Ignore default constructor in template class

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Hello everyone,

I have the following hierarchy:

std::map ->
assoc_map ->
_tActionMap

_tActionMap maps actions (Action) with moods (Mood). The problem is: Mood has an intensity value which is mandatory, so there is no default constructor, and I don't want one.

Now, templates only compile when a specialization is created. In order to get around this, in "assoc_map" I force the passage of a default value for the mapped type so that the default constructor is not mandatory on mapped types (instead, a copy of the default value is created). Then, I overload operator[] to use the default value instead of a new instance (created with the default constructor).

This "trick" works in VS08, but I don't know if this is standard compliant. So... Is it? Will it work in other compilers?

Best regards

Posted 7-Apr-12 9:28am

AndreFratelli

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Lakamraju Raghuram 8-Apr-12 3:22am

i am lost ... do elaborate

AndreFratelli 8-Apr-12 10:02am

I was kind of fearing that :P

std::map::operator [] does one of two things:

1) Return a reference to an existing value with the given key; or
2) Instantiate a new value using the default constructor and associate it with the key.

So what to do when a default constructor is not available? I inherited from std::map (assoc_map) and allow the passage of a default value at construction time.

Then I overload operator[] and copy the default instance instead of propagating to std::map::operator[], when the value does not exist.

This works because it's a template class and, therefore, its methods only get compiled when a specialization exists for them. What I would like to know is if this is indeed true. If an attempt exists to try to compile std::map::operator[] it will complain that the default constructor is not available, even though that does not happen in VS08.

Thank you

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Aescleal · Accepted Answer · 2012-04-13T21:40:00

It sounds like it would work, however without seeing the code I can't tell for sure. As you're a VC++ user perhaps download mingw (gcc for windows) and give it a whirl. If something compiles on VC++2005 or later and gcc 3.3 or later I'm usually happy it's standard code.

Just to satisfy my curiosity...

Is there a reason you don't want a default constructor? I assume it's something like you're using someone else's type and you've got the .h but not the .cpp.

Have you considered not deriving from a map but containing and forwarding instead? That will work on every compiler I know. You won't be able to do this if you've got existing code that wants a map but you avoid the whole "STL collections aren't meant to be derived from" panic that wells up in the souls of most experienced C++ programmers.

Anyway, I'm not sure that the weak advice of "try it on another compiler" is really a solution but it's a good way forward.

Cheers,

Ash

PS: If you can get hold of a compiler that uses the Edison Group's front end (http://www.edg.com/[^]) and it compiles on that you'll know it's standard compliant. Unfortunately all the compilers based on that I know of (Intel and Comeau) you have to pay for.

PPS: VC++2010 Intellisense uses EDG's front end. If you download VC++2010 express, stick your code in and see if you get any red wiggly lines under any part of the code. If you don't you're probably going to be okay. It's a good check anyway.

Ignore default constructor in template class

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Ignore default constructor in template class

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