220 32567 <1620864.HL4j5jKv18@tjmaciei-mobl1> article Path: news.gmane.org!.POSTED!not-for-mail From: Thiago Macieira Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: relaxing rules for ternary operator. Allow incompatible types. 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[78.47.120.188]) by mx.google.com with ESMTP id s7si10365952wrb.187.2017.05.21.17.12.18 for ; Sun, 21 May 2017 17:12:18 -0700 (PDT) Received-SPF: pass (google.com: domain of thiago@macieira.org designates 78.47.120.188 as permitted sender) client-ip=78.47.120.188; Original-Received: from tjmaciei-mobl1.localnet (unknown [IPv6:2601:1c0:4501:5f9f:8466:3462:d05f:7085]) by gondolin.macieira.info (Postfix) with ESMTPSA id C443D12C001 for ; Sun, 21 May 2017 17:12:17 -0700 (PDT) In-Reply-To: X-Original-Sender: thiago@macieira.org X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of thiago@macieira.org designates 78.47.120.188 as permitted sender) smtp.mailfrom=thiago@macieira.org 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:32567 Archived-At: On domingo, 21 de maio de 2017 13:54:51 PDT ma.kalbfuss@web.de wrote: > Yeah, i realize that. This is something you have to consider if you're > using templates extensively. No, it isn't. Today, the use of templates does not imply exponential growth of generated code. That is something completely new with your proposal. > This is not the right solution for everything. > I think in most cases, you use one or at most two ternary operators in one > place. For three or more ternary operators the runtime overhead is > assumably the lesser of two evils. But even if it grows exponentially, the > constants are pretty small. Normaly only a few instructions are involved. Not necessarily. Remember that you may not reevaluate the condition, so either this generates completely separate code paths (and thus the exponential expansion) or the result of the evaluation is stored in a hidden variable that is evaluated every time you access that automatic variable. That is, when you write: auto x = runtime_condition1 ? A{} : B{}; auto y = runtime_conditoin2 ? C{} : D{}; x.f(); This expands to either: if (runtime_condition1) { auto x = A{}; if (runtime_condition2) { auto y = C {}; x.f() + y.f(); } else { auto y = D {}; x.f() + y.f(); } } else { auto x = B{}; if (runtime_condition2) { auto y = C {}; x.f() + y.f(); } else { auto y = D {}; x.f() + y.f(); } } or to: variant x; variant y; bool hidden1 = runtime_condition1; if (hidden1) x.create(); else x.create(); bool hidden2 = runtime_condition2; if (hidden2) y.create(); else y.create(); if (hidden1) { if (hidden2) x.as().f() + y.as().f(); else x.as().f() + y.as().f(); } else { if (hidden2) x.as().f() + y.as().f(); else x.as().f() + y.as().f(); } -- Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org Software Architect - Intel Open Source Technology Center -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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