Delphi CompareMem with pointers
delphi
Solution
The difference is that `@s1` is address of the variable `s1` while PByte(s1) is value of the variable `s1`.
A string variable internally is a pointer to a string data, so `@s1` is a pointer to pointer. The line
CompareMem(@s1,@s2,1);
compares two least significant bytes of the addresses of `s1` and `s2` strings; it makes no sense and have no relation to the string data referenced by `Pointer(s1)` and `Pointer(s2)`.
Problem
I've been developing some software in Delphi 5 using DUnit as a driver for TDD but I found that when using CheckEqualsMem it kept failing even though in the debugger I could see that the objects being compared (two arrays of longwords in this case) were identical. Internally, CheckEqualsMem uses CompareMem and found that this was what was returning false. Delving a bit deeper I found that if I call CompareMem with a pointer to the address of the objects using @ or Addr CompareMem fails even when the memory is identical, but if I use PByte (from windows) or PChar it will compare the memory properly. Why? Here is an example ``` var s1 : String; s2 : String; begin s1 := 'test'; s2 := 'tesx'; // This correctly compares the first byte and does not return false // since both strings have in their first position if CompareMem(PByte(s1), PByte(s2), 1) = False then Assert(False, 'Memory not equal'); // This however fails ?? What I think I am doing is passing a pointer // to the address of the memory where the variable is and telling CompareMem // to compare the first byte, but I must be misunderstanding something if CompareMem(@s1,@s2,1) = False then Assert(False,'Memory not equal'); // Using this syntax correctly fails when the objects are different in memory // in this case the 4th byte is not equal between the strings and CompareMem // now correctly fails if CompareMem(PByte(s1),PByte(s2),4) = False then Assert(False, 'Memory not equal'); end; ``` As you can see in the comments, I come from a C background, so I thought that @s1 is a pointer to the first byte of s1 and PByte(s1) should be the same thing, but its not. What am I misunderstanding here? What is the difference between @ / Addr and PByte ??