Difference between (double) and double()

Question

I think this question is not new, but I can not find an answer anywhere.

Is there a difference between (double)myInt and double(myInt) in c++?

No difference in effect. The former is a C-style cast, and is in general disfavor. The latter is a constructor-style cast, and is in general disfavor as well. The generally advocated way is to use a static cast, `static_cast(myInt)` — Eljay, Feb 13 '20 at 13:15
Though it's okay in this case, but please bless others by using `static_cast` instead. Always. — theWiseBro, Feb 13 '20 at 13:21
As the answers say, `double(myInt)` and `(double)myInt` mean the same thing. But with `long double` they're not the same. — Pete Becker, Feb 13 '20 at 14:49

score 5 · Answer 1 · answered Feb 13 '20 at 13:18

5

Is there a difference between (double)myInt and double(myInt) in c++?

Only difference is syntactical. The meaning is the same.

The latter "functional" syntax cannot be used with compound types: int*(x) does not work.

answered Feb 13 '20 at 13:18

eerorika

NutCracker · Accepted Answer · 2020-02-13T13:38:49.623

2

(double)myInt is type conversion from int to double. In modern C++, it is strongly advised to use static_cast<double>(myInt).

double(myInt) calls double's "constructor" which also does the type conversion.

Essentially, they are the same and will result in the same ASM output.

Check the following code:

#include <iostream>

int main() {
    int a = 5;

    double b(a);           // line 1
    double c = (double)a;  // line 2

    return 0;
}

Both lines result with the same ASM:

cvtsi2sd        xmm0, DWORD PTR [rbp-4]
movsd   QWORD PTR [rbp-24], xmm0

Check here

edited Feb 13 '20 at 13:38

answered Feb 13 '20 at 13:19

NutCracker

Compiler Explorer is a nice tool thank you for your link. Are you sure, that `double(myInt)` calls a constructor. I always thought that constructors only exists for classes. And in c++ double is not a class is it? – NelDav Feb 13 '20 at 13:35
1

@NelDav built-in types like `double` don't have constructors but this type of [value initialization](https://en.cppreference.com/w/cpp/language/value_initialization) reminds on constructor. Check another answer [here](https://stackoverflow.com/a/5113429/5517378) – NutCracker Feb 13 '20 at 13:38

2 Answers2