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digitalmars.D.learn - not expected pointers for struct members from foreach

reply "deed" <none none.none> writes:
import std.stdio;


struct Test
{
     static Test[]  objects;
     static Test*[] psObject;
     static int[]   ints;
     static int*[]  psInt;
	
     int  a;
     int	 b;
     int* pa;
	
     this(int a)
     {
         this.a = a;
         this.pa = &this.a;
         this.b = 2 * a;
		
         objects  ~= this;
         psObject ~= &this;
         ints     ~= b;
         psInt    ~= &this.b;
     }
}


void main()
{
     Test(20);               // /*
     Test(21);               //  * As expected.
     Test(22);               //  */
	
     foreach(int i; 0 .. 3)  // /*
     {                       //  * What happens here?
         Test(30 + i);       //  *
     }                       //  */
	
     Test.psObject.writeln;
         // Prints [7FFFA4E84530, 7FFFA4E84540, 7FFFA4E84550,
         //         7FFFA4E84570, 7FFFA4E84570, 7FFFA4E84570]
         // Why are the three last adresses the same?

     Test.ints.writeln;
         // Prints [40, 42, 44, 60, 62, 64]
         // As expected.

     Test.psInt.writeln;
         // Prints [7FFFA4E84534, 7FFFA4E84544, 7FFFA4E84554,
         //         7FFFA4E84574, 7FFFA4E84574, 7FFFA4E84574]
         // Again, why are the three last adresses the same?
}
Oct 09 2012
next sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
deed:

         // Again, why are the three last adresses the same?

The D language and its compiler is acting correctly here, so the output you see is correct. All those structs are allocated on the stack. The first three Test are allocated on the stack. In the loop it allocates the first struct on the stack, and it gets destroyed. Then when the successive loop enters, it creates a new struct, and it uses the same stack space. See: import std.stdio; struct Test { static Test*[] psObject; int x; this(int a) { x = a; psObject ~= &this; } ~this() { writeln(x); } } void main() { Test(0); foreach (i; 1 .. 3) Test(i); Test.psObject.writeln(); } 0 1 2 [12FE48, 12FE54, 12FE54] Bye, bearophile
Oct 09 2012
prev sibling next sibling parent "deed" <none none.none> writes:
On Tuesday, 9 October 2012 at 16:21:47 UTC, bearophile wrote:
 deed:

        // Again, why are the three last adresses the same?

The D language and its compiler is acting correctly here, so the output you see is correct. All those structs are allocated on the stack. The first three Test are allocated on the stack. In the loop it allocates the first struct on the stack, and it gets destroyed. Then when the successive loop enters, it creates a new struct, and it uses the same stack space. See: import std.stdio; struct Test { static Test*[] psObject; int x; this(int a) { x = a; psObject ~= &this; } ~this() { writeln(x); } } void main() { Test(0); foreach (i; 1 .. 3) Test(i); Test.psObject.writeln(); } 0 1 2 [12FE48, 12FE54, 12FE54] Bye, bearophile

Meaning struct pointers are unusable or at least highly unreliable?
Oct 09 2012
prev sibling next sibling parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Wed, Oct 10, 2012 at 12:59:08AM +0200, deed wrote:
 On Tuesday, 9 October 2012 at 16:21:47 UTC, bearophile wrote:
deed:

       // Again, why are the three last adresses the same?

The D language and its compiler is acting correctly here, so the output you see is correct. All those structs are allocated on the stack. The first three Test are allocated on the stack. In the loop it allocates the first struct on the stack, and it gets destroyed. Then when the successive loop enters, it creates a new struct, and it uses the same stack space.


 Meaning struct pointers are unusable or at least highly unreliable?

No, meaning if you want the struct to survive the end of the scope it's declared in, you need to use new: struct S { // ... stuff } void main() { S*[] ptrs; foreach (i; 0..10) { auto s = new S; ptrs ~= s; // this is OK } // foreach (i; 0..10) { // S s; // ptrs ~= &s; // this is NOT OK //} } T -- Let X be the set not defined by this sentence...
Oct 09 2012
prev sibling next sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
deed:

 Meaning struct pointers are unusable or at least highly
 unreliable?

Struct pointers are useful and reliable, but before using them you need to know the difference between heap and stack, what a stack frame is, and how structs are handled when they are on the stack. Learning the basics of such things ideas requires only few minutes and it will be useful for many years. Bye, bearophile
Oct 09 2012
prev sibling parent "deed" <none none.none> writes:
 Struct pointers are useful and reliable, but before using them 
 you need to know the difference between heap and stack, what a 
 stack frame is, and how structs are handled when they are on 
 the stack. Learning the basics of such things ideas requires 
 only few minutes and it will be useful for many years.

 Bye,
 bearophile

Thanks for useful feedback.
Oct 10 2012