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POWhy doesn't scala have C++-like const-semantics?
 

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  1. POWhy doesn't scala have C++-like const-semantics?
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    <p>In C++. I can declare most things as const, for example:<br> Variables: <code>const int i=5;</code><br> Scala has <code>val i=5</code>, however this will only prevent reassigning, not changing the object as the following exampe shows:<br> C++: <pre><code>const int i[]={1,2,3,4}; i[2]=5; //error</code></pre> Scala: <pre><code>val a=Array(1,2,3,4) a(2)=5 //a is now Array(1, 2, 5, 4)</pre></code></p> <p>It gets even worse with member functions:<br> C++: <pre><code>class Foo { int i; int iPlusFive() <b>const</b> {return i+5;} int incrementI(){ return ++i; } }</pre></code> I can be sure, that calling iPlusFive won't change the object and that I won't accidentally call incrementI on a const object.</p> <p>When it comes to collections, C++ continues it's const-correct streak with const collections: simply declare your vector as const and you can't change it. Assign a <code>non-const vector&lt;Int&gt;</code> to a <code>const vector&lt;Int&gt;</code> and the compiler won't copy anything and will prevent you from changing anything in the now const collection.</p> <p>Scala has scala.collection.mutable.whatever and scala.collection.immutable.whatever, you can't just convert mutable collections to immutable collections, furthermore you're still allowed to change the collected objects with their non-const member functions.</p> <p>Why does scala, which has an otherwise really great type-system, not have anything comparable to C++ const-keyword?</p> <p><b>Edit: </b> Margus suggested using <code>import scala.collection.mutable</code>. My solution was to use <pre><code>import scala.collection.mutable.HashMap import scala.collection.immutable.{HashMap => ConstHashMap}</code></pre> This will make the mutable HashMap available as HashMap and the immutable als ConstHashMap, however I still like the C++ approach better.</p>
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    1. COI am adding this as a comment, as it is not really an answer. The only answer I can come up with is that it does not "by design". The fact is that very few programming languages have the same const-semantics as C++. Some don't have at all, others like Java/C# have a limited form (similar to scala) that blocks code from reassigning to the *reference*, but that provide no guarantee on the *referred* object (primitive types are handled differently in some, and can be made constant).
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      1. USDavid Rodríguez - dribeas
    2. CO+1 Interesting question for a C++ programmer / Scala noob
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    3. CO@David Rodríguez +1
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      1. USVasil Remeniuk
 

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