catching template errors

[bigearsbilly]

Now I have made a template to slurp a file into an STL list container
perl style

template <typename T>
void slurp(T & v, std::istream & in)
{
    std::string s;
    while (in >> s) {

        v.push_back(s);
    }
}

which works beautifully for deque and vector and list:

    vector<string> v;
    slurp(v, cin);

Now obviously this won't work for, say, a map, which makes sense.
but when you erroneously pass a map you get a long compiler error trace.

NOW is there a way to catch this?
i.e. if there isn't a push_back member I can give a nice error message rather
than a scary long list of compiler errors?

(I am bound to forget what this template does in the future of course)

[Bazzy]

You can specialize the template to work only with vector, list and deque or you can change it to something like this:

template <typename T>
void slurp( deque<T> &v, std::istream & in)
{
    T s;
    while (in >> s) {
        v.push_back(s);
    }
}

And overload it also for vector and list

[bigearsbilly]

yes, that would work but it's not generic enough.
if there was a later container that had a correct interface (i.e. a push_back())
I would have to add it manually.

[R0mai]

template <typename T>
void slurp(T & v, std::istream & in)
{
    T::value_type s;
    while (in >> s) {
        v.insert(v.back(), s);
    }
}

This will work on any STL containers. Note: std::map and std::multimap needs completely different care, as it is has key-value pairs, not just values. You can make the exact same code to work with std::map<K, V> and std::multimap<K, V> by overloading std::istream& operator>>(std::istream&, std::pair<K,V>&);

Hope this helps.

EDIT : see corrected code in my next post

[bigearsbilly]

no that doesn't even compile for me.

T is-a vector<string>

[R0mai]

sry, my bad

Correct code

template <typename T>
void slurp(T & v, std::istream & in)
{
    typename T::value_type s;
    while (in >> s) {
        v.insert(v.end(), s);
    }
}

[bigearsbilly]

much better, but still doesn't solve the problem.
catch wrong containers neatly with a nice error message like:
"oops cannot use a map here" rather than the usual compiler errors.

I tried to use template meta programming and iterator traits but my head exploded!

maybe it cannee be done

[R0mai]

I don't think you can do it automatically. But there are softwares for this purpose :
For example http://www.bdsoft.com/tools/stlfilt.html

[Bazzy]

This may interest you: http://www.generic-programming.org/languages/conceptcpp/
Unluckily looks as concepts will not be included in C++0x