Instantiating Function Object with unbound Function Objects leads to l-value error

Question

I have the following function objects:

/// function object wrapping node and label data into strings of DOT code
/// requires as template parameters 2 function objects that define behaviour
/// converting node and lable into strings. Requires as first template
/// argument the graph edge type
template<typename GRAPHEDGE, typename NodeToStringConverter,
         typename LabelToStringConverter>
struct DOTWrapper
{

    typedef NodeToStringConverter NodeConverter;
    typedef LabelToStringConverter LabelConverter;

    /// initialises to-string-converter objects
    DOTWrapper(NodeConverter& nc, LabelConverter& lc): nts(nc), lts(lc){}

    std::string start() const
    {
        return ("digraph  {\n");
    }

    // wrap an egde into dot code
    std::string wrap_edge(const GRAPHEDGE& e) const
    {
        // n needs to be to_string-able
        return ("\t\""+nts(e.source())+"\""+" -> "+"\""+nts(e.target())+"\""+
                " [ label = \""+lts(e.label())+"\" ];\n");
    }

    std::string finish() const { return "}\n"; }

    const NodeToStringConverter nts;
    const LabelToStringConverter lts;

};

template<typename Node>
struct NTSint
{

    std::string operator()(const Node& node) const { return node; }
};

template<typename Label>
struct LTSstr
{

    std::string operator()(const Label& label) const { return label; }
};

When I try to instantiate a Wrapper object like this:

typedef NTSint<Edge::Node> NConv;
typedef LTSstr<Edge::Label> LConv;
typedef DOTWrapper<Edge, NConv, LConv> DOTf;
const DOTf dotf((NConv()), (LConv()));

I get

"error: no matching constructor for initialization of 'const DOTf' (aka 'const DOTWrapper') const DOTf dotf((NConv()), (LConv()));"

Why is that? NConv and LConv have a place in the memory after all... so... shouldn't they be l-values?

`NConv` is a *type*, not an object. No, it does not "have a place in the memory". — Kerrek SB, Jul 04 '15 at 23:40
But am I not creating an object of that type with `NConv()` ? — lo tolmencre, Jul 05 '15 at 00:18
Yes, so what? You're creating a temporary object, which is a prvalue. — Kerrek SB, Jul 05 '15 at 00:30
Ah, I thought objects are always l-values. Ho come I don't get this error for all function object that I pass to some function as temporary object? like to std::sort(), in something like `sort(obj1.begin(), obj1.end(), functionObjectType());` — lo tolmencre, Jul 05 '15 at 00:40
Okay, but which do you refer to by "those"? I also don't understand what "mutable lvalue reference parameters" means. — lo tolmencre, Jul 05 '15 at 01:13

Instantiating Function Object with unbound Function Objects leads to l-value error

0 Answers0