Evaluating __FILE__ and __LINE__ of caller?

[Andrej Mitrovic]

Well, there is a way but it might wreak havoc on your object size
since it will instantiate a lot of templates. Therefore it would be a
good thing to only do it in debug mode:

import std.string;

class A
{
    int opIndexDebug(int line = __LINE__, string file = __FILE__)(int x)
    {
        /* do a bunch of checks */
        throw new Exception(format("%s: %s", file, line));

        int value;
        /* compute value */
        return value;
    }

    debug
        alias opIndexDebug opIndex;
    else
        alias opIndexDebug!(__LINE__, __FILE__) opIndex;
}

void main()
{
    auto a = new A;
    auto b = a[1];
}

You see, when using debug mode it gets instantiated on every call
site, whereas in release mode it uses one hardcoded template instance.
Of course you might want the file/line in release mode too, in that
case get rid of the aliases and just use the templated opIndex (but
then again DMD doesn't merge templates so this could be a hit on
performance).