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POHow do I assign a data object with const members?
 

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  1. POHow do I assign a data object with const members?
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    <p>Hope this is not a duplicate. If so, please point me to it in a comment and I'll remove the question again.</p> <p>I have a data object with data that's only valid in a bundle - i.e. there's no sense in changing the value of one member without invalidating the other members.<br> This data object describes some image information:</p> <pre><code>struct ImageInfo { ImageInfo(const double &amp;ppix, ...) : PpiX(ppix), ... { } const double PpiX; const double PpiY; const int SizeX; const int SizeY; }; </code></pre> <p>In my image object I have a <em>non-const</em> member of type <code>ImageInfo</code>:</p> <pre><code>class MyImageObject { ... private: ImageInfo mMyInfo; } </code></pre> <p>I want to be able to change <code>mMyInfo</code> at runtime, but only so that it will take a new ImageInfo(...) instance.</p> <p>In the <code>MyImageObject::Load()</code> function, I'd like to read this data from the file info and then create an <code>ImageInfo</code> instance with the correct set of data:</p> <pre><code>double ppix = ImageFile.GetPpiX(); ... mMyInfo = ImageInfo(ppix, ...); </code></pre> <p>But I couldn't manage to write a valid assignment operator (copy constructor is possible of course). My solution left <code>mMyInfo</code> empty, because I didn't reference <code>this</code>:</p> <pre><code>ImageInfo operator=(const ImageInfo &amp;other) { // no reference to *this return ImageInfo(other); } </code></pre> <p>Out of curiosity I'd like to know how the assignment operator for such a class would need to look like.</p> <p>I'm using plain C++.</p> <p><strong>EDIT</strong><br> Possible solutions (the goal is to keep the data transportable, but coherent):</p> <ul> <li>Use private members together with <code>Get...()</code> functions -> simple, but I'd like to avoid the parentheses.</li> <li>Store a pointer to ImageInfo: <code>ImageInfo *mpMyInfo;</code> (I'd like to avoid the heap.)</li> <li>Use serialization and store the serialized ImageInfo, then create local instances from the serialized data.</li> </ul>
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    1. USMartin Hennings
    1. USMartin Hennings
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    1. COYou could try casting away the 'constness' of the member variables in order to make the assignment, asfaik, this is valid to do when the object itself is not const.
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      1. USGearoid Murphy
    2. COWhy are those fields `const` to begin with?
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    3. CO@delnan They don't need to be `const`, they need to be `readonly`. But `readonly` exists in `C#`, but not in `C++`.
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      1. USMartin Hennings
 

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