First off :
STRCAT :
Cplusplus - strcat
When clearly the definition says :
char * strcat ( char * destination, const char * source );
Why'd they use char str[80] in the example???
Shouldn't they have used a character pointer?
That is because arrays decay into pointers in C/C++. If you define char s[80] the value of s will be the address of the first character i.e &s[0]
array can also be used as pointer. what strcat needs is the pointer to a memory in which it copies the destination string. In this case str[80] will give you the memory that can hold 80 chars.
char str[80];
declares an array of 80 characters.
However, in C and C++, arrays are implicitly converted to pointers. When you pass an array to a function (such as strcat), it automatically "decays", forming a pointer to the first element of the array.
That's not the same as saying that arrays and pointers are the same thing. They aren't. For example, sizeof() yields different results on the above array, and a char*.
An array is, strictly speaking, a pointer to the beginning of a block of memory. So str is a char * that points to the beginning of 80 characters.
When you index into an array, say position 53, the following is equivalent: str[53] is the same as *(str + 53) as str is just a char * and adding 53 to a character pointer will return a pointer so to get the value inside you have to use an asterisk to dereference the pointer. In effect array notation just makes code more readable in certain circumstances.
Actually a great little trick with arrays of char is when you want to skip over some leading text when copying a string. E.g. let's say your array str[80] contains the string "1023: error in code!". And you want to display just the string without the number in front. In this case you could say printf( "%s", str + 6 ) and only "error in code!" would be printed.
char str[80];
Edit:
Opps, I rushed my answer. As dribeas says the statement declares an array and str can be implicitly converted into a pointer when it is used. For instance:
++str;
is an invalid operation while:
char* ptr;
++ptr;
isn't.
Related
I am looking at some code I did not write and wanted help to understand an element of it. The code stores character arrays, creates pointers to these arrays (assigning the pointers the arrays addresses). It looks like it then creates an array to store these characters pointers addresses and I just wanted some clarification on what I am looking at exactly. I also am confused about the use of the double (**) when creating the array.
I have included a stripped down and simplified example of what I am looking at below.
char eLangAr[20] = "English";
char fLangAr[20] = "French";
char gLangAr[20] = "German";
char* eLangPtr = eLangAr;
char* fLangPtr = fLangAr;
char* gLangPtr = gLangAr;
char **langStrings [3]=
{
&eLangPtr,
&fLangPtr,
&gLangPtr
};
When using the array they pass it as an argument to a function.
menu (*langStrings[0]);
So the idea is that the character array value "English" is passed to the function but I'm having trouble seeing how. They pass the menu function a copy of the value stored in the langStrings function at location 0, which would be the address of eLandPtr? If someone could explain the process in English so I could get my head around it that would be great. It might be just because it has been a long day but its just not straight in my head at all.
You are correct that langStrings contains pointers to pointers of array of characters. So each langString[i] points to a pointer. That pointer points to an array. That array contains the name of a language.
As others point out, it looks a bit clumsy. I'll elaborate:
char eLangAr[20] = "English"; is an array of 20 chars and the name "English" is copied to it. I don't expect that variable eLangAr will ever contain something else than this language name, so there is no need to use an array; a constant would be sufficient.
char **langStrings [3]= ... Here, it would be sufficient to have just one indirection (one *) as there seems no need to ever have the pointer point to anything else (randomly shuffle languages?).
in conclusion, just having the following should be sufficient:
const char *langStrings [3]=
{
"English",
"French",
"German"
};
(Note the const as the strings are now read-only constants/literals.)
What the given code could be useful for is when these language names must be spelled differently in different languages. So
"English",
"French",
"German" become "Engels", "Frans", "Duits". However, then there still is one level of indirection too many and the following would be sufficient:
char *langStrings [3]=
{
aLangArr,
fLangAr,
gLangAr
};
Ok, here goes.
The **ptrToptr notation means a pointer to a pointer. The easiest way to think on that is as a 2D matrix - dereferencing one pointer resolves the whole matrix to just one line in the matrix. Dereferencing the second pointer after that will give one value in the matrix.
This declaration:
char eLangAr[20] = "English";
Declares an array of length 20, type char and it contains the characters 'E', 'n', 'g', 'l', 'i', 's', 'h' '\0'
so it is (probably) null terminated but not full (there are some empty characters at the end). You can set a pointer to the start of it using:
char* englishPtr = &eLangAr[0]
And if you dereference englishPtr, it'll give the value 'E'.
This pointer:
char* eLangPtr = eLangAr;
points to the array itself (not necessarily the first value).
If you look at
*langStrings[0]
You'll see it means the contents (the dereferencing *) of the pointer in langStrings[0]. langStrings[0] is the address of eLangPtr, so dereferencing it gives eLangPtr. And eLangPtr is the pointer to the array eLangAr (which contains "English").
I guess the function wants to be able to write to eLangAr, so make it point to a different word without overwriting "English" itself. It could just over-write the characters itself in memory, but I guess it wants to keep those words safe.
