220 4598 article Path: news.gmane.org!not-for-mail From: DeadMG Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Changing lambda scope Date: Sun, 26 May 2013 06:59:40 -0700 (PDT) Lines: 235 Approved: news@gmane.org Message-ID: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_2067_6843425.1369576780507" X-Trace: ger.gmane.org 1369576783 4187 80.91.229.3 (26 May 2013 13:59:43 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 26 May 2013 13:59:43 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDFMT44JSEMBBTNKRCGQKGQE3OZF3RY@isocpp.org Sun May 26 15:59:43 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vb0-f72.google.com ([209.85.212.72]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1UgbU7-0004HQ-1m for gclcip-std-proposals@m.gmane.org; Sun, 26 May 2013 15:59:43 +0200 Original-Received: by mail-vb0-f72.google.com with SMTP id w15sf5937429vbf.3 for ; Sun, 26 May 2013 06:59:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-beenthere:date:from:to:message-id:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-google-group-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=wmbVWUtulxEoZqtc7UZ7WWoM7U3rdr6tNQQFAfYrfHM=; b=b1E70tg+u0ogEim2rZZM2l41lWwJxEWE5jTFg8YXCXi5RiYtUQYR+nqGBpxaZJaszo AGtcf1z7+3AT2mFcx+83eLFX07B1774D1k9loJYJrPKftasBEQTz2GSXqnL0X75k29HJ oRsNcMALIg/VS9oGbF51C9kjsJ6Z81eyzWpP7XYZ4EEgEhlI+O+tdTtB7l/ydvxAIYS+ A4ZVgNVcV9MCYyO2T68gNrekPv9Ywbhq+MC7WfaITXzZZ3Ij0wZdbDkV8PXZVBQ5tCEp Asq8h5fgksjm1suyjagpsFm8j5hAq/Qlv+N/XnVA1K1AWfXNJUbEU0ht4t6kr+ZoZslc wzLQ== X-Received: by 10.224.174.145 with SMTP id t17mr13067491qaz.4.1369576781889; Sun, 26 May 2013 06:59:41 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.131.228 with SMTP id op4ls2456121qeb.99.gmail; Sun, 26 May 2013 06:59:40 -0700 (PDT) X-Received: by 10.49.30.105 with SMTP id r9mr2104147qeh.27.1369576780812; Sun, 26 May 2013 06:59:40 -0700 (PDT) X-Original-Sender: wolfeinstein@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:4598 Archived-At: ------=_Part_2067_6843425.1369576780507 Content-Type: text/plain; charset=ISO-8859-1 Pursuant to a recent discussion, I have discovered that lambdas are a local class to whatever scope they are nested in. This means that ADL can't find operators for them. For example, namespace X { template L operator+(L l, R r) { //... } auto f() { return [] {}; } }; int main() { 1 + X::f(); // Fail } I propose that the wording be changed so that lambdas' associated namespace should be the most nested namespace containing whatever scope they are declared in. Having operators available to work on lambdas considerably simplifies certain functional APIs, as you can just return a lambda directly. However, the current wording would require using hand-created types purely for the purpose of ADL. Because the operators cannot be members, nor found by ADL in the current wording, the only way for this to break existing code would be if they had something like template L operator+(L l, R r) { //... } namespace X { template L operator+(L l, R r) { //... } auto f() { return [] {}; } }; int main() { 1 + X::f(); // Fail- global operator *and* ADL operator found, so it's now ambiguous } However, having a global operator like this is a pretty pathological case. -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/?hl=en. ------=_Part_2067_6843425.1369576780507 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Pursuant to a recent discussion, I have discovered that lambdas are a local= class to whatever scope they are nested in. This means that ADL can't find= operators for them. For example,

= namespace = X {=
   = template<= /span><= typename L, typename R> L operator+(L l= , R r) {
        //...
    }
    auto f() {
        return [] {};
&n= bsp;   }
};
int main() {
    1 = + X= ::f(); // Fail
}

I propose = that the wording be changed so that lambdas' associated namespace should be= the most nested namespace containing whatever scope they are declared in. = Having operators available to work on lambdas considerably simplifies certa= in functional APIs, as you can just return a lambda directly. However, the = current wording would require using hand-created types purely for the purpo= se of ADL.

Because the operators cannot be members= , nor found by ADL in the current wording, the only way for this to break e= xisting code would be if they had something like

template<typename L, typename= R> L operator+(L l, R r) {
    //...
}
namespace X {
    template<typename L, = typename R> L operator+(L l, R r) {=
   =     //...=
    }<= br>    auto f() {
        <= /span>return [] {};
    }
};
int = main() {
    1 + X::f(= ); = // Fail- global o= perator *and* ADL operator found, so it's now ambiguous
}


However, having a global operator like thi= s is a pretty pathological case.

--
 
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