use an int to call that function. But why can't the compiler use the normal function Cents operator+(int v, const Cents &c) to resolve 4+c3 as it did for Cents c3{c+3}; using Cents operator+(const Cents &c, int v)?
--- update ---
I am using Clion on macOS, but when I use an online compiler https://www.onlinegdb.com/online_c++_compiler the code (member function + normal function) compilers and runs successfully. But the online compiler also raises a warning
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Language
main.cpp
input
main.cpp:54:14: warning: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
main.cpp:38:7: note: candidate 1: Cents operator+(const Cents&, int)
main.cpp:33:7: note: candidate 2: Cents Cents::operator+(const Cents&)
3
6
10
My follow-up question, since the friend function and normal function doesn't need a specific object to call the function, thus, they won't have parameters ordering issue. Does this make us prefer friend function or normal function over a member function when we are overloading an operator in C++?
