static const in class

[Champion]

Hi,

This question may have been asked before, but I would like some further clarification. Is it always correct to declare a constant this way?

class A
{
    private:
           static const float var;

}

const float A::var = 1.2f;

What I want is:
1) a constant that can be used by class A member functions
2) only one copy is created
3) Basically to replace #define in C.

I know you can initialize const integral in class, but I am looking for a standard way to declare const var that can be used for array/float etc.

Thanks.

[Elysia]

This is correct for a static const variable.
Note that if you intend to replace #define, you should avoid static. The compiler is smart enough to understand that, unless you do something funny with the constants, like take their address, it can optimize them away and replace any references to that constant with its value instead.

[laserlight]

Note that if you intend to replace #define, you should avoid static. The compiler is smart enough to understand that, unless you do something funny with the constants, like take their address, it can optimize them away and replace any references to that constant with its value instead.

Except in special cases, I don't think compilers can perform constant propagation for non-static const member variables because the value of such member variables may still vary at run time (the time of object creation), though it is constant for the lifetime of each object.

If the member is intended to be a class constant rather than a per-object constant, declare it static const.

[Champion]

This is correct for a static const variable.
Note that if you intend to replace #define, you should avoid static.

So would using static prevent the compiler from optimizing it out? Thanks.

[Elysia]

This might be a tricky case as Laserlight points out. If you really want it be a constant, try placing it outside the class (you could put it in a namespace, perhaps).
Otherwise looking at the assembly is really the only way to know.

[whiteflags]

So would using static prevent the compiler from optimizing it out? Thanks.

No, if it's a class constant, static actually makes sense, and encourages optimizations based on constness. Constant member data that could be different among instances of the class cannot be made static, obviously. It is a lot harder to optimize some non-static constant out. That is how I read laserlight's post.

Of course, Elysia is right that namespaces provide a good solution, as well, if it's OK that the constant isn't member data anymore.

[laserlight]

If you really want it be a constant, try placing it outside the class (you could put it in a namespace, perhaps).

In my opinion, if a compiler can prove that constant propagation can be applied to a const qualified variable in namespace scope, then it can also prove that constant propagation can be applied to a static const member variable used in the same way as that const qualified variable in namespace scope.

Once again, if you want a class constant, declare it as a static const member of the class. If you want a namespace level constant, declare it as a constant in namespace scope. Do not reach for one or the other as a premature optimisation that might not be an optimisation at all.