Invalid object after elision of copy operation?

Question

Possible Duplicate:
Why is it an error to use an empty set of brackets to call a constructor with no arguments?
Most vexing parse: why doesn't A a(()); work?

This one gets me mad. Maybe its just too simple.

struct Foo
{
  Foo() {}
  Foo(const Foo& f) {}
  void work() {}
};

int main()
{
  Foo f( Foo() );
  f.work();
}

GCC 4.6 gives me:

error: request for member ‘work’ in ‘f’, which is of non-class type ‘Foo(Foo (*)())’

After elision of the copy operation the effective code might look like:

int main()
{
  Foo f;
  f.work();
}

But why can't i call work() ??

Edit:

Yes, duplicate (see below). Didn't find the original post when search first because the source of the symptoms of this is located where i didn't expect that.

Why do you write `Foo f( Foo() );` in the first place, what is wrong with writing `Foo f;`, which should work as expected. Your first version creates a completely different type, as you can see in the error message. — Nobody moving away from SE, Jul 26 '12 at 11:12
Its of course a bit academic. Comes from a study what constructors are called — ritter, Jul 26 '12 at 11:13

score 3 · Accepted Answer · answered Jul 26 '12 at 11:12

3

Because Foo f( Foo() ); is a function declaration.

I think you want: Foo f;

Or in case you want to copy-construct:

Foo f( (Foo()) );

answered Jul 26 '12 at 11:12

Andrew

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A function declaration is not allowed within a basic block. How can it be interpreted as such? – ritter Jul 26 '12 at 11:15
@Frank: take a look here: http://stackoverflow.com/questions/1424510/most-vexing-parse-why-doesnt-a-a-work – Andrew Jul 26 '12 at 11:16
and actually you can write something like this inside the funciton: `int bar(int a, int b);`. It will compile – Andrew Jul 26 '12 at 11:18
Yes, seems a function declaration is allowed/accepted by the compiler. Not a definition. All with a basic block – ritter Jul 26 '12 at 11:22
@Frank: yeah, definition is not allowed – Andrew Jul 26 '12 at 11:24
Thanks, very good and quick answer! You were the first to answer so i give you the mark. – ritter Jul 26 '12 at 11:25

score 1 · Answer 2 · answered Jul 26 '12 at 11:13

1

f is effectively a function declaration within main function. Try

Foo f((Foo())); // to make the definition of f explicit enough.

answered Jul 26 '12 at 11:13

nurettin

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score 1 · Answer 3 · answered Jul 26 '12 at 11:17

n3337 8.2

The ambiguity arising from the similarity between a function-style cast and a declaration mentioned in 6.8 can also occur in the context of a declaration. In that context, the choice is between a function declaration with a redundant set of parentheses around a parameter name and an object declaration with a function-style cast as the initializer. Just as for the ambiguities mentioned in 6.8, the resolution is to consider any construct that could possibly be a declaration a declaration. [ Note: A declaration can be explicitly disambiguated by a nonfunction-style cast, by an = to indicate initialization or by removing the redundant parentheses around the parameter name. — end note ] [ Example:

struct S {
S(int);
};
void foo(double a) {
S w(int(a));
//function declaration
S x(int());
//function declaration
S y((int)a);
//object declaration
S z = int(a);
//object declaration
}

— end example ]

score 1 · Answer 4 · answered Jul 26 '12 at 11:18

1

C++ parser interprets Foo f(Foo()); expression as the function declaration with the signature Foo(Foo(*)()), i.e. a function returning Foo and taking a function pointer to the function returning Foo. Adding explicit parenthesis around the argument like so Foo f((Foo())); will resolve the ambiguity. But consider actually just doing Foo f; which avoids redundant code.

answered Jul 26 '12 at 11:18

yuri kilochek

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A function declaration is not allowed within a basic block. How can it be interpreted as such? – ritter Jul 26 '12 at 11:19
@Frank That is a C legacy [link](http://stackoverflow.com/questions/6089452/is-there-a-use-for-function-declarations-inside-functions) – yuri kilochek Jul 26 '12 at 11:23

Invalid object after elision of copy operation?

4 Answers4