Compiler Error C2088

'operator' : illegal for class-key

The specified operator was not defined for the class, structure, or union.

This error can be eliminated by defining a conversion to convert the operands to the type for which the operator is defined. Alternatively, an overloaded conversion operator can be defined.

An example of this error and two solutions are:

class A
{
public:
   int i;
} a;
int i = 1 + a;       // this line causes the error
class B
{
public:
   operator int() { return i; }
   int i;
} b;
int j = 1 + b;       // OK, uses conversion operator
class C
{
public:
   int operator+( int j ) { return (j+i); }  // solution
   int i;
} c;
int k = c + 1;       // OK, uses overloaded operator+
+ c;                 // still an error, no conversion
                     // operator defined for class C

Note that the last line requires that a conversion operator be defined since overloaded operators cannot be defined for built-in types.