but rather the default ctor of the template type. complex(const T& re = T(), const T& im = T());
Do I understand correctly that the default constructor does not set real and imag parts to zero? Well, it's appears to be more subtle than we might want it to be. ISO/IEC 14882:2011 specifies the T() form for the generalized complex template. The language specification goes on, however, to specify that only float, double and long double are valid template parameters for complex. And the subsequent template specializations for the built-in floating-point numbers have default values of 0.0F, 0.0, 0.0L.
For example, as in my other post, the specified constructor for float is:
complex(float re = 0.0F, float im = 0.0F);
I also believe that the value of, say, the default constructor of float is zero. In particular
void do_something() { // F1 is initialized to zero. float f1 = float(); ... }
So ISO/IEC 14882:2011 ensures that all valid uses of complex are default initialized to zero. Without judgement, this is the spec.
What a potential Boost.Complex should do for user-defined types that may potentially default construct to non-zero is an open issue.
In my opinion, any sensible number library (aiming at extending real/integer numbers) should provide a default constructor that initialize the value of the number to 0. It is the behavior that the end-user will most probably expect. There are three options: (a) Stick to T() and document that the type T should have a default constructor building a zero-valued T. (b) Change to T(0.) and not respect the standard. This does also imply that T must have a constructor taking a double as an argument. (c) Do not initialize the number when the default constructor is called. This would allow the allocation of an array of complex<T> without having to actually copy numbers. A zero-valued complex number could still be built using one of the other constructors. I would prefer (a) in order to stick to the standard or (c) to allow optimized allocations. For what it's worth, boost::rational does not rely on the default constructor of its template type and explicitly calls the constructor of the numerator with the value 0 in its default constructor. By the way, the constructors of boost::rational do not use constant references for their arguments. There might be some useless costly copies when the type used is not POD. Regards, Matthieu -- Matthieu Schaller