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digitalmars.D.learn - Why use .init for associative arrays?

reply Gilles G. <schaouette free.fr> writes:
Hello,
I am just wondering why we should use init for associative arrays of structs.
See this code for example:
    struct S
    {
        int nb;
    }
    void main()
    {
        S[char[]] aa; // associative array of struct S
        aa["this_is_good"] = S.init;
        aa["this_is_good"].nb = 1;      // all right
        aa["this_is_an_error"].nb = 1; // runtime error, compiles fine...
    }

I can't see a use case for _not_ using init before accessing a new key, so why
do we have to explicitely use S.init?
Why does this code compile fine (it gives an access violation error at
runtime), shouldn't it always give an error?

Regards.

--
Gilles
Jul 10 2007
next sibling parent Robert Fraser <fraserofthenight gmail.com> writes:
Possibly related to this:
http://d.puremagic.com/issues/show_bug.cgi?id=1315

Try with a runtime value.

Gilles G. Wrote:

 Hello,
 I am just wondering why we should use init for associative arrays of structs.
See this code for example:
     struct S
     {
         int nb;
     }
     void main()
     {
         S[char[]] aa; // associative array of struct S
         aa["this_is_good"] = S.init;
         aa["this_is_good"].nb = 1;      // all right
         aa["this_is_an_error"].nb = 1; // runtime error, compiles fine...
     }
 
 I can't see a use case for _not_ using init before accessing a new key, so why
do we have to explicitely use S.init?
 Why does this code compile fine (it gives an access violation error at
runtime), shouldn't it always give an error?
 
 Regards.
 
 --
 Gilles
 

Jul 10 2007
prev sibling parent reply Chris Nicholson-Sauls <ibisbasenji gmail.com> writes:
Gilles G. wrote:
 Hello,
 I am just wondering why we should use init for associative arrays of structs.
See this code for example:
     struct S
     {
         int nb;
     }
     void main()
     {
         S[char[]] aa; // associative array of struct S
         aa["this_is_good"] = S.init;
         aa["this_is_good"].nb = 1;      // all right
         aa["this_is_an_error"].nb = 1; // runtime error, compiles fine...
     }
 
 I can't see a use case for _not_ using init before accessing a new key, so why
do we have to explicitely use S.init?
 Why does this code compile fine (it gives an access violation error at
runtime), shouldn't it always give an error?
 
 Regards.
 
 --
 Gilles
 

I would say this is because, until /assigned to/, AA pairs "don't exist." Since the member-access of the dot operator occurs "before" the member-assignment, there is an attempt to dereference null. The `= S.init` idiom is just a convenient way to insert the new AA value. -- Chris Nicholson-Sauls
Jul 10 2007
parent Chris Nicholson-Sauls <ibisbasenji gmail.com> writes:
Chris Nicholson-Sauls wrote:
 Gilles G. wrote:
 Hello,
 I am just wondering why we should use init for associative arrays of 
 structs. See this code for example:
     struct S
     {
         int nb;
     }
     void main()
     {
         S[char[]] aa; // associative array of struct S
         aa["this_is_good"] = S.init;
         aa["this_is_good"].nb = 1;      // all right
         aa["this_is_an_error"].nb = 1; // runtime error, compiles fine...
     }

 I can't see a use case for _not_ using init before accessing a new 
 key, so why do we have to explicitely use S.init?
 Why does this code compile fine (it gives an access violation error at 
 runtime), shouldn't it always give an error?

 Regards.

 -- 
 Gilles

I would say this is because, until /assigned to/, AA pairs "don't exist." Since the member-access of the dot operator occurs "before" the member-assignment, there is an attempt to dereference null. The `= S.init` idiom is just a convenient way to insert the new AA value. -- Chris Nicholson-Sauls

Which later made me think of something. Might it be useful for AA's to explose a .add(K) counterpart to the existing .remove(K) property? Usage being as follows: V[K] aa ; aa.add(K, someValue); // equivelant to 'aa[K] = someValue' aa.add(K); // equivelant to 'aa[K] = V.init' Or did something like this come along and I missed it? -- Chris Nicholson-Sauls
Jul 13 2007