Constructor issue

Question

Possible Duplicate:
Default constructor with empty brackets

This is the code that I worked on and I don't understand what it's happening on constructor Package obj2(); On output are displayed only the values 4 (Package obj1(4)) and 2 (Package obj3(2))

#include <iostream>
using namespace std;
class Package
{
private:
    int value;
public:
    Package()
    {
        cout<<"constructor #1"<<endl;
        value = 7; cout << value << endl;

    }
    Package(int v)
    {
        cout<<"constructor #2"<<endl;
        value = v; cout << value << endl;

    }
    ~Package()
    {
        cout<<"destructor"<<endl;
        cout << value << endl;
    }
};

int main()
{
    Package obj1(4);
    Package obj2();
    Package obj3(2);

}

score 8 · Answer 1 · answered Nov 06 '12 at 17:25

8

This does not declare an object:

Package obj2();

Believe it or not, it declares a function that returns a Package object. It's called "the most vexing parse."

answered Nov 06 '12 at 17:25

Fred Larson

60,987
18
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hyde · Accepted Answer · 2012-11-06T17:53:18.023

4

Line

Package obj2();

needs to be

Package obj2;

More info

http://www.parashift.com/c++-faq/empty-parens-in-object-decl.html

or, alternative take on this (from Google cache, real site was down, and take it with a grain of salt, it raises good points but does its best to make them sound worse than they are):

http://webcache.googleusercontent.com/search?q=cache:http://yosefk.com/c%2B%2Bfqa/ctors.html#fqa-10.2

edited Nov 06 '12 at 17:53

answered Nov 06 '12 at 17:28

hyde

60,639
21
115
176

I gave the reason, you gave the fix! – Fred Larson Nov 06 '12 at 17:30
Don't need `()` when calling default contructor – Emmanuel N Nov 06 '12 at 17:32
1

@EmmanuelN it's not just "don't need", it's "must not use". – hyde Nov 06 '12 at 17:44
So how does he initialize the constructor parameter? – David G Nov 18 '12 at 17:12
@David default constructor call does not, by definition, have any parameters (default constructor can take parsmeters though, as long as all have default value). – hyde Nov 18 '12 at 21:23

score 4 · Answer 3 · answered Nov 06 '12 at 17:30

4

If you are using C++11, and want to solve the 'most vexing parse' problem, you can replace

Package obj2();

with

Package obj2{};

This is part of the uniform initialization syntax of C++11, which was designed primarily to get around this problem.

answered Nov 06 '12 at 17:30

Robin McCorkell

744
6
22

Constructor issue

3 Answers3