How to construct objects through pointer to function using map

Question

Base class

class Io_obj
{
public:
    virtual Io_obj* clone() const =0;
    virtual ~Io_obj(){}
};

Derived classes

class Io_circle : public Io_obj
{
    string c;
public:
    Io_circle(string& s):c{s}{cout << c << '\n';}
    Io_circle* clone() const override{return new Io_circle{*this};}
    static Io_obj* new_circle(string& s){return new Io_circle{s};}
};

class Io_triangle : public Io_obj
{
    string t;
public:
    Io_triangle(string& s):t{s}{cout << t << '\n';}
    Io_triangle* clone() const override{return new Io_triangle{*this};}
    static Io_obj* new_triangle(string& s){return new Io_triangle{s};}
};

main functions

using Pf = Io_obj*(string&);
map<string,Pf> io_map{{"circle",&Io_circle::new_circle},{"triangle",&Io_triangle::new_triangle}};
vector<string> vs{"circle","triangle"};

Io_obj* get_obj(int i){
    string word = vs[i];

    if(auto f=io_map[word]){
        return f(word);
    }else{
        throw runtime_error{"shape not found"};
    }
}

I get an error couldn't deduce template parameter '_InputIterator' map<string,Pf> io_map{{"circle",&Io_circle::new_circle},{"triangle",&Io_triangle::new_triangle}};

Code from the C++ programming language -- Bjarne Stroustrup Ch22.2.4

I also tried the technique presented in the book

io_map["circle"] = &Io_circle::new_circle;
io_map["triangle"] = &Io_triangle::new_triangle;

It doesn't work either [map] does not name a type

have you added "#include <map>" line?

Valery Shevchuk
– Valery Shevchuk

2015-11-06 10:41:17 +00:00
Commented Nov 6, 2015 at 10:41 — Valery Shevchuk
– Valery Shevchuk, Commented Nov 6, 2015 at 10:41

Valery Shevchuk · Accepted Answer · 2015-11-06 10:52:32Z

1

the problem is your function pointer declaration is invalid. Good one must look like following:

using Pf = Io_obj*(*)(string&);

I've checked your code with this correction and it's compiled OK. code is here

UPD: Also, I can recommend you to use the std::function instead of raw function pointers as more type-safe alternative

answered Nov 6, 2015 at 10:52

Valery Shevchuk

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