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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: Initialization of std::vector<std::string> throws
Date: Wed, 18 Sep 2019 08:37:31 -0700
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Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> writes:

> Ralf Goertz wrote:
>
>> Am Wed, 04 Sep 2019 08:20:51 -0400
>> schrieb Sam <sam@email-scan.com>:
>>
>>> Ralf Goertz writes:
>>>
>>>> why does the declaration
>>>>
>>>> std::vector<std::string> pt({{"One","Two"}});
>>>>
>>>> throw:
>>>>
>>>> terminate called after throwing an instance of 'std::length_error'
>>>>   what():  basic_string::_M_create
>>>
>>> Because this is undefined behavior.
>>>
>>>> whereas neither
>>>>
>>>> std::vector<std::string> pt({{"One"}});
>>>>
>>>> nor
>>>>
>>>> std::vector<int>  pt({{1,2}});
>>>
>>> If you actually examine the end result the last declaration, you will
>>> discover, to your surprise, a std::vector<int> with just one value,
>>> and that would hopefully be a big honking clue as to what this is
>>> doing.
>>
>> Hm, I did that and it failed to surprise me.
>>
>>
>> #include <vector>
>> #include <string>
>> #include <iostream>
>>
>> std::vector<int>  pt({{1,2}});
>> int main() {
>>     for (auto p:pt) std::cout<<p<<std::endl;
>> }
>>
>> gives:
>> 1
>> 2
>
> my guess is the above pt declarator is roughly equivalent to
> pt(vector<int>(initializer_list<int>(initializer_list<int>(1,2)))) --
> which just happens to give you an expected result.  I think it is
> undefined or implementation-specific what default copy constructor does
> on initializer_list because the data members of it are not defined by
> the standard.

This seems wrong to me.  Please note I am not saying it IS wrong,
only that it seems to me to be wrong.  More specifically I don't
think undefined behavior plays a role.  I tried reading the
Standard to verify that but found it impenetrable in this area.
Can someone help find the relevant passages (and an accompanying
explanation would also be appreciated)?

>> here.  Just as I expected.
>>
>>> { /* something */ } introduces a braced initialization list.
>>>
>>> { { /* something */ } } introduces a braced initialization list with
>>> one value.
>>
>> But didn't we need to use two braces for container intialization?
>
> for vector<int>, because there is constructor with count and value, you
> need something like = { 1, 2}; (note =) for {1,2} to be interpreted as
> initializer_list;  for vector<string> you should be able to initialize
> with {"One", "Two"} directly.

I'm pretty sure what you say about vector<int> isn't right.  A
declaration like

    vector<int> foo { 10, 1 };

gives a vector of length 2, not a vector of length 10.  If the
braces were parentheses

    vector<int> bas ( 10, 1 );

we would get a vector of length 10, using the ( count, value )
constructor.

Disclaimer:  the comments above are based solely on experimental
results, not on understanding what the Standard says about it.