Fail to initialise a pointer inside a class constructor scope

Question

In c++ 17 when attempting to initialize a member pointer through the scope (not in the member initializer list) of the non-default constructor an error is thrown, as stated below:

error: expression cannot be used as a function

Why is this invalid? I understand from the error that the pointer has been implicitly initialized, hence the constructor cannot be called. I have read the relevant passage from the standard and it's still unclear, as it states that it should have indeed not been initialised yet and therefore my code should be valid.

class Container{
    private:
        int length;
        double* data;
    public:
        Container(): length(0), data(nullptr) {}

        Container(const int length): length(length) {data(nullptr)}
};

int main()
{
  Container a(2);
}

What do you consider to be the relevant passage from the standard? — molbdnilo, Dec 02 '19 at 12:23
Why did you decide to put the initialisation of _one_ of your members randomly inside a `{}`? You got it right in the overload immediately above. Take more care. — Lightness Races in Orbit, Dec 02 '19 at 12:35
@molbdnilo The accepted answer from [this post](https://stackoverflow.com/questions/1144264/c-member-pointer-initialised) — EmVee, Dec 03 '19 at 13:17

score 9 · Accepted Answer · answered Dec 02 '19 at 12:21

9

Initialization can only be in the constructor initialization list. In the function body data(nullptr) is parsed as a function call (as told by the compiler in the message).

If you want to set a value inside the function body you must use plain assignment:

data = nullptr;

answered Dec 02 '19 at 12:21

Some programmer dude

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That explains it, thank you. Is this because the pointer has already been implicitly initialized in the initialization list? – EmVee Dec 02 '19 at 12:24
1

@EmVee no it is because `some_variable(some_value);` is wrong syntax, unless `some_variable` can be called like a function – 463035818_is_not_an_ai Dec 02 '19 at 12:28
@EmVee Also there's no "implicit initialization" of variables. Until the assignment the value of `data` will be *indeterminate*. – Some programmer dude Dec 02 '19 at 12:31

score 3 · Answer 2 · answered Dec 02 '19 at 12:18

3

This:

    Container(const int length): length(length) {data(nullptr)}

Should be:

    Container(const int length): length(length), data(nullptr) {}

answered Dec 02 '19 at 12:18

John Zwinck

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Reason for my initialization structure is that there will be conditional statements within the constructor scope, the snipped I showed was to simplify the question. My question is why initialising it within the scope is invalid. Thanks! – EmVee Dec 02 '19 at 12:20
2

Because it is not valid syntactical element. `data` is not a function and cannot be used like `data(whatever)`. Member initializer list uses `member-name(init-value)` which is very specific and cannot be used elsewhere. – Tanveer Badar Dec 02 '19 at 12:23
1

Don't show otherwise-broken code "to simplify the question". Show a real, actual, concrete question, so as to avoid completely wasting our time. – Lightness Races in Orbit Dec 02 '19 at 12:35
@EmVee There is a notion here that you should present _a concrete problem_ so that we do not go in circles chasing ghosts. This is not "attitude", we get thousands of questions a day and you are _competing_ for our free time, so it's in your best interests to make it as easy as possible to give you the best help possible. If that's not a mission you're interested in, that's your choice! In this case, it would have been easier to understand what you were doing had you included this detail about conditionals to begin with; the solution in this answer does not actually allow for that..... – Lightness Races in Orbit Dec 03 '19 at 13:59

Fail to initialise a pointer inside a class constructor scope

2 Answers2