Why is this code gives an Debug Assertion Fail?
std::string query;
int ClientID = 666;
query = "select logged from login where id = ";
query.append((char *)ClientID);
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can you add to definition of ClientID ? is it a std::string or a char * ?dwalter– dwalter2012-05-09 12:37:02 +00:00Commented May 9, 2012 at 12:37
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says invalid null-pointer by the wayHakon89– Hakon892012-05-09 12:39:45 +00:00Commented May 9, 2012 at 12:39
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4 Answers
The std::string::append() method expects its argument to be a NULL terminated string (char*).
There are several approaches for producing a string containg an int:
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#include <sstream> std::ostringstream s; s << "select logged from login where id = " << ClientID; std::string query(s.str()); std::to_string(C++11)std::string query("select logged from login where id = " + std::to_string(ClientID));-
#include <boost/lexical_cast.hpp> std::string query("select logged from login where id = " + boost::lexical_cast<std::string>(ClientID));
11 Comments
luke
+1 for std::to_string
ervinbosenbacher
+1 for clear, clean and complete answer
Hakon89
std::to_string has no function that requiers an int...
luke
It does according to the standard (check the reference link). Your compiler may not implement it yet, however.
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You cannot cast an int to a char* to get a string. Try this:
std::ostringstream sstream;
sstream << "select logged from login where id = " << ClientID;
std::string query = sstream.str();
I have a feeling that your ClientID is not of a string type (zero-terminated char* or std::string) but some integral type (e.g. int) so you need to convert number to the string first:
std::stringstream ss;
ss << ClientID;
query.append(ss.str());
But you can use operator+ as well (instead of append):
query += ss.str();
Comments
You are casting ClientID to char* causing the function to assume its a null terinated char array, which it is not.
from cplusplus.com :
string& append ( const char * s ); Appends a copy of the string formed by the null-terminated character sequence (C string) pointed by s. The length of this character sequence is determined by the first ocurrence of a null character (as determined by traits.length(s)).
