Re: Struct hash issues with string fields

[Ali Çehreli]

On 05/26/2012 12:53 PM, Andrej Mitrovic wrote:
I don't understand this:
import std.stdio;

struct Symbol { string val; }

void main()
{
     int[string] hash1;
     hash1["1".idup] = 1;
     hash1["1".idup] = 2;
     writeln(hash1);  // writes "["1":2]"

     int[Symbol] hash2;
     Symbol sym1 = Symbol("1".idup);
     Symbol sym2 = Symbol("1".idup);
     hash2[sym1] = 1;
     hash2[sym2] = 1;
     writeln(hash2);  // writes "[Symbol("1"):1, Symbol("1"):1]"
}

Why are sym1 and sym2 unique keys in hash2? Because the hash
implementation checks the array pointer instead of its contents? But
then why does hash1 not have the same issue?

I can't override toHash() in a struct,

But you can define it.

> so what am I supposed to do in
order to make "sym1" and "sym2" be stored into the same hash key?

I don't know all aspects of this issue. I know that there has been 
recent discussions about the hash functions. You may be seeing some 
shortcomings.

What I am pretty sure about is though, once you define toHash(), you 
must also define opCmp() and opEquals() that are all consistent with 
each other.

When I do that, the following code works as you expect:

import std.stdio;

struct Symbol
{
    string val;

    hash_t toHash() const nothrow @safe
    {
        return typeid(val).getHash(&val);
    }

    bool opEquals(const ref Symbol that) const
    {
        return val == that.val;
    }

    int opCmp(const ref Symbol that) const
    {
        return val < that.val ? -1 : (val > that.val ? 1 : 0);
    }
}

void main()
{
    int[Symbol] hash2;
    Symbol sym1 = Symbol("1".idup);
    Symbol sym2 = Symbol("1".idup);
    hash2[sym1] = 1;
    hash2[sym2] = 1;
    writeln(hash2);             // writes [Symbol("1"):1]
}

Ali

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