How to get the real and total length of char * (ch

Question

For a char [], I can easily get its length by:

char a[] = "aaaaa";
int length = sizeof(a)/sizeof(char); // length=6

However, I cannot do like this to get the length of a char * by:

char *a = new char[10];
int length = sizeof(a)/sizeof(char);

because, I know, a here is a pointer, such that length here will be always be 4 (or something other in different systems).

My question is that how can I get the length of a char * afterwards? I know someone may challenge me that you already know its 10 because you just created it. I want to know this because this step of getting its length may come long long way from its creation and I don't want to come long long way back to check this number. Moreover, I also want to know its real length.

To be more specific

• how can I get its real length=5?
• how can I get its total length=10?

for the following example:

char *a = new char[10]; 
strcpy(a, "hello");

Answer 1

• In C++:

Just use std::vector<char> which keep the (dynamic) size for you. (Bonus, memory management for free).

Or std::array<char, 10> which keep the (static) size.

• In pure C:

Create a structure to keep the info, something like:

typedef struct {
    char* ptr;
    int size;
} my_array;

my_array malloc_array(int size)
{
    my_array res;
    res.ptr = (char*) malloc(size);
    res.size = size;
    return res;
}

void free_array(my_array array)
{
    free(array.ptr);
}

Answer 2

You can make a back-tracker character, ex, you could append any special character say "%" to the end of your string and then check the occurrence of that character.
But this is a very risky way as that character can be in other places also in the char*

char* stringVar = new char[4] ; 
stringVar[0] = 'H' ; 
stringVar[1] = 'E' ; 
stringVar[2] = '$' ; // back-tracker character.
int i = 0 ;
while(1)
{
   if (stringVar[i] == '$')
     break ; 
   i++ ; 
}
//  i is the length of the string.
// you need to make sure, that there is no other $ in the char* 

Otherwise define a custom structure to keep track of length and allocate memory.

Answer 3

char *a = new char[10];

My question is that how can I get the length of a char *

It is very simply.:) It is enough to add only one statement

size_t N = 10;
char *a = new char[N];

Now you can get the size of the allocated array

std::cout << "The size is " << N << std::endl;

Many mentioned here C standard function std::strlen. But it does not return the actual size of a character array. It returns only the size of stored string literal.

The difference is the following. if to take your code snippet as an example

char a[] = "aaaaa";
int length = sizeof(a)/sizeof(char); // length=6

then std::strlen( a ) will return 5 instead of 6 as in your code.

So the conclusion is simple: if you need to dynamically allocate a character array consider usage of class std::string. It has methof size and its synonym length that allows to get the size of the array at any time.

For example

std::string s( "aaaaa" );

std::cout << s.length() << std::endl;

or

std::string s;
s.resize( 10 );

std::cout << s.length() << std::endl;

Answer 4

You can find the length of a char* string like this:

char* mystring = "Hello World";
int length = sprintf(mystring, "%s", mystring);

sprintf() prints mystring onto itself, and returns the number of characters printed.

Answer 5

You could try this:

int lengthChar(const char* chararray) {
   int n = 0;
   while(chararray[n] != '\0')
     n ++;
   return n;  
}

Answer 6

If the char * is 0-terminated, you can use strlen

Otherwise, there is no way to determine that information