invalid user-defined conversion - c++

Question

please have a look at this program and the error it is generating:
#include <iostream>
using namespace std;
class A
{
public:
virtual void f(){}
int i;
};
class B : public A
{
public:
B(int i_){i = i_;} //needed
B(){} //needed
void f(){}
};
int main()
{
//these two lines are fixed(needed)
B b;
A & a = b;
//Assignment 1 works
B b1(2);
b = b1;
//But Assignment 2 doesn't works
B b2();
b = b2; // <-- error
}
upon compilation, I get the following error:
$ g++ inher2.cpp
inher2.cpp: In function ‘int main()’:
inher2.cpp:32:10: error: invalid user-defined conversion from ‘B()’ to ‘const B&’ [-fpermissive]
inher2.cpp:14:6: note: candidate is: B::B(int) <near match>
inher2.cpp:14:6: note: no known conversion for argument 1 from ‘B()’ to ‘int’
inher2.cpp:32:10: error: invalid conversion from ‘B (*)()’ to ‘int’ [-fpermissive]
inher2.cpp:14:6: error: initializing argument 1 of ‘B::B(int)’ [-fpermissive]
Can you help me find the problem? thank you

Answer 1

Your "B b2();" is the 'vexing parse' problem of C++ (see here - the 'most vexing parse' takes the ambiguous syntax further).
It looks to the C++ compiler that you are declaring a function (a pre-declaration).
Check it out:
int foo(); //A function named 'foo' that takes zero parameters and returns an int.
B b2(); //A function named 'b2' that takes zero parameters and returns a 'B'.
When you later do:
b = b2;
It looks like you are trying to assign a function (b2) to a variable (b).
To call a constructor with zero parameters, call it without the parentheses and you'll be fine:
B b2;
For more information, see:
Understanding 'most vexing parse' - why allow ambiguous syntax?
Most vexing parse: why doesn't A a(()); work?

Answer 2

B b2();
It is a function declaration, not a variable declaration!
The function name is b2 which takes no argument, and returns object of type B.
Search for vexing parse in C++.