How to set char* to NULL using default constructor in C++?

Question

I am trying to set this char* to NULL using a default constructor. But I am facing trouble. The problem is with the line where I am using a default constructor and setting m_name=NULL;

class Car {
    char *m_name;
    int m_carnum;
public:
    Car() :m_carnum(0), m_name = NULL {} //this line has the error
    ~Car(){}
};

While this code seems to be working:

Car() :m_carnum(0) {
    m_name = NULL;
}

So I why isn't this working ?

m_name = NULL;

If you have a problem, **EXPLAIN YOUR PROBLEM**. And read [ask] — Amit, Mar 13 '16 at 20:06
You say `m_name = NULL;` is working. You then ask why `m_name = NULL;` isn't working. That doesn't make sense. You don't have `m_name = NULL;` in your non-working version, you just have `m_name = NULL`. Not that it would work if you inserted a semicolon there, but it's important to be precise. The compiler is, and if you're not, you're sure to get a lot of other problems in the future. — , Mar 13 '16 at 20:11

score 1 · Answer 1 · edited May 23 '17 at 12:15

1

Instantiate it like your int. You also need to make sure you instantiate member variables in the order they are declared (see Why should I initialize member variables in the order they're declared in?)

Car() : m_name(0), m_carnum(0) 
{}

edited May 23 '17 at 12:15

Community

1
1

answered Mar 13 '16 at 20:04

Ryan

1,797
12
19

1

Nice answer. But some explanation on this advice `make sure you instantiate member variables in the order they are declared` would be nice. – HolyBlackCat Mar 13 '16 at 20:08

score 1 · Accepted Answer · edited May 23 '17 at 11:45

1

Given that you are dealing with a pointer, the following might be advisable (considered good practise) if using C++ 11 and beyond:

Car() : m_name(nullptr), m_carnum(0) {}

The reasoning behind this is outlined here (among other places with a quick search): What are the advantages of using nullptr?

edited May 23 '17 at 11:45

Community

1
1

answered Mar 13 '16 at 20:54

berilac

48
1
7

Good practice would be not using `char *` to store a name – M.M Mar 13 '16 at 21:03
@berilac Thank you for a great answer. – Isan Rivkin Mar 13 '16 at 21:20
@M.M fair point. Might you recommend `std::string`? Is it still reasonable to consider `char*` for variable length names; or would it be better to allocate `char []` of a set size if you know the names will never be larger? – berilac Mar 13 '16 at 21:32
`std::string` is certainly the simplest option – M.M Mar 13 '16 at 21:33

How to set char* to NULL using default constructor in C++?

2 Answers2