Please explain the colon in my structure definition?

Question

Possible Duplicate:
What does a colon following a C++ constructor name do?

I'm reading a book about CUDA & I'm having trouble reading this C++ syntax. I'm not sure what to search for so that's why I'm posting here.

struct cuComplex {
    float   r;
    float   i;
    cuComplex( float a, float b ) : r(a) , i(b)  {}
}

What does the cuComplex statement do? Specifically:

cuComplex( float a, float b ) : r(a) , i(b)  {}

what is this called so I can learn about it?

Answer 1

: r(a) , i(b) in cuComplex ctor construct memory at allocation with value between parentheses.

struct cuComplex {
    const float   r;
    const float   i;
    cuComplex( float a, float b ) : r(a) , i(b)  {} // ok 
}

struct cuComplex {
    const float   r;
    const float   i;
    cuComplex( float a, float b ) {
        r = a;
        i = b;
    } // fail because once allocated, const memory can't be modified
}

Answer 2

That is C++, not C, as C structs cannot contain functions in that manner (they could contain a function pointer, but that is irrelevant to the question). That is a constructor for the type "cuComplex" that takes two floats. It initializes the two member variables 'r' and 'r' with the passed in values.

EDIT per comment: The r(a) and i(b) parts are initializing the member variables with the values of the parameters to the constructor.

Answer 3

This is C++ syntax.

cuComplex( float a, float b )

is the constructor defined for this struct.

: r(a) , i(b)

is called member initialization. Here the local members r and i are set to the parameters a and b passed to the constructor.

The rest is an empty function implementation.