Compiling error on template method, return is instance from inner class

Question

Here is a simplified example:

template<typename T>
class MyTemplate
{
  class Inner {};
  Inner met();
};

template<typename T>
MyTemplate<T>::Inner  MyTemplate<T>::met()
{ }

I get the following compilation error:

expected constructor, destructor, or type conversion before 'met'

I use GCC. It seems the compiler doesn't recognize MyTemplate<T>::Inner as a proper class. How can I fix this? I've tried sticking the typename keyword here and there to no avail. Right now, the only way I can manage to compile this is to inline the method definition in the class declaration, which I would like to avoid.

`typename MyTemplate::Inner MyTemplate::met()` and my GCC 4.6 is happy with your code. Which version of GCC are you using? — mkaes, Nov 23 '11 at 17:06

score 32 · Accepted Answer · answered Nov 23 '11 at 17:05

32

Clang reports the following:

error: missing 'typename' prior to dependent type name
'MyTemplate<T>::Inner' MyTemplate<T>::Inner  MyTemplate<T>::met()
^~~~~~~~~~~~~~~~~~~~ typename  1 error generated.

And placing typename in the appropriate place fixes it.

template<typename T>
class MyTemplate
{
  class Inner {};
  Inner met();
};

template<typename T>
typename MyTemplate<T>::Inner  MyTemplate<T>::met()
{ }

Did you put typename in the correct location? If so, then this must be a bug in G++.

answered Nov 23 '11 at 17:05

Michael Price

8,088
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Crap… I could swear I had tried that. Thank you (it works)! – lampyridae Nov 23 '11 at 17:13
13

Although that answers the question, it would be nice if you'd give the questioner a hint towards dependent names and why that's relevant. – Sebastian Mach Nov 23 '11 at 17:37
@phresnel Thank you for setting me on the trail of some additional googling, it was quite enlightening! – lampyridae Nov 28 '11 at 15:06

Praetorian · Answer 2 · 2011-11-23T17:21:39.157

13

The return type of MyTemplate::met is a dependent name, so you need to add the typename keyword before it. The following compiles on gcc-4.5.1

template<typename T>
struct MyTemplate
{
  class Inner {};
  Inner met();
};

template<typename T>
typename MyTemplate<T>::Inner  MyTemplate<T>::met()
{
  return typename MyTemplate<T>::Inner();
}

int main()
{
  MyTemplate<int> foo;

  MyTemplate<int>::Inner bar = foo.met();
}

edited Nov 23 '11 at 17:21

answered Nov 23 '11 at 17:05

Praetorian

106,671
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Well, it almost compiles. `met` needs to return a value. – John Dibling Nov 23 '11 at 17:10

score 2 · Answer 3 · answered Nov 23 '11 at 17:06

2

You need to add typename keyword before MyTemplate<T>::Inner because MyTemplate<T> is a dependent scope.

answered Nov 23 '11 at 17:06

Prasoon Saurav

91,295
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Compiling error on template method, return is instance from inner class

3 Answers3