4 ms·

I'd argue that you are conflating implementation with semantics. The semantic meaning of const is 'this value will not change from here on out'. The C# compiler

by kedean 10y ago

I'd argue that you are conflating implementation with semantics. The semantic meaning of const is 'this value will not change from here on out'. The C# compiler implements this by doing a substitution, but that is not mandated by nature of it being 'const'. Calling these variables 'config const' says 'this will be instantiated by some configuration parameter, but from here on it it will cannot change'. If someone were to patch your C# compiler such that it didn't substitute, and instead pulled from a read-only memory location, would it in any way harm the correctness of your code?

nightski 10y ago
Actually I think it's the other way around. Since there is no guarantee that it is not copied, you can't assume either way. const means just that - const at compile time. As you say how the compiler chooses to utilize that information is up to the implementation. In C# you can't assign a const to a value not known at compile time for this very reason. However, you can assign a variable value to a static readonly field.
- kedean 10y ago
  That's because of the implementation details of C#. If I make a const pointer in C, it's obviously not a compile-time value, because the memory location can't be guaranteed at compile time. Const values in C are definitely able to be assigned from variables. Very few languages share C#'s requirement that the assignment be known at compile-time (the only other one I can find is Nim).
  - kazinator 10y ago
    Nitpick: (scoped) const variables in C can be initialized from run-time values, not assigned. RAAI: Really, assignment ain't initialization. A file scope const object in C can have a value which is not known at the point of declaration. extern const int var; This one is not known until either a definition is supplied later in the same translation unit, or else not until linkage with another translation unit which supplies a definition. static const int var; In this case, the value is not determined until either a definition is seen which provides an initializer, or else the the end of the translation unit is reached (whereupon the tentative definition becomes actual, with a value of zero). In this situation, though, the static value can be propagated as a true literal constant into code which precedes the point where the value is known. Just not in a "one-pass compiler". This is also the case when a block-scoped const is initialized with a literal constant: { const int x = 5; /* almost "true" constant */ ... } In C, x isn't considered a constant expression, but de facto it is, which is good enough for optimizing.
  - nightski 10y ago
    It was an exception to your blanket statement (no language uses this definition of const). Apparently the only language that you consider relevant is C, which is fine. But I wouldn't take many cues from C in reference to modern language design and proper semantics. As useful as it is C has many holes and broken semantics.

by kedean 10y ago

nightski 10y ago

Actually I think it's the other way around. Since there is no guarantee that it is not copied, you can't assume either way. const means just that - const at compile time. As you say how the compiler chooses to utilize that information is up to the implementation. In C# you can't assign a const to a value not known at compile time for this very reason. However, you can assign a variable value to a static readonly field.

kedean 10y ago
That's because of the implementation details of C#. If I make a const pointer in C, it's obviously not a compile-time value, because the memory location can't be guaranteed at compile time. Const values in C are definitely able to be assigned from variables. Very few languages share C#'s requirement that the assignment be known at compile-time (the only other one I can find is Nim).
- kazinator 10y ago
  Nitpick: (scoped) const variables in C can be initialized from run-time values, not assigned. RAAI: Really, assignment ain't initialization. A file scope const object in C can have a value which is not known at the point of declaration. extern const int var; This one is not known until either a definition is supplied later in the same translation unit, or else not until linkage with another translation unit which supplies a definition. static const int var; In this case, the value is not determined until either a definition is seen which provides an initializer, or else the the end of the translation unit is reached (whereupon the tentative definition becomes actual, with a value of zero). In this situation, though, the static value can be propagated as a true literal constant into code which precedes the point where the value is known. Just not in a "one-pass compiler". This is also the case when a block-scoped const is initialized with a literal constant: { const int x = 5; /* almost "true" constant */ ... } In C, x isn't considered a constant expression, but de facto it is, which is good enough for optimizing.
- nightski 10y ago
  It was an exception to your blanket statement (no language uses this definition of const). Apparently the only language that you consider relevant is C, which is fine. But I wouldn't take many cues from C in reference to modern language design and proper semantics. As useful as it is C has many holes and broken semantics.