Question

It’s considered a bad idea/bad design, have a class with a constructor accepting a reference, like the following?

class Compiler
{
public:
  Compiler( const std::string& fileName );
  ~Compiler();
//etc
private: 
  const std::string& m_CurrentFileName;
};

or should I use values?
I actually do care about performance.

Answer 1

If you used a value parameter in this case, you would have a reference in the class to a temporary, which would become invalid at some point in the future.

The bad idea here is probably storing a reference as a member in the class. It is almost always simpler and more correct to store a value. And in that case, passing the constructor a const reference is the right thing to do.

And as for performance, you should only care about this where it matters, which you can only find out by profiling your code. You should always write your code firstly for correctness, secondly for clarity and lastly for performance.