Appending an Integer to a std::string: Why the Assertion Fails
In C , when attempting to append an integer value directly to a std::string as seen below:
std::string query; int ClientID = 666; query = "select logged from login where id = "; query.append((char *)ClientID);
you may encounter a Debug Assertion Fail. This occurs because the std::string::append() method requires its argument to be a NULL-terminated string (char*). However, passing an integer as a pointer to a character array without terminating it with a null character will result in undefined behavior and potentially a crash.
To correctly append an integer to a std::string, several approaches are available:
1. Using std::ostringstream:
#include <sstream> std::ostringstream s; s << "select logged from login where id = " << ClientID; std::string query(s.str());
This method creates a stringstream object, inserts the text and the integer value into it, and retrieves the resulting string.
2. Using std::to_string (C 11):
std::string query("select logged from login where id = " + std::to_string(ClientID));
This approach uses the std::to_string function to convert the integer to a string and then performs the concatenation.
3. Using boost::lexical_cast:
#include <boost/lexical_cast.hpp> std::string query("select logged from login where id = " + boost::lexical_cast<std::string>(ClientID));
This method utilizes the Boost library's lexical_cast function to convert the integer to a string and perform the concatenation.
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