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Apr 25th, 2005

Trying to overload + for class to add to string 'is illegal'

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Hi,

I'm trying to write code such that my class can be used like so:
To accomplish this,
I'm trying to overload the + operator of my class to return a string. I am able to do this, but when I try and do the operation above, I get:

Error: The operation "std::basic_string<char, std::char_traits<char>, std::allocator<char>> + Dan" is illegal.

What does this mean and how do I make it legal?

class Dan
{
public:
string first;
string last;

const string operator + (Dan & a)
{
return a.toString();
}

string toString()
{
return first + ", " + last;
}

....

Dan temp;
string myString = "Test" + temp;

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winbatch

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Apr 25th, 2005

Re: Trying to overload + for class to add to string 'is illegal'

C Syntax (Toggle Plain Text)
string operator+ ( const string& s, const Dan& d )
{
  return s + d.toString();
}
string operator+ ( const string& s, const Dan& d )
{
  return s + d.toString();
}

Since Dan doesn't have a suitable conversion to std::string, you also would need to provide another operator+ with the commutative operation:

C Syntax (Toggle Plain Text)
string operator+ ( const Dan& d, const string& s )
{
  return s + d.toString();
}
string operator+ ( const Dan& d, const string& s )
{
  return s + d.toString();
}

toString should also be const since it doesn't modify the state of your object:

C Syntax (Toggle Plain Text)
string toString() const
{
  return first + ", " + last; 
}
string toString() const
{
  return first + ", " + last; 
}

Narue

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Apr 25th, 2005

Re: Trying to overload + for class to add to string 'is illegal'

Narue,

I tried what you posted, but it complains:

Test.cpp", line 13: Error: Illegal number of arguments for Dan::operator+(const std::basic_string<char, std::char_traits<char>, std::allocator<char>>&, const Dan&).

winbatch

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Apr 25th, 2005

Re: Trying to overload + for class to add to string 'is illegal'

>I tried what you posted
No, you didn't. You tried some oddball variation of what I posted after filtering it through your n00b brain. Neither my code nor my comments suggested that operator+ should be a member function. Since toString is a public member function of Dan, neither of the overloads for operator+ should even be friends of Dan. Like this:

C Syntax (Toggle Plain Text)
class Dan {
  // Data members
public:
  string toString() const;
  // No operator+ here
};
 
string operator+ ( const string& s, const Dan& d )
{
  return s + d.toString();
}
 
string operator+ ( const Dan& d, const string& s )
{
  return s + d.toString();
}
class Dan {
  // Data members
public:
  string toString() const;
  // No operator+ here
};

string operator+ ( const string& s, const Dan& d )
{
  return s + d.toString();
}

string operator+ ( const Dan& d, const string& s )
{
  return s + d.toString();
}

>but it complains
I'm not surprised. When implemented as a member function, operator+ expects only one argument. When implemented as a non-member function, operator+ expects two arguments. The two are not interchangeable.

Narue

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Apr 25th, 2005

Re: Trying to overload + for class to add to string 'is illegal'

I appreciate the help, (and the insults not so much). In either case, it works now. By the way, I only seemed to need one of the functions, not both. Why do you think I need both?

Also, I changed it to return the first + ", " + last directly since first and last are not const so I couldn't return them using the toString.

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Apr 25th, 2005

Re: Trying to overload + for class to add to string 'is illegal'

>and the insults not so much
I can live with that.

>Why do you think I need both?
Users of your class will rightfully expect Dan + string to work just as well as string + Dan. Of course, in hindsight I should have written:

C Syntax (Toggle Plain Text)
string operator+ ( const Dan& d, const string& s )
{
  return d.toString() + s;
}
string operator+ ( const Dan& d, const string& s )
{
  return d.toString() + s;
}

Instead of pasting the body of the other operator+. This way users will get the expected behavior for string concatenation.

>I changed it to return the first + ", " + last directly
Not a good idea. Eventually you'll learn that public data members are a bad thing and make them private, then your code will break. You'll either have to use a public interface for the operators, or make them friends of your class. Both of those entail work on your part, so it's better to do it right from the start.

>since first and last are not const so I couldn't return them using the toString.
That shouldn't matter. A non-const object can be converted to a const object implicitly. The inverse is not true, but that's not the case here. Show me the code that's causing the error, preferably something I can compile directly so I don't accidentally fix something by writing framework code.

Narue

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