Explain GCC error after using an object accidentally declared as a function

Question

The following is a common typo with language newcomers, who think that they are defining an object but are actually declaring a function:

struct T
{
   void foo() {}
};

int main()
{
   T obj();
   obj.foo();
}

GCC 4.1.2's error is:

In function 'int main()':
Line 9: error: request for member 'foo' in 'obj', which is of non-class type 'T ()()'
compilation terminated due to -Wfatal-errors.

Why is the reported type in the message T ()()? I'd have expected T ().

score 5 · Accepted Answer · answered Mar 10 '11 at 11:47

5

IIRC this is just a compiler bug. GCC 4.4 says T() while 4.2 says T()() for me.

answered Mar 10 '11 at 11:47

etarion

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Oh, meh. Yea, 4.4.1 gives me the output I expected too. Thanks! – Lightness Races in Orbit Mar 10 '11 at 11:48

score 2 · Answer 2 · answered Mar 10 '11 at 12:33

2

The error is best understood when you realize that you usually don't write out function types without naming at least the function, but it's a bit more common for function pointers.

For instance, int (*fooPtr)() names the pointer. If you omit the name, you have int (*)(). Now, going from function pointer to function type would give you int ()().

There's no real standard here, because ISO C++ doesn't define canonical names for all types. For instance, const volatile int is the same type as volatile const int, and neither form is canonical.

answered Mar 10 '11 at 12:33

MSalters

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1

OK, but then when you provide a "function type" to, say, std::function, you write `std::function(...)`. – Lightness Races in Orbit Mar 10 '11 at 13:28
Indeed. And in fact `int()()` could (should?) be parsed as a function that returns an `int()` (!). But the question was to explain a GCC error. – MSalters Mar 10 '11 at 13:47

Explain GCC error after using an object accidentally declared as a function

2 Answers2