Cast derived class to base

Question

What type of cast takes place here (in B::get())?

class A {
public:
    A() : a(0) {}
    int a;
};

class B : public A {
public:
    A* get() {
        return this; //is this C-style cast?
    }
};

int main()
{
    B b;
    cout << b.get()->a << "\n";

    system("pause");
    return 0;
}

I've seen this kind of code in a famous API. Is it better practice to do static_cast<A*>(this);?

There's no casting at all, only an implicit conversion. A cast is an explicit conversion. — molbdnilo, Dec 14 '15 at 14:06

TartanLlama · Accepted Answer · 2015-12-14T14:15:27.743

13

This is a standard derived-to-base pointer conversion. The rules are that a pointer to D with some const/volatile qualifications can be converted to a pointer to B with the same qualifiers if B is a base class of D.

The standard conversions are implicit conversions with built-in meanings and are separate concepts to things like static_cast or C-style casts.

Generally it's best to rely on implicit conversions when you can. Explicit conversions add more code noise and may hide some maintenance mistakes.

edited Dec 14 '15 at 14:15

answered Dec 14 '15 at 14:05

TartanLlama

63,752
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Strictly, "can be converted to a pointer to `B` with the same *or more* qualifiers" – Martin Bonner supports Monica Dec 14 '15 at 14:09
3

I suggest specifically answering his question: "No, it is not better practise to use `static_cast` - in fact, it is worse practise. Always avoid casts if you don't need them." – Martin Bonner supports Monica Dec 14 '15 at 14:11
@MartinBonner Disagree! Implicit casts are the source of all sorts of wacky bugs. Sometimes I wish C++ didn't even have them at all... Nothing wrong with being explicit. – Barry Dec 14 '15 at 14:34
@Barry are you referring to standard conversions or user-defined conversion operators or both? – TartanLlama Dec 14 '15 at 14:35
@Barry can you give an example of a bug caused by implicit cast? – McLovin Dec 14 '15 at 14:36
I'd prefer the phrasing "implicit conversions". The trouble is that they are still there, but static_cast gives you the opportunity to be explicitly wrong. – Martin Bonner supports Monica Dec 14 '15 at 14:36
@Barry Also, shouldn't the compile error out if you are about to make an implicit mistake? – McLovin Dec 14 '15 at 14:38
Some kinds of implicit conversions are indeed error-prone, one famous example being the signed-to-unsigned promotion on comparisons. Many tools warn on such conversions, and a cast to alert the reader is a good idea. For a derived-to-base conversion i agree that the cast does more harm than good. If a coding convention requieres a cast here, `static_cast` is the way to go, but i would argue that the convention is broken. – Michael Karcher Dec 14 '15 at 15:57

score 3 · Answer 2 · answered Dec 14 '15 at 14:05

3

It is implicit conversion to ancestor. Implicit conversions are generally safe and they cannot do stuff static_cast cannot do. They actually even more restricted: you can do an unchecked downcast with static_cast, but not with implicit conversion.

answered Dec 14 '15 at 14:05

Revolver_Ocelot

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Cast derived class to base

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