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Conditional template specialization

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Clockowl

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Conditional template specialization

Apr 25th, 2009

Hey guys,

How would I partially specilize a template in a manner as below? Is that even possible?

I have these two functions:

 Help with Code Tags 
 cpp Syntax (Toggle Plain Text) 
bool node::read(const char *filename) {
    ifstream f(filename);
 
    if (!f.good()) return false;
 
    bool success = read(f);
    f.close();
 
    return success;
}
 
bool node::write(const char *filename) {
    ofstream f(filename);
 
    if (!f.good()) {
        return false;
    }
 
    bool success = write(f);
    f.close();
 
    return success;
}
bool node::read(const char *filename) {
    ifstream f(filename);

    if (!f.good()) return false;

    bool success = read(f);
    f.close();

    return success;
}

bool node::write(const char *filename) {
    ofstream f(filename);

    if (!f.good()) {
        return false;
    }

    bool success = write(f);
    f.close();

    return success;
}

And I'd like to create a function like this:

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 cpp Syntax (Toggle Plain Text) 
template <typename T>
bool node::read_or_write(const char *filename) {
    T f(filename);
 
    if (!f.good()) {
        return false;
    }
 
    bool success;
    if(T == ifstream){
        success = write(f);
    } else if (T == ofstream){
        success = read(f);
    }
    f.close();
 
    return success;
}
template <typename T>
bool node::read_or_write(const char *filename) {
    T f(filename);

    if (!f.good()) {
        return false;
    }

    bool success;
    if(T == ifstream){
        success = write(f);
    } else if (T == ofstream){
        success = read(f);
    }
    f.close();

    return success;
}

How would I do that?

Thanks in advance,

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tux4life

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Re: Conditional template specialization

Apr 25th, 2009

I think you'll need RunTime Type Identification (RTTI) for that purpose ...

"Never argue with idiots, they just drag you down to their level and then beat you with experience."

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Join Date: May 2008

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Clockowl

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Re: Conditional template specialization

Apr 25th, 2009

Well, a design wouldn't need it. I don't know if its needed but can't you say something like: T can only be ifstream, ofstream

Code could look a bit like this:

 Help with Code Tags 
 cpp Syntax (Toggle Plain Text) 
template <typename T = {ifstream, ofstream}>
bool node::read_or_write(const char *filename) {
    T f(filename);
 
    if (!f.good()) {
        return false;
    }
 
    bool success;
    if(T == typename ifstream){
        success = write(f);
    } else if (T == typename ofstream){
        success = read(f);
    }
    f.close();
 
    return success;
}
template <typename T = {ifstream, ofstream}>
bool node::read_or_write(const char *filename) {
    T f(filename);

    if (!f.good()) {
        return false;
    }

    bool success;
    if(T == typename ifstream){
        success = write(f);
    } else if (T == typename ofstream){
        success = read(f);
    }
    f.close();

    return success;
}

Compiler should throw an error when you try to instantiate any other type of it.. I'm trying to create such a class now, but I'm not very good at it.

This is the best I could come up with, but I didn't compile/test it yet due to diner being ready.

 Help with Code Tags 
 cpp Syntax (Toggle Plain Text) 
template <typename T>
class filestuffs {
    filestuffs(const char *filename) : filename(filename) {
        f(filename);
        success = f.good();
        success = read_or_write();
        f.close();
    }
 
    //can't do this unless we know T
    bool read_or_write(){ return false; }
 
    T f;
    bool success;
    const char *filename;
    const char [] errmsg = "Can't do this on anything but: ifstream, ofstream";
}
template <>
class filestuffs<ifstream> {
    //now we know T! :D
    bool read_or_write(){
        return read(f);
    }
}
 
template<>
class filestuffs<ofstream> {
    bool read_or_write(){
        return write(f);
    }
}
template <typename T>
class filestuffs {
    filestuffs(const char *filename) : filename(filename) {
        f(filename);
        success = f.good();
        success = read_or_write();
        f.close();
    }
    
    //can't do this unless we know T
    bool read_or_write(){ return false; }
            
    T f;
    bool success;
    const char *filename;
    const char [] errmsg = "Can't do this on anything but: ifstream, ofstream";
}
template <>
class filestuffs<ifstream> {
    //now we know T! :D
    bool read_or_write(){
        return read(f);
    }
}

template<>
class filestuffs<ofstream> {
    bool read_or_write(){
        return write(f);
    }
}

Last edited by Clockowl; Apr 25th, 2009 at 3:20 pm.

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tux4life

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Re: Conditional template specialization

Apr 25th, 2009

And what's the result ?
Did it work ?

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Clockowl

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Re: Conditional template specialization

Apr 25th, 2009

Oh I'm sure that doesn't work, never seen it, just made it up. But I want to know how I'd do that in C++.

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tux4life

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Re: Conditional template specialization

Apr 25th, 2009

Did you try searching google (e.g: typeid , c++ rtti )?

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Clockowl

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Re: Conditional template specialization

Apr 25th, 2009

I am not looking for a runtime solution, that's why I hadn't.

Here's a working example:

 Help with Code Tags 
 cpp Syntax (Toggle Plain Text) 
        template <typename T>
        bool read_or_write(const char *filename) {
            T f(filename);
            if (!f.good()) {
                return false;
            }
 
            bool success;
            if (typeid(T) == typeid(ofstream)) {
                success = write(f);
            } else if (typeid(T) == typeid(ifstream)) {
                success = read(f);
            }
            f.close();
 
            return success;
        }
        template <typename T>
        bool read_or_write(const char *filename) {
            T f(filename);
            if (!f.good()) {
                return false;
            }

            bool success;
            if (typeid(T) == typeid(ofstream)) {
                success = write(f);
            } else if (typeid(T) == typeid(ifstream)) {
                success = read(f);
            }
            f.close();

            return success;
        }

But this way, I also have to define write(ifstream ... ), while I know 100% sure that the compiler will never have to call such a function. It kinda defeats it's use (making the code smaller) for this small function, but for bigger functions this sort of template is very usable I think.

Any compiler-time suggestions?

PS: For anyone reading this and thinking, why the hell are you creating such a function for 20 lines of code? It's because I'm not going to use read_or_write(), but would like to know how I'd accomplish maximum code re-usage in C++.

Last edited by Clockowl; Apr 25th, 2009 at 4:16 pm.