struct constructor

[slurper]

hi,

i'm studying some stl. i saw the pair implementation in a header-file but
what i wonder is if a struct can have constructors as it seems in following
snippet from the stl library. the snippets i wonder about are indicated
with --------> : i thought a struct doesn't have constructors (but here
there are four, actually three)

template <class _T1, class _T2>
struct pair {
typedef _T1 first_type;
typedef _T2 second_type;

_T1 first;
_T2 second;
#ifdef _GLIBCPP_RESOLVE_LIB_DEFECTS
---------> pair() : first(), second() {}
#else
---------> pair() : first(_T1()), second(_T2()) {}
#endif
---------> pair(const _T1& __a, const _T2& __b) : first(__a), second(__b)
{}

template <class _U1, class _U2>
--------> pair(const pair<_U1, _U2>& __p) : first(__p.first),
second(__p.second) {}
};

 

[Cy Edmunds]

hi,

i'm studying some stl. i saw the pair implementation in a header-file but
what i wonder is if a struct can have constructors as it seems in following
snippet from the stl library. the snippets i wonder about are indicated
with --------> : i thought a struct doesn't have constructors (but here
there are four, actually three)

[snip]

In C++, the only difference between a struct and a class is that all members
of a struct are public by default. Thus a struct can have a constructor or
anything else a class can have.

 

[Ron Natalie]

hi,

i'm studying some stl. i saw the pair implementation in a header-file but
what i wonder is if a struct can have constructors as it seems in following
snippet from the stl library. the snippets i wonder about are indicated
with --------> : i thought a struct doesn't have constructors (but here
there are four, actually three)

You thought wrong. Structs are classes. The only difference is that classes
defined with the struct keyword have public access by default. Classes defined
with the class keyword have private access by default.

That is:
struct x { .... };
is the same as:
class x { public: ... } ;

 

[Nick Hounsome]

hi,

i'm studying some stl. i saw the pair implementation in a header-file but
what i wonder is if a struct can have constructors as it seems in following
snippet from the stl library. the snippets i wonder about are indicated
with --------> : i thought a struct doesn't have constructors (but here
there are four, actually three)

[snip]

In C++, the only difference between a struct and a class is that all members
of a struct are public by default. Thus a struct can have a constructor or
anything else a class can have.

I suspect that the poster was thinking that struct = POD (plain old (C
compatible) data).

Of course this isn't true but what may need pointing out is that even POD
can have some C++ stuff such as ctors and member functions.

Off the top of my head the only things it can't have (and still be POD) are:
- access specifiers
- virtual methods
- destructor
- references
- any member that isn't POD
There may also be restrictions on ctors

 

[David White]

hi,

i'm studying some stl. i saw the pair implementation in a header-file but
what i wonder is if a struct can have constructors as it seems in following
snippet from the stl library. the snippets i wonder about are indicated
with --------> : i thought a struct doesn't have constructors (but here
there are four, actually three)

[snip]

In C++, the only difference between a struct and a class is that all members
of a struct are public by default.

And public inheritance by default (as against private for classes).

DW

 

[Martijn Lievaart]

You thought wrong. Structs are classes. The only difference is that
classes defined with the struct keyword have public access by default.
Classes defined with the class keyword have private access by default.

s/access/access and inheritance/g

HTH,
M4

 

[Ron Natalie]

- destructor

User declared constructors, destructors, or copy-assignment operators.

- references
- any member that isn't POD

non-static members of reference or other non-pod type.

 

[Ron Natalie]

s/access/access and inheritance/g

Sorry, my statement is right as it stands. I didn't say anything that limits
the access control to members. Access refers to both members and
base classes.

Inheritance only exists in the concept of name lookup.

 

[Martijn Lievaart]

Sorry, my statement is right as it stands. I didn't say anything that
limits the access control to members. Access refers to both members
and base classes.

Inheritance only exists in the concept of name lookup.

Yes, now you mention it, but I think that explicitly stating it applies to
inheritance as well makes it clearer. I don't think everyone will
immediately come your (correct) interpretation. I know I didn't.

M4