** represents pointer to pointer. It's used when you have to store pointer for another pointer.
Value of eLangPtr will be pointer to eLangAr which will has the value "English"
Here:
char eLangAr[20] = "English";
an array is created. It has capacity of 20 chars and contains 8 characters - word "English" and terminating NULL character. Since it's an array, it can be used in a context where pointer is expected - thanks to array-to-pointer decay, which will construct a pointer to the first element of an array. This is done here:
char* eLangPtr = eLangAr;
Which is the same as:
char* eLangPtr = &eLangAr[0]; // explicitly get the address of the first element
Now, while char* represents pointer to a character (which means it points to a single char), the char** represents a pointer to the char* pointer.
The difference:
char text[] = "Text";
char* chPointer = &ch[0];
char** chPointerPointer = &chPointer; // get the address of the pointer
std::cout << chPointer; // prints the address of 'text' array
std::cout << *chPointer; // prints the first character in 'text' array ('T')
std::cout << chPointerPointer; // prints the address of 'chPointer'
std::cout << *chPointerPointer; // prints the value of 'chPointer' which is the address of 'text' array
std::cout << *(*chPointerPointer); // prints the first character in 'text' array ('T')
As you can see, it is simply an additional level of indirection.
Pointers to pointers are used for the same reason "first-level" pointers are used - they allow you to take the address of a pointer and pass it to a function which may write something to it, modifying the content of the original pointer.
In this case it is not needed, this would be sufficient:
const char *langStrings [3]=
{
eLangPtr,
fLangPtr,
gLangPtr
};
And then:
menu (langStrings[0]);
#include <iostream>
using namespace std;
int main() {
int * a[5];
char * b[5];
cout<<a[1]; // this works and prints address being held by second element in the array
cout<<b[1]; // this gives run time error . why ?
return 0;
}
Can anyone please explain to me cout<<b[1] gives run-time error ?
Shouldn't both int and char array behave similar to each other ?
Because IOStreams are designed to treat char* specially.
char* usually points to a C-string, so IOStreams will just assume that they do and dereference them.
Yours don't.
As others have said, iostream formatted output operators consider char* to point to C-style string and attempt to access this string.
What others have not said so far, is that if you are interested in the pointer, you need to cast the pointer in question to void*. For example:
std::cout << static_cast<const void*>(buf[1]);
An output stream such as cout gives special consideration to char * that it does not give to other pointers. For pointers other than char *, it will simply print out the value of the pointer as a hexadecimal address. But for char *, it will try to print out the C-style (i.e. null terminated array of char) string referred to by the char *. Therefore it will try to dereference the char pointer, as #AlexD points in the comment to your post.
C++ (inheriting it from C) treats character pointers specially. When you try to print a[1] of type int* the address is printed. But when you try to print b[1] of type char* the iostream library - following the rest of the language - assumes that the pointer points to the first character of zero-terminated string of characters. Both your output statements are initialised behaviour, but in the case of char* crash is much more likely because the pointer is dereferenced.
I saw this example:
const char* SayHi() { return "Hi"; }
And it works fine, but if I try to remove the pointer it doesn't work and I can't figure
out why.
const char SayHi() { return "Hi"; } \\Pointer removed
It works if I assign it a single character like this:
const char SayHi() { return 'H'; } \\Pointer removed and only 1 character
But I don't know what makes it work exactly. Why would a pointer be able to hold more than one character? Isn't a pointer just a variable that points to another one? What does this point to?
That is because a char is by definition a single character (like in your 3rd case). If you want a string, you can either use a array of chars which decays to const char* (like in your first case) or, the C++ way, use std::string.
Here you can read more about the "array decaying to pointer" thing.
You are correct that a pointer is just a variable that points somewhere -- in this case it points to a string of characters somewhere in memory. By convention, strings (arrays of char) end with a null character (0), so operations like strlen can terminate safely without overflowing a buffer.
As for where that particular pointer (in your first example) points to, it is pointing to the string literal "Hi" (with a null terminator at the end added by the compiler). That location is platform-dependent and is answered here.
It is also better practice to use std::string in C++ than plain C arrays of characters.
Consider a simple example
Eg: const char* letter = "hi how r u";
letter is a const character pointer, which points to the string "hi how r u". Now when i want to print the data or to access the data I should use *letter correct?
But in this situation, shouldn't I only have to use the address in the call to printf?
printf("%s",letter);
So why is this?
*letter is actually a character; it's the first character that letter points to. If you're operating on a whole string of characters, then by convention, functions will look at that character, and the next one, etc, until they see a zero ('\0') byte.
In general, if you have a pointer to a bunch of elements (i.e., an array), then the pointer points to the first element, and somehow any code operating on that bunch of elements needs to know how many there are. For char*, there's the zero convention; for other kinds of arrays, you often have to pass the length as another parameter.
Simply because printf has a signature like this: int printf(const char* format, ...); which means it is expecting pointer(s) to a char table, which it will internally dereference.
letter does not point to the string as a whole, but to the first character of the string, hence a char pointer.
When you dereference the pointer (with *) then you are referring to the first character of the string.
however a single character is much use to prinf (when print a string) so it instead takes the pointer to the first element and increments it's value printing out the dereference values until the null character is found '\0'.
