Forgive what might seem to some to be a very simple question, but I have this use case in mind:
struct fraction { fraction( size_t num, size_t denom ) : numerator( num ), denominator( denom ) {}; size_t numerator; size_t denominator; }; What I would like to do is use statements like:
fraction f(3,5); ... double v = f; to have v now hold the value represented by my fraction. How would I do this in C++?
One way to do this is to define a conversion operator:
struct fraction { size_t numerator; size_t denominator; operator float() const { return ((float)numerator)/denominator; } }; Most people will prefer not to define an implicit conversion operator as a matter of style. This is because conversion operators tend to act "behind the scenes" and it can be difficult to tell which conversions are being used.
struct fraction { size_t numerator; size_t denominator; float as_float() const { return ((float)numerator)/denominator; } }; In this version, you would call the as_float method to get the same result.
Assignment operators and conversion constructors are for initializing objects of your class from objects of other classes. You instead need a way to initialize an object of some other type with an object of your class. That's what a conversion operator is for:
struct fraction { //other members here... operator double() const { return (double)numerator / denominator;} //other members here... }; You can use the operator double to convert:
struct fraction { operator double() const { //remember to check for denominator to 0 return (double)numerator/denominator; } }; operator= has nothing to do with it, rather you want to add to your struct a public operator double something like:
operator double() { return ((double) numerator))/denominator; } 
With that much of code it will be compiler error as the compiler doesn't how to convert struct fraction into a double. If you want to provide the conversion then you have to define the operator double() which will be used by the compiler for this conversion.
