ofstream ambiguity problem

[rince]

Hi! I have a complex class like

class Complex
{
    double       re;
    double       im;
 
public:
    Complex();
    Complex( const double &r, const double &i );
    Complex( const double &d );
    friend ofstream &operator<<( ofstream &stream, const Complex &c );
    [other stuff]
};
 

If I now write

ofstream ofs( "file.txt" );
ofs << 1;
 

this generates C2666 in MSVC - overloads have similar conversions. Looks like ofs << 1 requires an implicit conversion and the compiler is unsure whether to use basic_ostream<_Elem,_Traits>::operator <<(int) or to convert int to Complex and use operator <<(std::ofstream &,const Complex &).

I could fix the problem in one of the following ways:
1. Remove operator<< from Complex.
2. Remove constructor Complex( const double &d ) from Complex.
3. Write ofs.basic_ostream<char, char_traits<char>>::operator<<( 1 ) instead of ofs << 1.

But nothing of the above seems acceptable. I can't believe things are really this screwed up, it doesn't seem like I'm doing anything wrong. Any suggestions?

[weaknessforcats]

Using Visual Sutdio.NET 2008 I get:

error C2666: 'operator <<' : 17 overloads have similar conversions

The real culprit here is the symbolic constant 1. You are no supposed to have constants like this in a position where the compiler has to play multiple choice.

If this is truly an int (the default), then assign this value to an int and << the int.

I would not typecast. I don't believe in it with C++ unless you are calling a relic C function. If you aren't then I believe your design is flawed.

[weaknessforcats]

I got to thinking about your problem and thought that maybe if your friend was:

  friend ostream &operator<<( ostream &stream, const Complex &c );
 

it would help since an ofstream IS-A ostream. In fact, every inserter I have seen has used and ostream& argument.

I tried this using your code and got a successful compile.

[rince]

Oh, thanks, it really compiles ok with ostream. Still it's strange that it doesn't work with ofsream, why would there be any conversions? Such stuff really pisses me off.

[weaknessforcats]

As long as you use defined types you will be OK. But if you use a symbolic constant you open yourself up to ambiguity.

That 1, for example:

Does the compiler:
1) use it as an int with operator<<(ostream&, int&)?

or,

2) does it use it as a char with operator<<(ostream&, char&)?

or,

3) does it convert it to a double and use operator<<(ostream&, double&)?

etc..
etc...

Don't give the compiler that latitude. If you have an int. Use an int variable.