trouble with some syntax of C++

Question

    #include<iostream>
using namespace std;

class A {
 public:
    A(int ii = 0) : i(ii) {}
    void show() { cout << "i = " << i << endl;}
 private:
    int i;
};

class B {
 public:
    B(int xx) : x(xx) {}
    operator A() const { return A(x); }
 private:
    int x;
};

void g(A a)
{  a.show(); }

int main() {
  B b(10);
  g(b);
  g(20);
  getchar();
  return 0;
}

In the above code can anyone explain what does the line

A(int ii = 0) : i(ii) {}

mean and how the output of the program

i = 10
i = 20

Assuming it's the colon part you're confused about, http://stackoverflow.com/questions/1711990/what-is-this-weird-colon-member-syntax-in-the-constructor — chris, Sep 13 '14 at 18:19
...and if it's `int ii = 0`: http://en.wikipedia.org/wiki/Default_argument — Ivan Aksamentov - Drop, Sep 13 '14 at 18:22
Time for a C++ tutorial/book [[suggested link](http://www.cplusplus.com/doc/tutorial/)] — 101010, Sep 13 '14 at 18:22
.. or: [The Definitive C++ Book Guide and List](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — Ivan Aksamentov - Drop, Sep 13 '14 at 18:23
It's good example of bad variable names, which is probably the actual reason for the confusion. — Sceptical Jule, Sep 13 '14 at 19:24

score 2 · Answer 1 · answered Sep 13 '14 at 18:25

2

A(int ii = 0) : i(ii) {}

is class A's constructor, and constructor has a value ii, and initializing i to ii.

Output

B b(10);
g(b);
g(20);

in this code, g(20) is pass temporary instance of class B.

because, the class B's constructor is not explicit so int to B is OK.

answered Sep 13 '14 at 18:25

Joy Hyuk Lee

716
9
22

Sorry sir, did not understand why g(20) is not an error ?? – iec2011007 Sep 13 '14 at 18:29
You need to understand c++ constructor. B'c constructor's parameter is int. So you pass int, than compiler can find B's Constructor. And automacally call B's constructor. – Joy Hyuk Lee Sep 13 '14 at 18:30
But A's Constructor also has int as the parameter ?? – iec2011007 Sep 13 '14 at 18:31
Oops. My mistake. But also class A's constructor has int. so if you pasa int, compiler will call A's conatructor. – Joy Hyuk Lee Sep 13 '14 at 18:33
Just one more question what does the line operator A() const { return A(x); } mean ?? and why can g function accept and object of class B ?? – iec2011007 Sep 13 '14 at 18:34
2

operator mean Implicitlly converting B to A. So, when you pasa B class to g function, B's converting functions has called. So you can use B instance. PS. I'm not sir i just 14 years old programmer: ) – Joy Hyuk Lee Sep 13 '14 at 18:44
Is this answer helpful? Than accept this answer please! – Joy Hyuk Lee Sep 13 '14 at 19:02

score 0 · Answer 2 · answered Sep 13 '14 at 18:22

This is declaring the constructor to class A

A(int ii = 0) : i(ii) {}

This part declares that the constructor can take a single int parameter, that has a default value of 0.

A(int ii = 0)

This part is initializing the member variable i with the value from the parameter ii.

i(ii)

score 0 · Answer 3 · answered Sep 13 '14 at 18:23

A(int ii = 0) : i(ii) {}

is the constructor of class A and it takes an argument ii, which in the initializer list, is assigned to i. Then the body of it does nothing.

Equivalently you have:

A(int ii = 0) : i(ii) {
 // do nothing
}

Equivalently -in terms of result- you have:

A(int ii = 0) {
  i = ii;
}

trouble with some syntax of C++

3 Answers3