Simple Constructor in C++

Question

I've downloaded simple code with constructors. Could anyone explain me this part of code:

class Myclass
{
private:
    int x;
    double *y;                      

public:
    Myclass(int X);              

};

Myclass::Myclass(int X) : x(X)
{
y = new double[x];
}

I don't understand "Myclass::Myclass(int X) :x(X)". Couldn't it be written simpler?

See http://stackoverflow.com/questions/1711990/what-is-this-weird-colon-member-syntax-in-the-constructor — Ilya Kobelevskiy, Apr 15 '15 at 16:14
@AdamMadamski get used to that syntax. For your `int` member is trivial, but there are entities in C++ where it is required (const members, reference members, non-default-constructibles, etc). It is fundamentally more efficient to construct-as-wanted rather then default-construct and assign-as-wanted. Don't try to avoid it: *embrace it*. — WhozCraig, Apr 15 '15 at 16:16

Mike Seymour · Answer 1 · 2015-04-15T16:46:20.157

4

I guess from the bold face that it's just the : x(X) part you don't understand.

That's the initialiser list, used to initialise member variables. In this case, it initialises the variable x to have the same value as the constructor parameter X. y is not included in the list, so it's left uninitialised at that point, then assigned a value in the constructor body.

There's no simpler way to initialise a member variable (in a class with a non-trivial constructor).

edited Apr 15 '15 at 16:46

answered Apr 15 '15 at 16:15

Mike Seymour

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Arguably, aggregate initialisation is simpler ;) – Lightness Races in Orbit Apr 15 '15 at 16:42
@LightningRacisinObrit: True, but not for this class since it needs a non-trivial constructor. – Mike Seymour Apr 15 '15 at 16:45
Right, but you didn't constrain your summary! hehe – Lightness Races in Orbit Apr 15 '15 at 16:46

score 4 · Answer 2 · edited May 23 '17 at 12:14

You can't write it any simpler without altering the effect of the code.

You could write it this way:

Myclass::Myclass(int X)
{
    x = X;
    y = new double[x];
}

But then it wouldn't be an initialization of x, it would be an assignment to x.

Each member variable is initialized before running the constructor body (the part between { and }), in the so-called member initialization list between the : and the {.

As a side note, it would be better to initialize y in the member initialization list as well, rather than assigning to it in the constructor body:

Myclass::Myclass(int X) : x(X), y(new double[x])
{
}

Try declaring the member as "const int x;", and then you'll see the difference between initialization and assignment. — Dan Korn, Apr 15 '15 at 16:27

Simple Constructor in C++

2 Answers2