220 14654 article Path: news.gmane.org!not-for-mail From: r.s.salkin@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: "const public:" and "const private:" to avoid to write "T Get() const" member functions Date: Thu, 20 Nov 2014 12:22:04 -0800 (PST) Lines: 190 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/mixed; boundary="----=_Part_7200_525277009.1416514924260" X-Trace: ger.gmane.org 1416514936 3714 80.91.229.3 (20 Nov 2014 20:22:16 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 20 Nov 2014 20:22:16 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC47DW5KYQGBB3M2XGRQKGQEPM3P3DA@isocpp.org Thu Nov 20 21:22:09 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ig0-f197.google.com ([209.85.213.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1XrYEx-00077B-28 for gclcip-std-proposals@m.gmane.org; Thu, 20 Nov 2014 21:22:07 +0100 Original-Received: by mail-ig0-f197.google.com with SMTP id r2sf16412291igi.0 for ; Thu, 20 Nov 2014 12:22:06 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:subject:mime-version:content-type :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=vodz3O/XAuYtCkHMBmRi3he+xXVAzCBaZX/NBpfzyaM=; b=QeQEPHHZfKPGgW9JbbMP0MXIt9WC/tlBw5HOTthL1JL0PPczpO55qmtLnNrsB9LvTC 1tVuzMKW+BOMhUag6/ceJlw+gwHXnakZAlbiFaSSbtLkJkVtJsVk8w+awnE4eXdex3yQ wmST/Op/mLLwNrlza0HeF9RqOcJq6f46bnrydGnVzYQeGndJH086R5Fkkti+TLTDJu4k Em4sphZzjlziTR7nLJQ3ZqHS/829K+9SSu/7KSZWKW43KnmHo1DNdhu8ACNBswOEsYNe qvImEA3s9Gi0hNJClvbhcWBSV1r4OahrJduFBxEz68E4Kyv9ianWKt689x4otQg7nAub Eubg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:subject:mime-version :content-type:x-original-sender:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=vodz3O/XAuYtCkHMBmRi3he+xXVAzCBaZX/NBpfzyaM=; b=gNmnNdP+jjyXVHRYtwX5r/6iokXkMhwEn8MUJW+/lfHST07eGdqz62EmrP14ZhScag lwXprhS0H9ZYzURj03T+r8/KxMvEPp/9EQy2yZfR8Pp6wPUAu1siDkhjFPo6uE42oNm8 ct57/21YWcQ+/RyrAaWyzfOaQrY+USI34YfdbKI1qD1p3n7NcJGOKjqBviwHrszIhk3t GAYTp51xfBa/mtw4KomV7u8544DGt08uxti8YwQkIjsIAE2yM7VJQ1O9RJ+mOuem4q88 CRp2739g/w1CCsXPZn7jK2IPWKWHCaync/9lbqL0ZNiSUiVU3KIWPRdsz4zjybW8Fu/1 9r6A== X-Gm-Message-State: ALoCoQmrOFeASLD9H2XEqQYpuVdiPhr05FTYn7Va8X29Q14uM9R9aJtSR0ILp5a2E7iuajNOmDWL X-Received: by 10.182.33.134 with SMTP id r6mr341772obi.34.1416514926123; Thu, 20 Nov 2014 12:22:06 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.97.200 with SMTP id m66ls489002qge.97.gmail; Thu, 20 Nov 2014 12:22:05 -0800 (PST) X-Received: by 10.140.102.117 with SMTP id v108mr4980qge.36.1416514925488; Thu, 20 Nov 2014 12:22:05 -0800 (PST) X-Original-Sender: r.s.salkin@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:14654 Archived-At: ------=_Part_7200_525277009.1416514924260 Content-Type: multipart/alternative; boundary="----=_Part_7201_25417446.1416514924260" ------=_Part_7201_25417446.1416514924260 Content-Type: text/plain; charset=UTF-8 In order to protect member variables from improper use, normally get-functions are used. If the variable in question is constant, the variable can also be declared const and be accessed directly by putting it into the "public:"-section. The disadvantage of the first approch is rather lengthy code for a simple task: read-only access outside the class, read/write-access inside. The second approach is not as general as the first since it can be only used only for constant variables. Why not introducing a keyword showing that a member variable can be altered within the class but not outside, still having read-access from outside? Proposal 1) Add "const public:" and "const protected:" to the existing "public:", "protected:", and "private:" keywords. Examples below. It is unclear what happens with functions after those new keywords. They could be forbidden (compiler error). Examples: Instead of: class A { public: int GetX() const { return m_iX; }; private: int m_iX; }; write class A { const public: int m_iX; }; This means m_iX is visible and readable from outside the class, but not writable. From within it is writable. Instead of class B { public: int GetX() const { return m_iX; }; protected: int m_iX; }; write class B { const protected: int m_iX; }; Here m_iX cannot be read outside the class, but it can be read by a derived class (but not written). Within A it can be read or written. class C : public B { void Function() { int iY=m_iX*3; // allowed m_iX=4; // forbidden } }; Proposal 2) Use a keyword like "readonly" preceding the member variable type. Example: class A { public: const int m_iW; // readonly access inside and outside class readonly int m_iX; // readonly access outside, read/write-access inside class int m_iY; // read/write access inside and outside (as usual) protected: readonly int m_iZ; // read but not write access from derived class private: readonly int m_iL; // not really meaningful, should be not allowed }; -- --- 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/. ------=_Part_7201_25417446.1416514924260 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
In order to protect member variables from improper use, no= rmally get-functions are used. If the variable in question is constant, the= variable can also be declared const and be accessed directly by putting it= into the "public:"-section.
The disadvantage of the first approch is ra= ther lengthy code for a simple task: read-only access outside the class, re= ad/write-access inside. The second approach is not as general as the first = since it can be only used only for constant variables.

Why not intro= ducing a keyword showing that a member variable can be altered within the c= lass but not outside, still having read-access from outside?

Proposa= l 1) 
Add "const public:" and "const protected:" to the existing "= public:", "protected:", and "private:" keywords. Examples below. It is uncl= ear what happens with functions after those new keywords. They could be for= bidden (compiler error).

Examples:

Instead of:

class A
{
public:
  &nbs= p; int GetX() const
    {
    &nbs= p;   return m_iX;
    };
private:
 &= nbsp;  int m_iX;
};

write

class A
{
const public:
 
  &= nbsp; int m_iX;
};

This means m_iX is visible and readable= from outside the class, but not writable. From within it is writable.
=
Instead of

class B
{public:
    int GetX() const
    {        return m_iX;
  &nb= sp; };
protected:
    int m_iX;
};

wr= ite

class B
{
const pro= tected:
 
    int m_iX;
};

Here = m_iX cannot be read outside the class, but it can be read by a derived clas= s (but not written). Within A it can be read or written.

class C : public B
{
    v= oid Function()
    {
     &nb= sp;  int iY=3Dm_iX*3;  // allowed
    &nbs= p;   m_iX=3D4;        &nb= sp;  // forbidden
    }
};


Prop= osal 2)
Use a keyword like "readonly" preceding the member variable typ= e.
Example:

class A
{public:
    const int m_iW;    &nbs= p;          // readonly access= inside and outside class
    readonly int m_iX; &nb= sp;          // readonly acces= s outside, read/write-access inside class
    int m_iY;&n= bsp;            = ;            // read= /write access inside and outside (as usual)

protected:
 &nbs= p; readonly int m_iZ;         =     // read but not write access from derived class

p= rivate:
   readonly int m_iL;     &nb= sp;      // not really meaningful, should be not a= llowed
};

--

---
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/<= /a>.
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