sorry
CMyReturn Foo()
{
if(int)
{
m_iVal = ...
m_eInternType=eIntType;
} else if(...
}
should be
CMyReturn Foo()
{
if(int)
{
m_iVal = ...
m_eInternType==eIntType
} else if(...
}
Main Topics
Browse All Topics
Loading Advertisement...
Question
C++, Can I return variant types via polymorphic mechanism?
A C++ Question; Is it possible to have a polymorphic function that returns various types, dependent on the derived object type?
I am trying to write some code to cache a database table. The table could have columns containing various basic SQL types, ie int, double, float, string etc.
I was hoping to create a 'cell' object for the table, similar to a spreadsheet cell, that would hold the data.
To be able to hold the cell in an stl container such as map, I need a common base pointer, so I start with an interface class that has the cell's column name, and some abstract functions that the derived type-specific cells will implement.
How can I create a derived cell type, ie IntCell, that has a 'get' type function to return the cells held int value, but which can be called via the base pointer? I need IntCells to return ints, DoubleCells to return double values, etc.
I can't have a get function, pure virtual, in the base class, because the return type will be different for each derived cell class.
I could use a template to derive the cell types, but then how would I call the get function via the base pointer?
In short, I need a class hierarchy that allows me to group and access types from an stl container, and that has a get function that returns the contained type.
I have a work around, to use text as the internal storage type, and have various conversion functions to produce types, but I have been tying myself in knots trying to work out a more efficient way to do this.
Thanks in advance for any help you can give me.
This Question has been solved and asker verified All Experts Exchange premium technology solutions are available to subscription members.
Subscribe now for full access to Experts Exchange and get
Instant Access to this Solution
- Plus...
- 30 Day FREE access, no risk, no obligation
- Collaborate with the world's top tech experts
- Unlimited access to our exclusive solution database
- Never be left without tech help again
Trusted by hundreds of thousands everyday for fast, accurate and reliable tech support.
- "The time we save is the biggest benefit of Experts Exchange to Warner Bros. What could take multiple guys 2 hours or more each to find is accessed in around 15 minutes on Experts Exchange." Mike Kapnisakis, Warner Bros.
- "Our team likes having a resource that is more secure than just using Google and most experts using this service really know their stuff. It's nice to look here first versus using Google." Dayna Sellner, Lockheed Martin
- "Anytime that I've been stumped with a problem, 9 out of 10 times Experts Exchange has either the accepted solution or an open discussion of the potential solution to the problem." Kenny Red, eBay Inc.
See what Experts Exchange can do for you.
Got a question?
We've got the answer.
Experts Exchange has been collecting answers to technology questions since 1996…3 million and counting! If you have a question, chances are we already have your answer.
Need individual assistance?
Our experts are ready to help.
If you can't find the exact answer you're looking for, ask our exclusive community of 50,000 experts. You’ll get a personalized answer from a trusted professional.
Want to learn from the best?
Read articles from industry experts.
Thousands of free tech tips, tricks, how-to’s and tutorials are available in our peer reviewed articles section. See for yourself how smart our experts are, no login required.
Working on a long term project?
Store your work and research.
Save solutions to your questions, answers you’ve discovered through searching plus helpful articles in your personal knowledgebase for easy future access.
Access the answers to your technology questions today.
30-day free trial. Register in 60 seconds.
What Makes Experts Exchange Unique?
Members of the expert community talk about why the experience at Experts Exchange is different than what you will find anywhere else.
Trusted by the world's most respected brands.
Faithfully serving IT professionals since 1996.
Try it out and discover for yourself.
30-day free trial. Register in 60 seconds.
Free Tech Articles
-
WARNING: 5 Reasons why you should NEVER fix a computer for free.
It is in our nature to love the puzzle. We are obsessed. The lot of us. We love puzzles. We love the challenge. We thrive on finding the answer. We hate disarray. It bothers us deep in our soul. W...
-
SCCM OSD Basic troubleshooting
SCCM 2007 OSD is a fantastic way to deploy operating systems, however, like most things SCCM issues can sometimes be difficult to resolve due to the sheer volume of logs to sift through and the dispe...
-
Migrate Small Business Server 2003 to Exchange 2010 and Windows 2008 R2
This guide is intended to provide step by step instructions on how to migrate from Small Business Server 2003 to Windows 2008 R2 with Exchange 2010. For this migration to work you will need the fo...
-
Create a Win7 Gadget
This article shows you how to create a simple "Gadget" -- a sort of mini-application supported by Windows 7 and Vista. Gadgets can be dropped anywhere on the desktop to provide instant information, ...
-
Outlook continually prompting for username and password
There have been a lot of questions recently regarding Outlook prompting for a username and password whilst using Exchange 2007. There are a few reasons why this would happen and I will try to cover t...
-
Backup Exchange 2010 Information Store using Windows Backup
There seems to be quite a lot of confusion around the ability to backup Exchange 2010 using the built in Windows Backup feature. This stems from the omission of this feature prior to Exchange 2007 s...
Cloud Class Webinars
- Avoiding Bugs in Microsoft Access
Alison Balter takes and in-depth look at avoiding bugs in Access. In this webinar you will learn about using the immediate window to debug your applications, invoking the debugger, using breakpoints to troubleshoot, stepping through code, setting the next statement to execute, ...
- Top 10 Best New Features in Visio 2010
Scott Helmers gives live demonstrations of the top 10 new features in Visio 2010. This webinar will teach you how to create compelling diagrams by adding shapes to the page with a single click, linking the shapes in a diagram to data in Excel (or SQL Server, or SharePoint), ...
- IT Consultant Business Secrets Revealed
Michael Munger, Experts Exchange tech pro and IT consultant, pulls back the curtain on his very successful businesses and answers question on every IT consultant and business owner should know about. He shares secrets on what he did to solve the 5 most common problems in IT, ...
- Disaster Recovery and Business Continuity
Quest CTO, Mike Billon, gives an overview of the steps involved in building a dunamic disaster recovery plan. Through case studies and an examination of software/hardware tooles for monitoring and testing, you'll gain a better understandin of where you are, where you want ...
- Organize Your Visio Diagrams with Containers and Lists
Scott Helmers uses cross functional flowcharts, wireframe diagrams, data graphic legends and seating charts to teach you: how to ustilize all three new structured diagram components in Visio 2010, the best practices for organizeing shapes in previous version of Visio, how to organize ...
- How to Us Objects, Properties, Events and Methods in Microsoft Access
Alison Dalter gives an in-depbth look at objects, properties, events and methods in Microsoft Access. In this webinar you will learn about using the object browser, referring to objects, working with properties and methods, working with object variables, understanding the ...
Join the Community
Give a Little. Get a Lot.
Join the community of experts here and help other tech pros by answering question in your area of expertise. You can earn FREE access to all Experts Exchange's premium features and resources.
Answers
>> I need IntCells to return ints, DoubleCells to return double values, etc.
There's a problem with your design.
The (abstract) base class should provide the interface that all derived classes have to implement. There is only one such interface, so you'll need to make sure that all implementations follow this one interface.
So, if your get function returns a certain type, then all get implementations will have to return that type. If you need that type to be able to represent different types, then you could use a combination of struct, union and enum, like AndyAinscow hinted on.
But even better would be to modify your design so you don't need to do this. If for example this get function will only be used for showing the cell contents to the user, then you could have it return a string, and each implementation would format that string depending on the cell contents. If for example this get function will be used for performing calculations between cells, then you could provide specific functions to perform these calculations instead (the multiple dispatch pattern comes to mind for example - good use of templates could also do the trick).
@Infinity08, Yep, I kind of suspected the design was not good, if I was coming up against a brick wall like this.
The 'get' function is/was going to be used in an SQL type query. I am effectively caching a database table to make fast queries on it in RAM, so I was hoping to emulate SQL's SELECT functionality by being able to compare int values in the cached table to int values in the code, string values in the table to string values in the code, etc. The aim was for flexibility. I thought about templates for the variant return type, but then I could not figure out how to call the templated function via the base pointer, there being no abstract function for it.
@AndyAinscow, I will try something that is similar to your suggestion too.
I will store the data as one type, and return variant types by named functions. The internal data type will be string (90% of the data will be string data anyway), and in the case of int/double/float data being required, I'll put a conversion method in the class. If the cell object is myCell, to get the int value use myCell.int(), to get the double call myCell.double(). I could use a numeric flag indicator to allow the cell to indicate it's preferred natural type.
I was concerned that I had missed an obvious or tried/tested way of tackling this problem, and it looks like I have not, so working around it looks like a reasonable way to progress. Many thanks for the swift and clear replies.
3 Ways to Join
Business Accounts
- Perfect for organizations
- Multiple users
- Save over 36%
- Create a Business Account
Answer for Membership
- Earn free access
- Showcase your knowledge
- No credit card required
- Sign Up to Answer
Loading Advertisement...
by: AndyAinscowPosted on 2009-09-16 at 02:22:27ID: 25343488
I don't think you can modify the return type in that way.
You could however return something that encapsulates different internal types such as the variant. (Your code that retrieves vales from the database probably has it as a variant internally.
A 'roll your own' could be (pseudo code)
class CMyReturn
{
public:
int m_iVal;
bool m_bVal;
...
enum InternType....
}
CMyReturn Foo()
{
if(int)
{
m_iVal = ...
m_eInternType==eIntType
} else if(...
}
CMyReturn r = Foo();
if(r.m_eIntenType==e
{
.
}