Databases
StackOverflow2013
Postsdbo
PODo non const internal members of class become const when I have const ref to an object of that class?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PODo non const internal members of class become const when I have const ref to an object of that class?
    primarykey
    data
    text
    <p>Well, I think my problem originates in a lake of knowledge of basic C++ concepts. The problem is, in my code (below) I have the classes Header and Register. For both, I pass a reference to a ifstrem file already opened. The Header reads some bytes from it. Register has a method to return a reference of Header (which is passed in Register constructor). </p> <p>The problem is, when I declare the reference to Header as const (in my Register class), I've got an error message saying:</p> <pre><code>error C2662: 'Header::Field_1' : cannot convert 'this' pointer from 'const Header' to 'Header &amp;' </code></pre> <p>The problem, I guess, is:</p> <p>Field_1 method changes the file cursor through seekg. therefore this member object cannot be const (its internal structure is being changed). Being fstream _stream declared inside Header class, and having a const reference for this class does it make all its internal members const as well?</p> <p>PS: I'm using VS C++ express. The code below doesn't do anything useful, is just an example.</p> <pre><code>// ********** HEADER.H *********** #ifndef __HEADER_H__ #define __HEADER_H__ #include &lt;fstream&gt; class Header { private: std::ifstream* _stream; unsigned long field1; public: Header(std::ifstream* stream); ~Header() { } unsigned long Field_1(void); }; class Register { private: const Header&amp; _header; std::ifstream* _stream; public: Register(const Header&amp; header, std::ifstream* stream); ~Register(){ } const Header&amp; GetHeader(void) { return _header; } }; #endif /*__HEADER_H__*/ //********* PROGRAM.CPP ************* #include &lt;iostream&gt; #include "header.h" using namespace std; Header::Header(ifstream* stream) : _stream(stream) { } unsigned long Header::Field_1(void) { _stream-&gt;seekg(0x00, fstream::beg); _stream-&gt;read(reinterpret_cast&lt;char*&gt;(&amp;field1), sizeof(field1)); return field1; } Register::Register(const Header&amp; header, std::ifstream* stream): _header(header), _stream(stream) { } int main(void) { ifstream file("test.dat", ios::binary | ios::in); Header h(&amp;file); Register reg(h, &amp;file); cout &lt;&lt; "Field 1 " &lt;&lt; reg.GetHeader().Field_1() &lt;&lt; endl; file.close(); return 0; } </code></pre>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USluke
    1. USAndres
    plurals
    1. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. COWhy do you declare `Register::GetHeader(void)` to be returning a *const* `Header&` in the first place?
      singulars
      1. PODo non const internal members of class become const when I have const ref to an object of that class?
      1. USRomain
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload