使用std :: map错误函数调用的boost :: factory

Question

I'm having a problem with understanding how boost::factory does its job. The following code will throw in the ctor of test2.

#include <boost/functional/factory.hpp>
#include <boost/function.hpp>

typedef boost::function<A*()> creator;
typedef std::map<string,creator> factory;

class A{
}

class AA : A {
}

class AB : A {
}

class C{
public: 
 C(factory& f);

 factory _f;
}

int main(){
 factory f;
 f["1"] = boost::factory<AA*>();
 f["2"] = boost::factory<AB*>();

 C test(f);
 C test2(f);
}

C::C(factory& f){
  _f = f;
  A* t = _f["1"]();
}

The message is

terminate called after throwing an instance of 'boost::exception_detail::clone_impl

' what(): call to empty boost::function

I think I don't understand copy/move behaviour here and that is causing the problem. As far as I understand, the factory gets copied in C::C, so every call to _f[something]() should get its own function to call. But somehow the function is moved out of the factory in the ctor of test, and then I get a bad function call, cause f["1"] is left in an undefined state. Please help.

Answer 1

As far as I understand, the factory gets copied in C::C

Yes.

But somehow the function is moved out of the factory in the ctor of test

Nope. If that seems to be the case you likely have Undefined Behaviour elsewhere. Or you might be running a different (buggy) version of boost (this seems highly unlikely).

See if you can reproduce it with the code shown here:

Live On Coliru

#include <boost/functional/factory.hpp>
#include <boost/function.hpp>
#include <map>
#include <iostream>

struct A {};
struct AA : A {
    AA() { std::cout << __PRETTY_FUNCTION__ << "\n"; }
};
struct AB : A {
    AB() { std::cout << __PRETTY_FUNCTION__ << "\n"; }
};

typedef boost::function<A*()> creator;
typedef std::map<std::string, creator> factory;

struct C {
    C(factory &f){
        _f = f;
        for (auto& e : _f)
            std::cout << "entry for " << e.first << " present " << (!!e.second) << "\n";
    }
    factory _f;
};

int main() {
    factory f;
    f["1"] = boost::factory<AA*>();
    f["2"] = boost::factory<AB*>();

    C test(f);
    delete f["1"]();
    delete f["2"]();
    delete f["1"]();
    delete f["2"]();

    C test2(f);
    delete f["1"]();
    delete f["2"]();
    delete f["1"]();
    delete f["2"]();
}

Prints

entry for 1 present 1
entry for 2 present 1
AA::AA()
AB::AB()
AA::AA()
AB::AB()
entry for 1 present 1
entry for 2 present 1
AA::AA()
AB::AB()
AA::AA()
AB::AB()