4 ms·

There are some magic keywords, like const, which definitely help the compiler. Unfortunately these kinds of optimisations are far too numerous, and soon become

There are some magic keywords, like const, which definitely help the compiler. Unfortunately these kinds of optimisations are far too numerous, and soon become tedious. After a certain point, more structured languages could allow much more rigid optimisation. These days JIT seems to have taken over though, which isn't really a bad thing.

AshleysBrain 14y ago
Const doesn't actually help compilers (at least in C/C++ based languages). Since it can be const_cast away and still work in a standards compliant way, any optimisation assuming const values never change is a bug. Instead the compiler will use other techniques, e.g. finding variables that are only written to on initialisation, and those don't even need to be marked const.
- koenigdavidmj 14y ago
  I've seen gcc turn a const bool& into a direct pass of a bool by value. Of course, I noticed this because it was the implementation of a web service binding, and the wrapper was going to pass me a bool& (as a pointer) whether I liked it or not. Instant segfault...but not always :(
- _kst_ 14y ago
  If an object is defined as const, casting away its constness and modifying the object causes the program's behavior to be undefined. Compilers are free to take advantage of that.
  - AshleysBrain 14y ago
    Do you have a spec reference for that? I was under the impression you could safely const_cast and modify pretty much anything. Perhaps it was just primitives like ints, or const member functions.
  - mikeash 14y ago
    That is true, but it's extremely rare to be able to take advantage of that. When people think of const as a potential optimization aid, it's almost always in the context of const pointers, which actually do nothing at all for optimization.
bitwize 14y ago
Oh God. Don't get me started on const. const char * const fuck_this(const char *shit) const; Thing #1: In C++ the word "const" can mean many things depending on position and inflection: declaring a symbol an actual constant; a pointer argument has in (as opposed to in-out) semantics; a function has no side effects. I keep forgetting which use of const does what. Thing #2: Each use of const, regardless of meaning, induces a different type. Which makes API design a pain in the ass since you can't pass in stuff that wasn't declared with the same constness if the procedure expects const parameters without casting the parameters. I actually worked on a codebase where a guy broke much of our code by checking in a "fix" for our APIs to make them comply with what he considered best practice regarding constness. Thing #3: Passed-in pointers and references are not 'const' by default. You should have to declare intent to clobber their referents. Thing #4: And as someone said downthread, you can cast around it in most cases anyways. This is why I say, if you're thinking of using C++, reach for Ada instead. Ada's reputation has been necessarily tainted by whiners complaining about its verbosity. But I swear that figuring out a page of Ada code is easier than figuring out a few lines of sufficiently complicated C++. For one thing, in Ada, function/procedure parameters are 'in' (i.e., const) by default, and you must declare intent to modify parameters in side effects by declaring them 'in out'.

AshleysBrain 14y ago

Const doesn't actually help compilers (at least in C/C++ based languages). Since it can be const_cast away and still work in a standards compliant way, any optimisation assuming const values never change is a bug. Instead the compiler will use other techniques, e.g. finding variables that are only written to on initialisation, and those don't even need to be marked const.

koenigdavidmj 14y ago
I've seen gcc turn a const bool& into a direct pass of a bool by value. Of course, I noticed this because it was the implementation of a web service binding, and the wrapper was going to pass me a bool& (as a pointer) whether I liked it or not. Instant segfault...but not always :(
_kst_ 14y ago
If an object is defined as const, casting away its constness and modifying the object causes the program's behavior to be undefined. Compilers are free to take advantage of that.
- AshleysBrain 14y ago
  Do you have a spec reference for that? I was under the impression you could safely const_cast and modify pretty much anything. Perhaps it was just primitives like ints, or const member functions.
- mikeash 14y ago
  That is true, but it's extremely rare to be able to take advantage of that. When people think of const as a potential optimization aid, it's almost always in the context of const pointers, which actually do nothing at all for optimization.

bitwize 14y ago

Oh God. Don't get me started on const. const char * const fuck_this(const char *shit) const; Thing #1: In C++ the word "const" can mean many things depending on position and inflection: declaring a symbol an actual constant; a pointer argument has in (as opposed to in-out) semantics; a function has no side effects. I keep forgetting which use of const does what. Thing #2: Each use of const, regardless of meaning, induces a different type. Which makes API design a pain in the ass since you can't pass in stuff that wasn't declared with the same constness if the procedure expects const parameters without casting the parameters. I actually worked on a codebase where a guy broke much of our code by checking in a "fix" for our APIs to make them comply with what he considered best practice regarding constness. Thing #3: Passed-in pointers and references are not 'const' by default. You should have to declare intent to clobber their referents. Thing #4: And as someone said downthread, you can cast around it in most cases anyways. This is why I say, if you're thinking of using C++, reach for Ada instead. Ada's reputation has been necessarily tainted by whiners complaining about its verbosity. But I swear that figuring out a page of Ada code is easier than figuring out a few lines of sufficiently complicated C++. For one thing, in Ada, function/procedure parameters are 'in' (i.e., const) by default, and you must declare intent to modify parameters in side effects by declaring them 'in out'.