Append an int to a std::string

Question

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);

Answer 1

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));
  

Answer 2

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

Answer 3

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();

Answer 4

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)).