Hello 🙌🏻 i have a question why such thing is allowed

Question

Hello 🙌🏻 i have a question why such thing is allowed

in c++ ? (calling method on a temporary object)

struct myStruct {
void method(){}
};
int main() {
myStruct{}.method();
return 0;
}

my analyze:
myStruct is temp obj and an rvalue so you can't take address of it to pass through methods that have this pointer which just refers to an lvalue kind

0

11.06.2023

15 ответов

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Mehrad Автор вопроса

Azadi
Right. A temp object is called temporary since it ...

yeah referencing is allowed but with an const lavlue reference my question is impilcit this is a pointer not a reference so you bind an rvalue it your example would like this void foo(const int* r) { std::cout << r; } int main() { foo(10); // not allowed }

0

11.06.2023

Mehrad Автор вопроса

Mehrad
yeah referencing is allowed but with an const lavl...

so why is not like this and can accept (its temp object)?

0

11.06.2023

Azadi

Mehrad
yeah referencing is allowed but with an const lavl...

10 is a value not an address!

0

11.06.2023

Mehrad Автор вопроса

Azadi
10 is a value not an address!

but "this" is a pointer so how can it take a value ? (myStruct{})

0

11.06.2023

Azadi

Mehrad
but "this" is a pointer so how can it take a value...

Your temp object myStruct{} has the implicit this pointer but the object is not stored in memory to get an actual address to be able to take and use it.

0

11.06.2023

Pavel

Azadi
Right. A temp object is called temporary since it ...

I don't think temporary doesn't have an address (can't find such a rule), it's more like explicitly taking this address doesn't make sense in most cases (so the compilers issue a warning for doing that)

0

11.06.2023

Azadi

Pavel
I don't think temporary doesn't have an address (c...

Not sure where temp objects are temporarily stored. Probably standard hasn't mandated using memory (parts developers are dealing with them mostly, such as heap, stack or static) for them. In what code does a compiler issue a warning when you take the address of a temp object?

0

11.06.2023

Manav | avoid unnecessary messaging me

Value categories are expressions, when you talk about those you're actually talking about the process of code being generated by compiler. They're not as concrete as the underlying machine instructions, that's why you have entities on which addressof operator fails, like in your case if you do &(mystruct{})

0

11.06.2023

Pavel

Azadi
Not sure where temp objects are temporarily stored...

Memory-wise it doesn't really matter if the object is temporary or if it's a normal named variable. Both can be stored on the stack or go directly to registers, depending on what compiler decides the best in the specific case, so there shouldn't be differences. What code? Here was an example https://stackoverflow.com/questions/2280688/taking-the-address-of-a-temporary-object

0

11.06.2023

Manav | avoid unnecessary messaging me

Pavel
Memory-wise it doesn't really matter if the object...

This exactly, for instance literals are usually stored in the data section of binary generated. But addressof operator can't be used on them.

0

11.06.2023

Azadi

Pavel
Memory-wise it doesn't really matter if the object...

I agree to some extent that's why I talked about standard not implementations and about parts of memory and not registers. What is clear is taking the address of a temp object is not allowed. Both MSVC and GCC issue an error for that line of code in that link.

0

11.06.2023

Mehrad Автор вопроса

Manav | avoid unnecessary messaging me
Value categories are expressions, when you talk ab...

based on your speech, why can't golang handle this but c++ can ? package main type myStruct struct {} func (ms *myStruct) function() {} func main () { myStruct{}.function() } ./main.go:11:13: cannot call pointer method function on myStruct

0

15.06.2023

Manav | avoid unnecessary messaging me

Mehrad
based on your speech, why can't golang handle this...

Well C++ and go have different grammar and implementation. Also I don't know much about go to even answer properly why it can't do that.

0

15.06.2023

Manav | avoid unnecessary messaging me

Mehrad
based on your speech, why can't golang handle this...

I guess can explain why I said that value categories are expressions and they're not as concrete as underlying machine instructions. I quickly wrote this, https://godbolt.org/z/3WsfvxKE5 You can see here that on x86-64 there's no difference between the different functions their calls and how values are passed to them when. It doesn't matter in this case as the notion of temporaries, rvalue, lvalue references dilute at this state. For it, simply they are all on stack, and their pointers can be passed for successful member function call.

0

15.06.2023

Azadi · Accepted Answer

Azadi

Right. A temp object is called temporary since it doesn't have an address. Referencing is possible, however.

0

11.06.2023

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