Where can we write code like
struct Foo
{
int bar;
int baz;
} foo()
{
}
C89/C90? C99? C11? Or maybe it's K&R only?
And what about this
void foo(bar, baz)
int bar;
int baz;
{
}
-
Not sure, what you are trying to achieve in first example but Second is K&R and format of function definition, it is not approved by C standard.Alok Save– Alok Save2012-09-02 07:21:57 +00:00Commented Sep 2, 2012 at 7:21
Add a comment
|
3 Answers
It's standard since C89. In K&R C, it was not possible to return structs, only pointers to structs.
2 Comments
FrozenHeart
Thanks! And i can't use this in any C++ standard, yes?
eq-
@NikitaTrophimov: You can certainly return structs in C++, too, but you can't define them in the function declaration.
It is possible and useful, certainly since C89, to return a struct.
Your second example foo is old K&R C and is deprecated in newer standards.
Notice that on Linux x86-64 the ABI defines calling conventions which returns a two membered structure directly thru registers. Other structures are returned thru memory, so may be a little slower to return than a single pointer.
For instance, you could define a point to be a struct of two numbers (e.g. int or double) called x and y and return such a struct from some getPosition routine. On Linux/x86-64, the two numbers would be returned in two registers (without bothering building some struct in memory, when optimization is enabled and possible), e.g.:
struct point_st { int x, y; };
struct point_st getPosition(void);
struct point_st getPosition () {
struct point_st res = {-1,-1};
res.x = input_x();
res.y = input_y();
return res;
};
You generally want to declare the returned struct before the function's prototype.
answered Sep 2, 2012 at 7:23
Basile Starynkevitch
231k18 gold badges323 silver badges578 bronze badges
Comments
In C89/C90, it is possible to have a function that returns a struct.
I didn't find any explicit definition in the standard, but a footnote in section 3.3.2.3 "Structure and Union Members" says:
EXAMPLE 1: If f is a function returning a structure or union, and x is a member of that structure or union, f().x is a valid postfix expression but is not an lvalue.
Section 3.6.6.4 "The return Statement" doesn't lose a word about structures and unions as return types though.
Section 3.5.4.3 "Function declarators (including prototypes)" says:
A function declarator shall not specify a return type that is a function type or an array type.
It does not mention any restriction on structure or union types, and since this would be the place where such a restriction would belong, they are allowed.
C99 has the same wording, it just renumbered the sections.
