MESSAGE
| DATE | 2016-11-21 |
| FROM | ruben safir
|
| SUBJECT | Subject: [Learn] Fwd: Re: the new {} syntax
|
From learn-bounces-at-nylxs.com Mon Nov 21 04:56:20 2016
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Subject: [Learn] Fwd: Re: the new {} syntax
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-------- Forwarded Message --------
Subject: Re: the new {} syntax
Date: Mon, 21 Nov 2016 10:10:23 +0200
From: Paavo Helde
Newsgroups: comp.lang.c++
References:
On 21.11.2016 6:33, ruben safir wrote:
> On 11/20/2016 11:06 PM, Louis Krupp wrote:
>> On Mon, 21 Nov 2016 03:10:01 +0000 (UTC), Popping mad
>> wrote:
>>
>>> why doesn't this enter the second constructor?
>>>
>>> 18 #include
>>> 19
>>> 20 class parent{
>>> 21 public:
>>> 22 parent():_something{nullptr}, _val{0} {
>>> 23 std::cout << "null constructor\n";
>>> 24 };
>>> 25 parent(parent *in):_something{in}, _val{10} {
>>> 26 std::cout << "here\n";
>>> 27 _something->_val = 0;
>>> 28 };
>>> 29 parent * _something;
>>> 30 int _val;
>>> 31 };
>>> 32 void discover(parent * in){
>>> 33 std::cout<< in->_val << std::endl;
>>> 34 std::cout<< in->_something->_val << std::endl;
>>> 35 }
>>> 36
>>> 37 int main(int argc, char **argv)
>>> 38 {
>>> 39 parent * tmp = new parent;
>>> 40 parent * tmp2{tmp};
>>> 41 //parent * tmp2 = new parent(tmp);
>>> 42 discover(tmp2);
>>> 43
>>> 44
>>> 45
>>> 46 return EXIT_SUCCESS;
>>> 47 }
>>> [ruben-at-flatbush max_path]$ g++ -Wall test.cpp
>>> [ruben-at-flatbush max_path]$ ./a.out
>>> null constructor
>>> 0
>>> Segmentation fault
>>
>> Because "parent * tmp2{tmp}" isn't constructing a new parent object;
>> if I'm not mistaken, tmp2 is going to be of type parent * and a copy
>> of tmp.
>>
>> Try this:
>>
>> 1 #include
>> 2
>> 3 class parent{
>> 4 public:
>> 5 parent():_something{nullptr}, _val{0} {
>> 6 std::cout << "null constructor\n";
>> 7 };
>> 8 parent(parent *in):_something{in}, _val{10} {
>> 9 std::cout << "here\n";
>> 10 _something->_val = 0;
>> 11 };
>> 12 parent * _something;
>> 13 int _val;
>> 14 };
>> 15 void discover(parent * in){
>> 16 std::cout<< in->_val << std::endl;
>> 17 std::cout<< in->_something->_val << std::endl;
>> 18 }
>> 19
>> 20 int main()
>> 21 {
>> 22 parent * tmp = new parent;
>> 23 parent tmp2{tmp};
>> 24 //parent * tmp2 = new parent(tmp);
>> 25 discover(&tmp2);
>> 26
>> 27 return 0;
>> 28 }
>>
>> Louis
>>
> Right but there seems to be no way, without calling new, to create an
> anonymous object which is pointed to by parent * myvar
You are about right, if I understand correctly what you mean by
"anonymous", and if you want the object to stay alive for longer term.
This is because in C++ the object lifetimes are deterministic. Either
you create the object on stack (automatic storage duration) and then it
needs a named object or at least a const reference to keep it alive, or
alternatively, you create an object via new or equivalent (dynamic
storage duration) and then you need a pointer for accessing and finally
destroying the object.
The nearest you can get to "a non-newed object and pointer" is "a
non-newed object and a const reference". There is a special rule that a
const reference keeps a temporary object alive during the lifetime of
the reference.
#include
class parent{
public:
parent():_something{nullptr}, _val{0} {
std::cout << "null constructor\n";
};
parent(parent *in):_something{in}, _val{10} {
std::cout << "here\n";
_something->_val = 0;
};
parent * _something;
int _val;
};
void discover(const parent * in){
std::cout<< in->_val << std::endl;
std::cout<< in->_something->_val << std::endl;
}
int main()
{
parent * tmp = new parent;
const parent& tmp2 = parent {tmp};
discover(&tmp2);
return 0;
}
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