Undefined reference to constructor problem

Question

See the below-given code

#include <iostream>
using namespace std;

class Number
{
    int a;

public:
    Number();

    Number(int num_1) {
        a = num_1;
    }

    void print_number(void) { cout << "Value of a is " << a << endl; }
};

int main()
{
    Number num_1(33), num_3;
    Number num_2(num_1);

    num_2.print_number();
    return 0;
}

In the above code, I am having 2 constructors in the same class but when compiling it,gives me the error

ccnd0o9C.o:xx.cpp:(.text+0x30): undefined reference to `Number::Number()'
collect2.exe: error: ld returned 1 exit status

Can anyone solve this problem? and I still need the 2 constructors but without replacing num_3 with num_3() the main function.

TYPO: change `Number(void);` to `Number(){}` – eyllanesc Sep 21 '21 at 05:46 — eyllanesc, Sep 21 '21 at 05:46

JeJo · Accepted Answer · 2021-09-21T09:02:42.300

5

In your class, you have declared the default constructor, but you have not defined it.

You can default it (since C++11) and you will be good to go:

Number() = default;

Otherwise:

Number() {}

As from @TedLyngmo liked post, both will behave the same, but the class will get different meaning as per the standard, though. More reads here: The new syntax "= default" in C++11

@Jarod42's Comment as side notes: The default constructor make sense when it provide a default value to the member a. Otherwise it will be uninitialized (indeterminate value) and reading them will cause to UB.

edited Sep 21 '21 at 09:02

answered Sep 21 '21 at 05:49

JeJo

30,635
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3

This is the preferable way to do it. See [reasons](https://stackoverflow.com/a/20828970/7582247) – Ted Lyngmo Sep 21 '21 at 06:05
Notice that `= default;` or `{}` is risky here, as `a` would not be initialized, and so `num_3.print_number();` would lead to UB. – Jarod42 Sep 21 '21 at 08:43
1

`Number(int num_1 = 0)` might make sense as alternative. – Jarod42 Sep 21 '21 at 09:04

score 1 · Answer 2 · answered Sep 21 '21 at 05:49

1

Use this code

#include <iostream>

using namespace std;
class Number
{
    int a;

public:
    Number(){};
    Number(int num_1)
    {
        a = num_1;
    }

    void print_number(void) { cout << "Value of a is " << a << endl; }
};

int main()
{
    Number num_1(33), num_3;

    Number num_2(num_1);
    num_2.print_number();

    return 0;
}

answered Sep 21 '21 at 05:49

Aman

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2
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3

Your answer would be more useful if you explained WHY this code is needed. – Remy Lebeau Sep 21 '21 at 06:03
3

People who don't know what's wrong in OPs code will find it hard to spot the change you have made. Answers should be more about explaining problems and solutions than about providing ready-to-use code. – Lukas-T Sep 21 '21 at 06:06
Notice that `{}` (or even `= default;`) is risky here, as `a` would not be initialized, and so `num_3.print_number();` would lead to UB. – Jarod42 Sep 21 '21 at 08:44

Undefined reference to constructor problem

2 Answers2