What is a complex data type and an Imaginary data type in C++?

I know what imaginary and complex numbers are in the math world but what about in C++, what are data types of complex and imaginary. In addition, I开发者_运维百科 see data types such as _Imaginary and _Complex. What is the difference and what are complex and imaginary data types?

_Imaginary and _Complex are keywords in the C99 language standard used for defining imaginary and complex floating-point variable types; they are not part of the C++ language. They are not data types in and of themselves -- they modify the float, double and long double types. For example:

float _Imaginary x;      // imaginary 32-bit number
double _Complex y;       // complex 64-bit number
long double _Complex z;  // complex 80-bit number

_Imaginary values are mostly equivalent to regular real values, except when you add a real with an imaginary value, you get a _Complex value.

The header file <complex.h> defines the macros imaginary as _Imaginary and complex as _Complex, as well as I as either _Complex_I or _Imaginary_I (the imaginary unit). This is so that legacy C programs can use complex and imaginary as identifiers; new programs should use complex and imaginary instead of _Complex and _Imaginary. Note that identifiers beginning with an underscore followed by an uppercase letter are reserved by the implementation, so legacy code should never use _Complex or _Imaginary.

C++, on the other hand, does not use this, and instead has the templated types std::complex<float>, std::complex<double>, and std::complex<long double> for dealing with complex numbers. Those classes function very similarly to the C99 types but are non-interchangeable.

These are actually C datatypes, not C++ datatypes.

When creating new C standards, the committee avoids using new keywords. So, they use reserved identifiers, that begin with an underscore. Then, the standard may also provide a header file that defines a nicer macro (complex, instead of _Complex).

Since C++ is newer that C, it has included the complex datatype since it was first standardized, and doesn't need to resort to the underscored version.

not this?

and from another source:

_Imaginary
Revision as of 11:59, 3 December 2010 by PyBot (Talk | contribs)

Category

Reserved Words

Syntax

_Imaginary

Description

Reserved for future use.

    Warning: C++Builder does not implement _Imaginary but does treat it as a keyword when the -An compiler flag is set. Do not use _Imaginary as an identifier.

There is a header <complex> which defines a convenient complex number data type for you, all part of the standard library. Here is the documentation.

In C++, there is a header called <complex> which defines a template class double<>, which can be instantiated for float, double, and long double. This allows you to define complex numbers and do the usual arithmetic operations and some functions.

There is no imaginary class or type in standard C++, just complex numbers with a real part of 0.0. Any imaginary class would be some sort of vendor extension. (Personally, I don't see that an imaginary class would be of much use. It's not even closed under multiplication.)

Any name that begins with an underscore and is followed by a capital letter is specifically reserved for use by the implementation, and so there isn't anything standard C++ has to say about them. Any such type would be implementation-specific, and you haven't specified an implementation. Moreover, it's probably not something you should be using yourself.