g ++ 4.8.2：无法从函数指针转换为std :: function <>

Question

I'm trying to write a class that takes, as a constructor parameter, a factory function that creates another instance of the same class on a transformed version of the input. For a trivial example, a functor class that takes an int, prints it, and returns another functor that prints the input's successor.

I'm getting errors of the form

error: could not convert 'foo' from 'Type(*)(int)' to 'std::function<Type(int)>'

The only seems to arise with factory functions passed into constructors.

Using a function pointer instead of a std::function<> works fine, but I was hoping that with C++11 I'd be able to avoid function pointers.

Here's an example:

#include <functional>
#include <iostream>

// std::function-returning-int param in ctor with default value
// This works fine

class PrInc; // "print and increment"

using Worker = std::function<int(int)>;

int foo(int i) {
    std::cout << i << std::endl;
    return i+1;
}

class PrInc {
  public:
    PrInc(int i, Worker fn = foo) : i_(i), fn_(fn) {}
    int operator()() { return fn_(i_); }
  private:
    int i_;
    Worker fn_;
};

// std::function-returning-PrInc2 param in ctor with default value
// This fails, at least on g++ 4.8.2 --std=c++11

class PrInc2;

using Factory = std::function<PrInc2(int)>;
// Use function ptrs (instead of std::function<>s) and it works fine
//typedef PrInc2 (*Factory)(int);

PrInc2 bar(int);

class PrInc2 {
  public:
    PrInc2(int i, Factory fn = bar) : i_(i), fn_(fn) {}
    PrInc2 operator()() { return fn_(i_); }
  private:
    int i_;
    Factory fn_;
};

PrInc2 bar(int i) {
    std::cout << i << std::endl;
    return PrInc2(i+1);
    // error: could not convert 'bar' from 'PrInc2 (*)(int) to 'Factory {aka std::function<PrInc2(int)>'
}

int main() {
    auto p1 = PrInc {1};
    auto p2 = PrInc{p1()};
    p2();

    auto p3 = PrInc2 {1};
    // error: could not convert 'bar' from 'PrInc2 (*)(int) to 'Factory {aka std::function<PrInc2(int)>'
    auto p4 = p3();
    p4();

    return 0;
}

What am I doing wrong?

EDIT: thelink2012's suggestion to try PrInc2(int i, Factory fn = Factory(bar)) works fine on the "repro" above, but fails on the motivating example (GitHub link; relevant code is erdos.h:35-54 around ParseStep and ParseFunction). Back to the drawing board for a better repro....

Answer 1

The following part of your code, which is the only part that appears to be relevant to the compiler error you're asking about, compiles cleanly with gcc 4.9.2 using the -std=c++11 mode:

#include <functional>
#include <iostream>

using Worker = std::function<int(int)>;

int foo(int i) {
    std::cout << i << std::endl;
    return i+1;
}

class PrInc {
  public:
    PrInc(int i, Worker fn = foo) : i_(i), fn_(fn) {}
    int operator()() { return fn_(i_); }
  private:
    int i_;
    Worker fn_;
};

Most likely a bug in the older gcc you're using. Upgrade to a more recent version of gcc.