Please explain the colon in my structure definition?

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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?

If you know C++ that look the same than constructor. I guess it have the same semantic — mathk, Jan 26 '11 at 08:12
I don't know CUDA, so I don't know if this is CUDA syntax, but this is definitely valid C++ syntax anyway :) — bdonlan, Jan 26 '11 at 08:15
I didn't know what CUDA was before I saw this question (had heard the term, never looked into it), so I answered it in terms of pure C as the question was originally tagged. — Ed S., Jan 26 '11 at 08:16

score 6 · Answer 1 · answered Jan 26 '11 at 08:11

6

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.

answered Jan 26 '11 at 08:11

BjoernD

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ah i see, so it's the same as doing r = a – ninjaneer Jan 26 '11 at 08:22
1

No, it is the same as doing r(a), but in this case the copy constructor will do the same thing as the assignment operator. – Ed S. Jan 26 '11 at 08:32

Ed S. · Answer 2 · 2011-01-26T08:33:53.847

1

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.

edited Jan 26 '11 at 08:33

answered Jan 26 '11 at 08:10

Ed S.

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i get the first part, but I still don't get the r(a), i(b). can you explain that a little more? Thanks – ninjaneer Jan 26 '11 at 08:12
Do those exist in C? I was under the impression that only C++ has those. – Joachim Sauer Jan 26 '11 at 08:13
No, they don't exist in C. I missed the "C" tag for about 30 seconds before editing my response :) – Ed S. Jan 26 '11 at 08:14

Errata · Answer 3 · 2012-02-10T20:11:24.863

: 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
}

Please explain the colon in my structure definition?

3 Answers3