Example of the perils of binding rvalues to const ref

[Szymon Gatner via Digitalmars-d]

On Wednesday, 17 September 2014 at 08:57:36 UTC, Szymon Gatner 
wrote:
> On Wednesday, 17 September 2014 at 08:52:58 UTC, Arjan wrote:
>> On Tuesday, 16 September 2014 at 15:30:49 UTC, Andrei 
>> Alexandrescu wrote:
>>> http://www.slideshare.net/yandex/rust-c
>>>
>>> C++ code:
>>>
>>> std::string get_url() {
>>>   return "http://yandex.ru";
>>> }
>>>
>>> string_view get_scheme_from_url(string_view url) {
>>>   unsigned colon = url.find(':');
>>>   return url.substr(0, colon);
>>> }
>>>
>>> int main() {
>>>   auto scheme = get_scheme_from_url(get_url());
>>>   std::cout << scheme << "\n";
>>>   return 0;
>>> }
>>>
>>> string_view has an implicit constructor from const string& 
>>> (see "basic_string_view(const basic_string<charT, traits, 
>>> Allocator>& str) noexcept;" in 
>>> https://isocpp.org/files/papers/N3762.html). The function 
>>> get_url() returns an rvalue, which in turn gets bound to a
>>
>> Forgive me my ignorance but  get_url() returns a std::string 
>> (on the stack), so its address can be token.
>> And iirc the explainer Scott Meyers explained once "iff you 
>> can take its address its not a rvalue its a lvalue". So isn't 
>> the get_scheme_from_url() not simply holding a const ref to 
>> temporary? (which most compiler warn about)
>>
>> ...Or am I missing the point?
>>
>>> reference to const and implicitly passed to string_view's 
>>> constructor. The obtained view refers to a dead string.
>>>
>>>
>>> Andrei


[ Sorry for double posting, i must have double clicked on "reply" 
button accidentally. ]

std::string returned from get_url() is a temporary and hence a 
"rvalue". In fact it's address cannot be taken. It is often 
helpful to think of lvalues as things that can appear on the left 
side of assignment expression.