As this is a C++ question it is also important to note that you should really store strings as the safe encapulated type std::string and you the type safe iostreams where possible:
std::string line="hi how r u";
std::cout << line << std::endl;
%s prints up to the first \0 see: http://msdn.microsoft.com/en-us/library/hf4y5e3w.aspx, %s is a character string format field, there is nothing strange going on here.
printf("%s") expect the address in order to go through the memory searching for NULL (\0) = end of string. In this case you say only letter. To printf("%c") would expect the value not the address: printf("%c", *letter);
printf takes pointers to data arrays as arguments. So, if you're displaying a string (a type of array) with %s or a number with %d, %e, %f, etc, always pass the variable name without the *. The variable name is the pointer to the first element of the array, and printf will print each element of the array by using simple pointer arithmetic according to the type (char is 1 or 2 bytes, ints are 4, etc) until it reaches an EOL or zero value.
Of course, if you make a pointer to the array variable, then you'd want to dereference that pointer with *. But that's more the exception than the rule. :)
Why do we need the *?
char* test = "testing";
From what I understood, we only apply * onto addresses.
This is a char:
char c = 't';
It can only hold one character!
This is a C-string:
char s[] = "test";
It can hold multiple characters. Another way to write the above is:
char s[] = {'t', 'e', 's', 't', 0};
The 0 at the end is called the NUL terminator. It denotes the end of a C-string.
A char* stores the starting memory location of a C-string.1 For example, we can use it to refer to the same array s that we defined above. We do this by setting our char* to the memory location of the first element of s:
char* p = &(s[0]);
The & operator gives us the memory location of s[0].
Here is a shorter way to write the above:
char* p = s;
Notice:
*(p + 0) == 't'
*(p + 1) == 'e'
*(p + 2) == 's'
*(p + 3) == 't'
*(p + 4) == 0 // NUL
Or, alternatively:
p[0] == 't'
p[1] == 'e'
p[2] == 's'
p[3] == 't'
p[4] == 0 // NUL
Another common usage of char* is to refer to the memory location of a string literal:
const char* myStringLiteral = "test";
Warning: This string literal should not be changed at runtime. We use const to warn the programmer (and compiler) not to modify myStringLiteral in the following illegal manner:
myStringLiteral[0] = 'b'; // Illegal! Do not do this for const char*!
This is different from the array s above, which we are allowed to modify. This is because the string literal "test" is automatically copied into the array at initialization phase. But with myStringLiteral, no such copying occurs. (Where would we copy to, anyways? There's no array to hold our data... just a lonely char*!)
1 Technical note: char* merely stores a memory location to things of type char. It can certainly refer to just a single char. However, it is much more common to use char* to refer to C-strings, which are NUL-terminated character sequences, as shown above.
The char type can only represent a single character. When you have a sequence of characters, they are piled next to each other in memory, and the location of the first character in that sequence is returned (assigned to test). Test is nothing more than a pointer to the memory location of the first character in "testing", saying that the type it points to is a char.
You can do one of two things:
char *test = "testing";
or:
char test[] = "testing";
Or, a few variations on those themes like:
char const *test = "testing";
I mention this primarily because it's the one you usually really want.
The bottom line, however, is that char x; will only define a single character. If you want a string of characters, you have to define an array of char or a pointer to char (which you'll initialize with a string literal, as above, more often than not).
There are real differences between the first two options though. char *test=... defines a pointer named test, which is initialized to point to a string literal. The string literal itself is allocated statically (typically right along with the code for your program), and you're not supposed to (attempt to) modify it -- thus the preference for char const *.
The char test[] = .. allocates an array. If it's a global, it's pretty similar to the previous except that it does not allocate a separate space for the pointer to the string literal -- rather, test becomes the name attached to the string literal itself.
If you do this as a local variable, test will still refer directly to the string literal - but since it's a local variable, it allocates "auto" storage (typically on the stack), which gets initialized (usually from a normal, statically allocated string literal) on every entry to the block/scope where it's defined.
The latter versions (with an array of char) can act deceptively similar to a pointer, because the name of an array will decay to the address of the beginning of the array anytime you pass it to a function. There are differences though. You can modify the array, but modifying a string literal gives undefined behavior. Conversely, you can change the pointer to point at some other chars, so something like:
char *test = "testing";
if (whatever)
test = "not testing any more";
...is perfectly fine, but trying to do the same with an array won't work (arrays aren't assignable).
The main thing people forgot to mention is that "testing" is an array of chars in memory, there's no such thing as primitive string type in c++. Therefore as with any other array, you can't reference it as if it is an element.
char* represents the address of the beginning of the contiguous block of memory of char's. You need it as you are not using a single char variable you are addressing a whole array of char's
When accessing this, functions will take the address of the first char and step through the memory. This is possible as arrays use contiguous memory (i.e. all of the memory is consecutive in memory).
Hope this clears things up! :)
Using a * says that this variable points to a location in memory. In this case, it is pointing to the location of the string "testing". With a char pointer, you are not limited to just single characters, because now you have more space available to you.
In C a array is represented by a pointer to the first element in it.