Chapter140:Pointers

Section140.1:Pointersforarrayaccess

ThisexampledemonstrateshowpointerscanbeusedforC-likeaccesstoC#arrays.

unsafe

{

varbuffer=newint[1024];

fixed(int*p=&buffer[0])

{

for(vari=0;i<buffer.Length;i++)

{

*(p+i)=i;

}

}

}

The unsafekeyword is required because pointer access will not emit any bounds checks that are normally emitted when accessing C# arrays the regular way.

ThefixedkeywordtellstheC#compilertoemitinstructionstopintheobjectinanexception-safeway.Pinningis requiredtoensurethatthegarbagecollectorwillnotmovethearrayinmemory,asthatwouldinvalidateany pointerspointingwithinthearray.

Section140.2:Pointerarithmetic

Addition and subtraction in pointers works differently from integers. When a pointer is incremented or decremented, the address it points to is increased or decreased by the size of the referent type.

Forexample,thetypeint(aliasforSystem.Int32)hasasizeof4.Ifanintcanbestoredinaddress0,the subsequentintcanbestoredinaddress4,andsoon.Incode:

varptr=(int*)IntPtr.Zero;

Console.WriteLine(newIntPtr(ptr));//prints0 ptr++;

Console.WriteLine(newIntPtr(ptr));//prints4

ptr++;

Console.WriteLine(newIntPtr(ptr));//prints8

Similarly,thetypelong(aliasforSystem.Int64)hasasizeof8.Ifalongcanbestoredinaddress0,thesubsequent

longcanbe stored in address8, and so on. Incode:

varptr=(long*)IntPtr.Zero;

Console.WriteLine(newIntPtr(ptr));//prints0 ptr++;

Console.WriteLine(newIntPtr(ptr));//prints8

ptr++;

Console.WriteLine(newIntPtr(ptr));//prints16

The type voidis special and voidpointers are also special and they are used as catch-all pointers when the type isn’t known or doesn’t matter. Due to their size-agnostic nature, voidpointers cannot be incremented or decremented:

varptr=(void*)IntPtr.Zero;

Console.WriteLine(new IntPtr(ptr));

ptr++;//compile-timeerror

Console.WriteLine(new IntPtr(ptr));

ptr++;//compile-timeerror

Console.WriteLine(new IntPtr(ptr));

Section140.3:Theasteriskispartofthetype

InCandC++,theasteriskinthedeclarationofapointervariableispartoftheexpressionbeingdeclared.InC#,the asterisk in the declaration is part of the type.

InC,C++andC#,thefollowingsnippetdeclaresanintpointer:

int*a;

InCandC++,thefollowingsnippetdeclaresanintpointerandanintvariable.InC#,itdeclarestwointpointers:

int*a,b;

InCandC++,thefollowingsnippetdeclarestwointpointers.InC#,itisinvalid:

int*a,*b;

Section140.4:void*

C#inheritsfromCandC++theusageofvoid*asatype-agnosticandsize-agnosticpointer.

void*ptr;

Anypointertypecanbeassignedtovoid*usinganimplicitconversion:

int*p1=(int*)IntPtr.Zero;

void*ptr=p1;

Thereverserequiresanexplicitconversion:

int*p1=(int*)IntPtr.Zero;

void*ptr=p1;

int*p2=(int*)ptr;

Section140.5:Memberaccessusing->

C#inheritsfromCandC++theusageofthesymbol->asameansofaccessingthemembersofaninstance through a typed pointer.

Considerthefollowingstruct:

structVector2

{

publicintX;

publicintY;

}

Thisisanexampleoftheusageof->toaccessitsmembers:

Vector2v;

v.X=5;

v.Y=10;

Vector2* ptr = &v;

intx=ptr->X;

inty=ptr->Y;

strings=ptr->ToString();

Console.WriteLine(x);//prints5

Console.WriteLine(y);//prints10

Console.WriteLine(s);//printsVector2

Section140.6:Genericpointers

Thecriteriathatatypemustsatisfyinordertosupportpointers(see Remarks)cannotbeexpressedintermsof generic constraints. Therefore, any attempt to declare a pointer to a type provided through a generic type parameter will fail.

voidP<T>(Tobj)

whereT:struct

{

T*ptr=&obj;//compile-timeerror

}