Surprising struct initialization

[Juha Nieminen]

I suppose you can never know C++ fully. I would have never guessed
this actually compiles and works:

struct Point { int x, y; };

struct Line
{
Point endpoint[2];
int weight;
};

Line createLine(int sx, int sy, int ex, int ey)
{
Line l = { sx, sy, ex, ey, 1 };
return l;
}

Both gcc and Visual Studio 2005 compile that happily.

My question would be: Is that *really* correct, or are both compilers
simply being lenient? What are the exact rules for the initialization
blocks of structs?

[James Kanze]

On Jul 5, 5:27 am, Greg Herlihy <[e-mail] wrote:

On Jul 4, 6:54 pm, Old Wolf <oldw...@inspir e.net.nzwrote:

On Jul 5, 4:00 am, Juha Nieminen <nos...@thanks. invalidwrote:

Old Wolf wrote:
>{
> Line l = { sx, sy, ex, ey, 1 };
A minor point; the above initialization is
illegal in C90 which requires initializers
to be constant expressions.

I thought that was only the case with array
initialization. (Or is it the other way around nowadays?)

Nowadays (C99) the above is allowed. But in C90
brace-enclosed initializers (for structs and
arrays) had to be constant expressions.

No, C99 has the same, const-expression initializer
requirements for arrays - as C90 did: namely, that the
initializers for arrays with static storage duration must be
constant expressions, whereas the initializers for arrays with
other-than-static storage duration have no such requirement

That's what I always thougt too, but on pulling out my old
copy of C90, I find (§6.5.7, in the Constraints section):

All of the expressions in an initializer for an object
that has static storage duration or in an initializer
list for an object that has aggregate or union type
shall be constant expressions.

And in §6.1.2.5, "Array and structure types are collectively
called aggregate types" (with "aggregate types" in italics,
so this is a definition).

Historically, of course, K&R C required constant expressions
for all aggregate initialization, and C99 only requires them
for variables with static lifetime. Apparently, the change
didn't take place until after C90 (although I remember it as
being before).

--
James Kanze (GABI Software) email:ja******* **@gmail.com
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