Manipulating Bits of Any Value Type

Question

Has anybody cooked up some generic functions that extend core.bitop bitmanipulations to work on any value type?

Something like

bool getBit(T)(in T a, int bitnum); // bt
T setBit(T)(in T a, int bitnum); // bts
auto ref setBitInPlace(T)(ref T a, int bitnum);

I know this is relatively easy to implement so that's why I'm curious to why its not already Phobos.

Update:

Here's my first try at this:

bool getBit(T, I)(in T a, I bitnum) @safe pure nothrow if (isIntegral!T &&
                                                           isIntegral!I) {
    return a & (((cast(I)1) << bitnum)) ? true : false;
}

bool getBit(T, I)(in T a, I bitnum) @trusted pure nothrow if ((!(isIntegral!T)) &&
                                                              isIntegral!I) {
    enum nBits = 8*T.sizeof;
    static      if (nBits ==  8) alias I = ubyte;
    else static if (nBits == 16) alias I = ushort;
    else static if (nBits == 32) alias I = uint;
    else static if (nBits == 64) alias I = ulong;
    return (*(cast(I*)&a)).getBit(bitnum); // reuse integer variant
}
alias bt = getBit;

My idea is to make getBit work on all types that have value semantics. That's why I need the cast (I think). Is there a traits to check if a type has value semantics or not?

Also is there a trait to check if a type supports a specific operation such as bitwise and &? I could always use __traits(compiles, ...) but standardization is good.

To make it even better I guess I need an explicit overload for T's that support bit manipulations in order to make this variant @safe right? In my generic solution above I need the cast and that's @unsafe.

See also: http://forum.dlang.org/thread/tekrnzkemcbujbivvfpv@forum.dlang.org#post-tekrnzkemcbujbivvfpv:40forum.dlang.org

Answer 1

Is there a traits to check if a type has value semantics or not?

There is no trait for value types, at least not in the documentation However, I have checked for value types before using the "is expression":

import std.stdio;

struct Foo {}

auto fn(T)(T type)
{
    static if (is(T == struct)) {
        writeln(T.stringof ~ " is a value type");   
    } else if (is(T == class)) {
        writeln(T.stringof ~ " is a reference type");   
    } else {
        writeln(T.stringof ~ " is something else"); 
    }
}

void main()
{
    Foo f;

    fn(f);
    fn(new Object());
    fn(1);
}

Also is there a trait to check if a type supports a specific operation such as bitwise and &?

Other than the trait for compiles, this can also be achieved with an is expression. This is similar to how it is currently done for ranges, for example:

import std.stdio;

struct Foo {
    auto opBinary(string op)(Foo rhs) 
        if (op == "&")
    {
        return rhs.init; // dummy implementation
    }
};

template canUseBitOps(T)
{
    enum bool canUseBitOps = is(typeof(
    (inout int = 0)
    {
        T t1 = T.init;
        T t2 = T.init;
        auto r = t1 & t2;
    }));
}

void main()
{
    assert(canUseBitOps!int);
    assert(!canUseBitOps!string);
    assert(canUseBitOps!Foo);
}

Answer 2

I do something like this in my serialisation library at https://github.com/atilaneves/cerealed. It's not quite what you're asking for but you may get ideas from it.