operator<< overload question

[Ian Lance Taylor]

Allen Wayne Best <gcc@atoka-software.com> writes:

> good idea; here is the complete file:

Well, this example doesn't compile because you commented out the
declaration of the relevant operator>>:

> //		friend ostream& operator<<( ostream& o , const Sensor& s ) ;

When I uncomment that line, the file compiles.

When I try to link it, I get

foo1.o(.text+0x6b): undefined reference to `Measure::operator<<(std::ostream&, Measure::Sensor const&)'

That's because of this line:

> ostream& operator<<( ostream& o , const Sensor& s) 

That defines operator<<, not Measure::operator<<.  Your friend
declaration was inside namespace Measure.

When I change the function definition to:

ostream& Measure::operator<<( ostream& o , const Sensor& s) 

then the file compiles and links.


Alternatively, if I add these lines at the top:

namespace Measure { class Sensor; }
extern ostream& operator<<( ostream& o , const Measure::Sensor& s );

and then change the friend declaration to

		friend ostream& ::operator<<( ostream& o , const Sensor& s ) ;

then the file compiles and links.

Ian