Error C2451 using boost::any

Question

class A {
public:
    void f()
    {
        cout << "A()" << endl;
    }
};

class B {
public:
    void f()
    {
        cout << "B()" << endl;
    }
};

class C {
public:
    void f()
    {
        cout << "C()" << endl;
    }
};

void print(boost::any& a)
{
    if(A* pA = boost::any_cast<A>(&a))
    {
        pA->f();
    }
    else if(B* pB = boost::any_cast<B>(&a))
    {
        pB->f();
    }
    else if(C* pC = boost::any_cast<C>(&a))
    {
        pC->f();
    }
    else if(string s = boost::any_cast<string>(a))
    {
        cout << s << endl;
    }
    else if(int i = boost::any_cast<int>(a))
    {
        cout << i << endl;
    }
}

int main() 
{
    vector<boost::any> v;
    v.push_back(A());
    v.push_back(B());
    v.push_back(C());
    v.push_back(string("Hello boy"));
    v.push_back(24);

    for_each(v.begin(), v.end(), print);
}

I'm getting this error in print() when testing for string using Visual Studio 2010:

 error C2451: conditional expression of type 'std::string' is illegal
          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

Answer 1

else if(string s = boost::any_cast<string>(a))

This line is causing you problems. string s is not a pointer, it's a stack variable. You can't do a check for null.

The reason you can do a check on the integer below is that integers implicitly map to bool.
0 -> FALSE
1 -> TRUE