note: 'person::person()' is implicitly deleted because the default definition would be ill-formed

Question

I'm working on an example program to help me learn structs in C++. Here's my code:

#include <stdio.h>
#include <iostream>
#include <string>

using namespace std;

int nextPersonID = 0;
int nextAddressID = 0;

struct date {
    int day;
    int month;
    int year;
};

struct address {
    int id;
    string address;
    date effectiveDate;
    date expirationDate;
};

struct person {
    int id;
    string name;
    date birthdate;
    const int numberOfAddresses;
    address addresses [1];
};

int main () {
    person bob;
    bob.name = "Bob";
    bob.id = nextPersonID;
    nextPersonID++;
    bob.birthdate.day = 1;
    bob.birthdate.month = 1;
    bob.birthdate.year = 1990;
    bob.numberOfAddresses = 1;
    bob.addresses[0].address = "31415 E. Pi Blvd.";
    bob.addresses[0].id = nextAddressID;
    nextAddressID++;
    bob.addresses[0].effectiveDate.day = 1;
    bob.addresses[0].effectiveDate.month = 1;
    bob.addresses[0].effectiveDate.year = 1990;
    bob.addresses[0].expirationDate.day = 1;
    bob.addresses[0].expirationDate.day = 1;
    bob.addresses[0].expirationDate.day = 2020;
    cout << bob.name;
}

But when I try to compile, it fails with note: 'person::person()' is implicitly deleted because the default definition would be ill-formed.. Here's my build log:

-------------- Build: Debug in DataStructures (compiler: GNU GCC Compiler)---------------

mingw32-g++.exe -Wall -g -std=c++11 -I"C:\Program Files (x86)\CodeBlocks\MinGW_Dev_Libs\include\SDL2" -c C:\Users\dxs\Documents\C++\Challenges\DataStructures\DataStructures.cpp -o obj\Debug\DataStructures.o
C:\Users\dxs\Documents\C++\Challenges\DataStructures\DataStructures.cpp: In function 'int main()':
C:\Users\dxs\Documents\C++\Challenges\DataStructures\DataStructures.cpp:32:12: error: use of deleted function 'person::person()'
C:\Users\dxs\Documents\C++\Challenges\DataStructures\DataStructures.cpp:23:8: note: 'person::person()' is implicitly deleted because the default definition would be ill-formed:
C:\Users\dxs\Documents\C++\Challenges\DataStructures\DataStructures.cpp:23:8: error: uninitialized non-static const member 'const int person::numberOfAddresses'
C:\Users\dxs\Documents\C++\Challenges\DataStructures\DataStructures.cpp:39:29: error: assignment of read-only member 'person::numberOfAddresses'
Process terminated with status 1 (0 minute(s), 1 second(s))
3 error(s), 0 warning(s) (0 minute(s), 1 second(s))

I can't find anything with Google relating to my problem. Any ideas? I'm using Code::Blocks with g++.

From your own error: `uninitialized const member 'const int person::numberOfAddresses'` — Drew Dormann, Jul 02 '15 at 23:18
You must initialize a const with a value at the same time you declare it: const int x = 0; NOT const x; x = 0; — SmugDoodleBug, Feb 17 '18 at 13:18

AnT stands with Russia · Accepted Answer · 2015-07-02T23:56:21.300

37

Well, the problem is not with that "note". The "note" simply explains the reason for the error. The error is that you are trying to default-construct your person object when class person does not have a default constructor.

Instead of trying to default-construct it, you can {}- initialize that const member and the code will compile

person bob = { nextPersonID++, "Bob", {}, 1 };
bob.birthdate.day = 1;
bob.birthdate.month = 1;
bob.birthdate.year = 1990;
...

Alternatively, you can simply write your own default constructor for the class.

edited Jul 02 '15 at 23:56

answered Jul 02 '15 at 23:34

AnT stands with Russia

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Okay, thanks. But one other thing, the reason I was using a const in the first place was to try to allow for a user to input the array size. Would this solution allow me to do that? – Dessa Simpson Jul 03 '15 at 00:14
@VirtualDXS: Well, it would, if you do it properly. But `const` has nothing to do with facilitating "user input". Frankly, I see no reason at all for that field to be `const`. – AnT stands with Russia Jul 03 '15 at 00:17
@VirtualDXS: "The same error"? I don't see such error in your original post. What "use" is it in this case? – AnT stands with Russia Jul 03 '15 at 00:31
@VirtualDXS: By "what use" I mean: what specific line of code triggers this error? – AnT stands with Russia Jul 03 '15 at 01:18
Thanks for the hint! The `const` member triggered this issue – MewX Mar 26 '18 at 15:51
Regarding: "Okay, thanks. But one other thing, the reason I was using a const in the first place was to try to allow for a user to input the array size"... you could either do it with a special constructor as suggested, or initialize the constant with an initializer list at construction. Eg: constructor Person() : numAddresses(1), or some override. Remember though, you are stuck with that value! – Quinn Carver May 08 '19 at 16:48

Quinn Carver · Answer 2 · 2022-02-08T15:13:59.593

The problem has not to do with the default-constructor. There is a constant in the class declaration but the default (no argument) constructor does not guarantee that the constant will be defined. Suggest using an "initializer list".

struct Person {
        int id;
        string name;
        date birthdate;
        const int numberOfAddresses;
        address addresses [1];
    
        Person(int); // constructor declaration
        Person() : numberOfAddresses(1) {} // constructor definition.
                      // ": numberOfAddresses(1)" is the initializer list
                      // ": numberOfAddresses(1) {}" is the function body
    };
    Person::Person(int x) : numberOfAddresses(x) {} // constructor definition. ": numberOfAddresses{x}" is the initializer list
    int main()
    {
        Person Bob; // calls Person::Person()
        Person Shurunkle(10); // calls Person::Person(int)
    }

note: 'person::person()' is implicitly deleted because the default definition would be ill-formed

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