Say I have a struct that looks like this (a POD):
struct Foo
{
int i;
double d;
};
What are the differences between the following two lines:
Foo* f1 = new Foo;
Foo* f2 = new Foo();
The first one leaves the values uninitialised; the second initialises them to zero. This is only the case for POD types, which have no constructors.
I suppose nothing at all. Foo()
is allowed, even if it makes no sense... I've tried to change struct
into class
and tried a diff on the generated exe, and they resulted to be the same, meaning that a class without method is like a struct from a practical and "effective" point of view.
But: if you use only one of the alternative, keeping struct
or class
no matter, it happens that new Foo
and new Foo()
gives executables which differ! (At least using g++) I.e.
struct Foo { int i; double d; } int main() { Foo *f1 = new Foo; delete f1; }
is compiled into somehing different from
struct Foo { int i; double d; } int main() { Foo *f1 = new Foo(); delete f1; }
and the same happens with class
instead of struct
. To know where the difference is we should look at the generated code... and to know if it is a g++ idiosincracy or not, I should try another compiler but I have only gcc and no time now to analyse the asm output of g++...
Anyway from a "functional" (practical) point of view, it is the same thing.
Add
At the end it is always better to know or do deeper investigation for some common human problems on Q/A sites... the only difference in the code generated by g++ in () and no () cases,
movl $0, (%eax) fldz fstpl 4(%eax)
which is a fragment that initializes to 0/0.0 the int and the double of the struct... so Seymour knows it better (but I could have discovered it without knowing if I had taken a look at the asm first!)
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