Question

I am trying to do the following:

template <class T>
std::ifstream& operator>> (std::ifstream& fin, List<T> l)
{
    T temp;
    l.resize(0);
    fin >> ignore(1,'\t');
    for(ListIterator<T> i=l.begin();i!=l.end();i++)
    {
        fin >> ignore(1,'\t') >> temp;
        l.push_back(temp);
    }
    return fin;
}

I have to read all the contents from a file. Each field is separated by '\t' character, so I have to ignore the '\t' characters.

The error log is the following:

/home/ramy/Documents/C++/Prova/Util.h||In function ‘std::ifstream& Util::operator>> (std::ifstream&, Util::List<T>)’:|
/home/ramy/Documents/C++/Prova/Util.h|431|error: there are no arguments to ‘ignore’ that  depend on a template parameter, so a declaration of ‘ignore’ must be available|
/home/ramy/Documents/C++/Prova/Util.h|431|note: (if you use ‘-fpermissive’, G++ will  accept your code, but allowing the use of an undeclared name is deprecated)|
||=== Build finished: 1 errors, 0 warnings ===|

Answer 1

For builtin types, argument dependent lookup (ADL) is not performed, therefore, an ignore symbol must be “imported” into the current namespace.

You can, for example, do this; from most preferred to least preferred (i.e. to most intrusive and name polluting):

• foobar::ignore (...)
• using foobar::ignore; ignore(...);
• using namespace foobar; ignore(...);

The error message comes up like this because in templates, you also enter the realm of dependent names and Two Phase Lookup. Names that depend on a template parameter, e.g.

template <typename T> void foo() {
    T x;
    x.frobnicate();
}

are looked up in phase 2, which is upon instantiation. Names that do not depend on template parameters, like

class Foo {};

template <typename T> void foo() {
    Foo foo;
    foo.frobnicate();
}

must be resolvable in the first phase.

This separation helps template authors to find bugs earlier and to find correct symbols, and it helps making templates more generic. For example, in C# generics, everything must be resolvable, which puts rather stringent limits on their flexibility (because everything that may be used by a generic must be defined). Oppositely, some old C++ compilers resolved in phase 2 only, i.e. at instantiation time, which had some subtle consequences for lookup and error finding.

The C++ 2-phase model combines the best of the eager-model (C#) and the lazy-model (some old C++ compilers).