Append an int to a std::string

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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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)).

小开

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:

  • std::ostringstream

    #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));

  • boost::lexical_cast

    #include <boost/lexical_cast.hpp>


std::string query("select logged from login where id = " +
boost::lexical_cast<std::string>(ClientID));
小开

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();

stringstream reference

小开

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();

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