Initialise const int and object with that const int in the same constructor

Question

I have an object that is created in a .h file that should be initialised in the constructor. This object is passed a COM port number that is 5 in our current application. For this reason I have created a const int in the .h file.

Edit: I added a more complete example

class ClassB
{
public:
    ClassB(int comPort);
private:
    int m_comPort;
};

ClassB::ClassB(int comPort) :
    m_comPort(comPort)
{
}

class ClassA
{
public:
    ClassA();
private:
    const int comPort;
    ClassB B;
};

ClassA::ClassA() :
    comPort(5),
    B(comPort)
{
}

int main()
{
    ClassA A;
    return 0;
}

Because the object is initialised before comPort is fully initialised, the value for comPort is garbage.

What would be the correct way to circumvent this? I can think of the following:

Initialise the const int in the header file
Create and initialise the object in the body of the constructor
use a #define

If `comPort` is defined in the class before `B` then it wouldn't be garbage. There isn't enough information here to answer your question or reproduce the problem. — wally, Jan 16 '17 at 15:46
I do not see how you are getting garbage here. The code looks correct. — NathanOliver, Jan 16 '17 at 16:00
Sorry I missed the const in the hurry of providing a proper example, should be fixed now. — YouKnowNothingJohn, Jan 16 '17 at 16:02
The example is correct, can you be more specific as to what you are asking? Are you asking for other ways of achieving the same thing? — François Andrieux, Jan 16 '17 at 16:03
It should still work just fine. Is it the `m_comPort` that you see the issue with? — NathanOliver, Jan 16 '17 at 16:03
Strange, I do not get garbage in my test program but I get it in my actual code. Might be something else in play. Indeed like you both say the code seems fine the way it is. I will check the main code again. @FrançoisAndrieux — YouKnowNothingJohn, Jan 16 '17 at 16:06
@YouKnowNothingJohn Are you sure you declare `comPort` before `B` in `classA`? — François Andrieux, Jan 16 '17 at 16:08
@FrançoisAndrieux Ah, you mean in the header file? That could be it. Maybe it is switched in the actual code. — YouKnowNothingJohn, Jan 16 '17 at 16:09

score 1 · Answer 1 · answered Jan 16 '17 at 16:14

It seems like a problem with the order your members are initialized. Class members are initialized in the order they are declared. The order they are initialized in the constructor does not override this. In the next example bar::my_foo is initialized before bar::my_const, which will cause problems. my_foo will be initialized with an unitialized my_const member;

struct foo {
    foo(int p_x) : x(p_x) {}
    int x;
}

struct bar {
    bar() : my_const(5), my_foo(my_const) {}
    foo my_foo;
    const int my_const;
}

The problem can be fixed by changing order the members are declared.

struct bar {
    bar() : my_const(5), my_foo(my_const) {}
    const int my_const; // my_const before my_foo
    foo my_foo;
}

A less brittle approach would be to not rely on the order at all. `bar(const int var = 5) : my_const(var), my_foo(var) {}` gets rid of any dependency. — NathanOliver, Jan 16 '17 at 16:18

score 0 · Accepted Answer · edited May 23 '17 at 12:24

0

You can reproduce your error if you exchange declaration of comPort and B in definition of ClassA. See this comment on SO concerning Constructor initialization-list evaluation order.

Hence, make sure that if an initializer list relies on a specific order of evaluation, then the declaration of the members to be initialized must adhere to this order.

edited May 23 '17 at 12:24

Community

1
1

answered Jan 16 '17 at 16:11

Stephan Lechner

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As Francois mentioned in the comments, I believe this was the culprit! – YouKnowNothingJohn Jan 16 '17 at 16:14

Initialise const int and object with that const int in the same constructor

2 Answers2