no match for operator*

Question

I\'m reading Effective C++ (Scott Meyers), and getting the error \" no match for operator* \" when trying to compile following code from this book:

rational.h

Answer 1

You haven't declared the operator* in your header file, so it isn't visible in main.cpp.

Answer 2

You forgot to tell your users about the existence of your operator:

rational.h

...
const rational operator*(const rational &lhs, 
                         const rational &rhs);
...

Generally, in C as well as in C++, we talk about "definitions" and "declaration". Declarations are annotations to make something visible to someone else, but in itself they do nothing. Definitions are the entititiess that actually do something:

int foo();              // <- we call it "declaration"
int foo() { return 0; } // <- we call it foo's "definition"

In your code, there is no declaration of operator* visible in main.cpp, so you need to provide one somewhere (ideally in your rational's header).

As a style advice: In almost all cases, if a constructor takes builtin types, you want to make it explicit:

explicit rational (int, int);

This prevents sometimes subtle bugs because (in your case) rationals might be unintentionally created (see Automatic Conversions).