将const * char []变量与char []连接

Question

I'm making a connection between C++ and MySql, and I was trying to do something like this:

int id=1;
const char *sql = "delete from register where id=";
strcat(*sql, itoa(id));

But I can't because itoa(id) does not generate a *char[] variable.

What can I do?

Answer 1

Double-check the docs for itoa . It takes three arguments and will write the value of your number to the string you provide. http://www.cplusplus.com/reference/cstdlib/itoa/

It would be easier to do the following though.

char string[MAX_LENGTH];
snprintf(string, MAX_LENGTH-1, "delete from register where id = %d", id);

Answer 2

You can't concatenate onto a string literal. You have to copy the string literal to a separate buffer, and then concatenate onto that. Also, itoa() requires a buffer of its own, too:

int id = 1;
char sql_buffer[50];
char id_buffer[16];
strcpy(sql_buffer, "delete from register where id=");
strcat(sql_buffer, itoa(id, id_buffer, 10));

Or:

int id = 1;
char sql_buffer[50] = "delete from register where id=";
char id_buffer[16];
strcat(sql_buffer, itoa(id, id_buffer, 10));

Alternatively, use snprintf() instead:

int id = 1;
char sql_buffer[50];
snprintf(sql_buffer, sizeof(sql_buffer), "delete from register where id=%d", id);

But, either way, these are C ways to do things. The C++ way is to use a std::ostringstream instead:

#include <string>
#include <sstream>

int id = 1;
std::ostringstream oss;
oss << "delete from register where id=" << id;
std::string sql = oss.str();