Re: Example of the perils of binding rvalues to const ref

[Olivier Grant via Digitalmars-d]

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 reference to const and implicitly passed to string_view's constructor. The obtained view refers to a dead string.

Andrei

I would say the problem with this code is not so much in this
usage example but in the implementation of string_view. Given
string_view aims to be a non-owning reference to a string, it
should prevent such assignments, which is pretty straights
forward to ensure in C++11. Just delete the corresponding
constructor:

    string_view( std::string && ) = delete;

This example, as stated in subsequent answers to your post, is no
different from returning a const reference to a temporary. In
this example, it just happens to be nicely hidden in the
string_view implementation instead of being explicit:

    std::string const & get_url( )
    { return "foo"; }

This has always been a no-no, and C++11 at least now adds the
possibility to refuse such code via deleted functions.

O.