From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <daniel.kruegler@gmail.com>
Received: from mail-ej1-x62c.google.com (mail-ej1-x62c.google.com
 [IPv6:2a00:1450:4864:20::62c])
 by sourceware.org (Postfix) with ESMTPS id 7BDC1389202B
 for <libstdc++@gcc.gnu.org>; Mon, 16 Nov 2020 07:26:44 +0000 (GMT)
DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 7BDC1389202B
Received: by mail-ej1-x62c.google.com with SMTP id o9so22974419ejg.1
 for <libstdc++@gcc.gnu.org>; Sun, 15 Nov 2020 23:26:44 -0800 (PST)
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
 d=1e100.net; s=20161025;
 h=x-gm-message-state:mime-version:references:in-reply-to:from:date
 :message-id:subject:to:cc;
 bh=9sSSeybcQOoatzeEu6gxqDQAxoHvvhG9NkrZofwOCUA=;
 b=A1IZj7ani7VUz0kRnyuxFZuC+xNQfCWb37qIMefaLFjYW9D/cXSKilfm1yJxlWt8Ee
 dsIbrbrl+OAgKfmDEA/2RJKT627etzEAuDEql5ZYu5p5fUeEO7dTw75FsS7gLMfBVrCu
 50yxm2LTfe5EZYzNVoSqe3mg3wABtR8ktJvbDMd7tPz2WjfsMeSOHcyt4dTVt03Cgbic
 lhAvSNuES7IT5JRzNouNOlVrjPVH4pbCORlWqnNjagXEa3xhOLhWTQqxXmx6baYHfVnC
 URsPthB8igm2PuWGp+GvXNPs8XbNcqh3lmB7RhZKV+we1abYo1nUAvQxtsM6QT0Z+yvc
 dF9Q==
X-Gm-Message-State: AOAM530CP5ASrwrArt4vwmQmRcoqLVo0DhjDWIrxRDfdjYsmLwnPCKvE
 DGWVieEqPbLNqMpe/XCOfwWggkI8xZmRaGUO26E=
X-Google-Smtp-Source: ABdhPJye7H589Ad3VQOu0XksfByoQnHnM9W7EAZkoPfnRUK9FrGKUPxQP220JU9Ou8DRb5gM5EjU1TpsB9x1t1g3vgo=
X-Received: by 2002:a17:906:1253:: with SMTP id
 u19mr13091918eja.288.1605511603598; 
 Sun, 15 Nov 2020 23:26:43 -0800 (PST)
MIME-Version: 1.0
References: <CH2PR02MB6522ACC88083B9E4DD88EDD3B2E30@CH2PR02MB6522.namprd02.prod.outlook.com>
In-Reply-To: <CH2PR02MB6522ACC88083B9E4DD88EDD3B2E30@CH2PR02MB6522.namprd02.prod.outlook.com>
From: =?UTF-8?Q?Daniel_Kr=C3=BCgler?= <daniel.kruegler@gmail.com>
Date: Mon, 16 Nov 2020 08:26:32 +0100
Message-ID: <CAGNvRgD_q0nAW+GbwY264EK3_99T_Toc+ojs37PGC3HYOSOPbA@mail.gmail.com>
Subject: Re: transparent proxy of string_view as key for associative
 containers.
To: sotrdg sotrdg <euloanty@live.com>
Cc: "libstdc++" <libstdc++@gcc.gnu.org>
Content-Type: text/plain; charset="UTF-8"
X-Spam-Status: No, score=-2.3 required=5.0 tests=BAYES_00, DKIM_SIGNED,
 DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, RCVD_IN_DNSWL_NONE,
 SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2
X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on
 server2.sourceware.org
X-BeenThere: libstdc++@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Libstdc++ mailing list <libstdc++.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/libstdc++>,
 <mailto:libstdc++-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/libstdc++/>
List-Post: <mailto:libstdc++@gcc.gnu.org>
List-Help: <mailto:libstdc++-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/libstdc++>,
 <mailto:libstdc++-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Mon, 16 Nov 2020 07:26:45 -0000

Am Mo., 16. Nov. 2020 um 07:44 Uhr schrieb sotrdg sotrdg via Libstdc++
<libstdc++@gcc.gnu.org>:
>
> The code fails to compile for both unordered_map and map. Is this a bug of libstdc++ or a defect of WG21?
> #include<unordered_map>
> #include<string>
> #include<string_view>
> #include<map>
>
> int main()
> {
>                 std::unordered_map<std::string,std::string> umap;
>                 std::string_view key{"a key"};
>                 auto p{umap.find(key)};
>                 std::map<std::string,std::string> map;
>                 auto p1{map.find(key)};
> }

I don't see any WG21 defect involved here, but a mislead user expectation:

Let's start with the observation that this code isn't valid. To
activate transparent comparison, you need to provide function objects
that have the is_transparent tag defined, e.g. aomething like the
following should be more appropriate:

#include<unordered_map>
#include<string>
#include<string_view>
#include<map>

struct hash_transparent
{
    using is_transparent = void;

    template<class T>
    std::size_t operator()(const T& s) const
    {
        return std::hash<T>()(s);
    }
};

int main()
{
    std::unordered_map<std::string, std::string, hash_transparent,
std::equal_to<>> umap;
    std::string_view key{ "a key" };
    auto p{umap.find(key)};
    std::map<std::string, std::string, std::less<>> map;
    auto p1{map.find(key)};
}

Nonetheless, when you test this, you will still get a compile error
with gcc/clang for the unordered_map (Not for the map), so I'm
assuming that this feature has not been implemented yet. Visual Studio
2019 accepts that code, though.

- Daniel