string to char converting problem

[bergziege]

hi,
first i ll want to tell you what i am planing to do:
i want to convert a cpp string to a char array
the string gets assembled with x = x + y and wors just fine.
then i want to convert this string into a dynamically sized char array.

        string sFinalQuery;

	char *cQuery;   // where everything should go at the end.
 
	sFinalQuery = "select * from gpsdestinations where name like '%";
	sFinalQuery = sFinalQuery + sMySQLQuery;
	sFinalQuery = sFinalQuery + "%' OR beschreibung like '%";
	sFinalQuery = sFinalQuery + sMySQLQuery;
	sFinalQuery = sFinalQuery + "%'";
	
	cout << sFinalQuery << endl; // test output of the string
	printf("\n\nLänge Query : %d\n\n", sFinalQuery.length()); // printing string lenght
	cQuery = (char *) malloc(sFinalQuery.length());
	for (int i=0; i < sFinalQuery.length(); i++){
		cQuery[i] = sFinalQuery[i];
		printf("%c", cQuery[i]);
	}
	printf("\n\nLänge Query : %d\n\n", strlen(cQuery));  // printing array lenght

at the end the lenght from the string and the array are different but they should not be.

may some of you can post a idea why the lenghts are not the same.
i am just going nuts with it.

greez bergziege

ps.: sMySQLQuery is just another string coming from an input device

[Bench82]

I would suggest being careful with the use of malloc/free in C++ code (its OK for built-in types, but for anything else you should use new/delete. For the sake of consistency, you should prefer new/delete unless you have some compelling reason to do otherwise. At the moment your code looks like a fairly ugly mix of C and C++)

you can get a const char* representation of a std::string with the string::c_str() function.

so if you have a function with a signature requiring a const char*, pass it as so

void some_function(const char* );

/* .... */

std::string my_string("Hello, World");
some_function( my_string.c_str() );

/* .... */

Perhaps the reason that your lengths are different is that you are allocating enough space for sFinalQuery.length() ... this will return the length of the string - however a C-style string requires a null terminating character, so you need enough space for sFinalQuery.length() + 1. At the moment I believe your program exhibits undefined behaviour.

[bergziege]

thanks for the fast response.
i skipped all the dynamic stuff and now using .c_str()
works just fine.

thanks again for help
greez bergziege