C#•4d ago

In String constructor `String(Char)`, `Char` is character pointer or character array pointer?

I was exploring System.String class here: https://learn.microsoft.com/en-us/dotnet/api/system.string?view=net-9.0 And one of the constructors is String(Char*) Its description says: Initializes a new instance of the String class to the value indicated by a specified pointer to an array of Unicode characters. However, I got error, when I tried following code to initialize string using pointer to an array of characters:

char[] cArray = {'a','r','r','a','y'};
char[]* cArray_ptr = &cArray;
string s = new string(cArray_ptr); // Error: cannot convert char[]* to char*

char[] cArray = {'a','r','r','a','y'};
char[]* cArray_ptr = &cArray;
string s = new string(cArray_ptr); // Error: cannot convert char[]* to char*

So, my question is: 1. Is String(Char*) takes character pointer as argument instead of character array pointer? 2. Or am I doing some mistake in declaring and initializing character array pointer?

String Class (System)

Represents text as a sequence of UTF-16 code units.

8 Replies

mg•4d ago

it's a pointer to an array in a C kind of sense the "Pointers and Arrays" section of The C Programming Language would be a good read if you can get a copy of it anything you can do with an array can be done with a pointer, so a char* is a char[], in a way. in other ways it's not

reflectronic•4d ago

in terms of the number of indirections, a char[]* is the same as char**, not char* what you want is to get the char* of a char[]. you need to use the fixed statement for this

reflectronic•4d ago

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/statements/fixed

fixed statement - pin a moveable variable - C# reference

Use the C# fixed statement to pin a moveable variable and declare a pointer to that variable. The address of a pinned variable doesn't change during execution of the statement.

Angius•4d ago

It's always interesting to me how people take a language that lets you avoid all that pointer tomfuckery, and decide to use pointer tomfuckery anyway :KEKW:

yourFriendOP•4d ago

Ok, so Char* in String(Char*) is pointer of first element of character array. If I understand right. Hence, both way declaring and initializing pointer works

unsafe
        {
            char[] cArray = {'a','r','r','a','y'};

            fixed(char* ptr = cArray)
            {
                string s = new string(ptr);
                // Do stuff with string s
            }

            fixed(char* ptr = &cArray[0])
            {
                string s = new string(ptr);
                // Do stuff with string s
            }
        }

unsafe
        {
            char[] cArray = {'a','r','r','a','y'};

            fixed(char* ptr = cArray)
            {
                string s = new string(ptr);
                // Do stuff with string s
            }

            fixed(char* ptr = &cArray[0])
            {
                string s = new string(ptr);
                // Do stuff with string s
            }
        }

Thanks, the link was very helpful 🙂

reflectronic•4d ago

right. well, it's a pointer to whichever element you want to be the first you can give a pointer to &cArray[2] and it will create a string starting from there but you have to be careful because this is not a valid usage of new string(char*) the way it calculates the length of the string is by searching for the first character which is '\0' in the array your pointer is pointing to, and stopping there you do not have a null character in your array. so your code only works because the memory after the array happens to be empty. it may not always be the case you should include a '\0' in your array. or, you can use new string(char*, int, int) to provide a start offset and length, so it does not have to search for the '\0'

yourFriendOP•4d ago

👍

mg•3d ago

also, like, never do this perfectly fine to play around and explore but unsafe is for when you're experienced enough to know how to break the rules the right way in the name of performance

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In String constructor `String(Char*)`, `Char*` is character pointer or character array pointer?

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In String constructor `String(Char)`, `Char` is character pointer or character array pointer?