enum VS static immutable

[Ali Çehreli]

On 03/13/2014 07:38 AM, ref2401 wrote:

 > Hi.
 > I have this structure:
 >
 > struct MyStruct {
 >      enum MyStruct VALUE = MyStruct(5f);
 >
 >      static immutable MyStruct value = MyStruct(5f);
 >
 >
 >      float data;
 >
 >      this(float v) { data = v; }
 > }
 >
 > What's the difference between MyStruct.VALUE and MyStruct.value? How
 > should I decide what to use?

enum defines a manifest constant, meaning that it is just a value; there 
is not one but many MyStruct objects constructed every time VALUE is 
used in the code. The following run-time asserts prove that two 
instances have different data members:

     assert(&MyStruct.VALUE.data != &MyStruct.VALUE.data);

On the other hand, there is just one MyStruct.value object:

     assert(&MyStruct.value == &MyStruct.value);

In other words, MyStruct.VALUE is an rvalue but MyStruct.value is an 
lvalue; you cannot take the address of an rvalue. The following static 
asserts pass.

     static assert(!__traits(compiles, &MyStruct.VALUE));
     static assert( __traits(compiles, &MyStruct.value));

It shouldn't matter much for such a type but prefer a static immutable 
for expensive types like arrays.

Ali