C++ SQLite3 query issue unsolved

Question

I'm somewhat new to SQL and C++. I do believe the question I have yields a simple solution.

I have to create a sql query to insert some values into an existing table (const char *sql_query), with mixed strings and float values. My issue is getting the string assembly operation nailed down, without getting compiler errors.

Here is my attempt:

int main(int argc, char* argv[])
{
    create_entry("mahut", "topa", "suma", 5.55, 6.66, 7.77);

    return 0;
}

void create_entry(char col1[], char col2[], char col3[], float col4, float col5, float col6)
{
    cout << ("create_entry") << endl;

    sqlite3 *db;
    char *zErrMsg = 0;
    int rc;
    char *sql;
    char db_name[] = "db_test.db";
    stringstream sql_query;

    // assemble string
    sql_query << "INSERT INTO Department1 (Date,Time,Accept,Factor1,Factor2,Factor3) VALUES ('" << col1 << "','" << col2 << "','" << col3 << "','" << col4 << "','" << col5 << "','" << col6 << "')";
    string str = sql_query.str();

    rc = sqlite3_open(db_name, &db);

    if( rc )
    {
        fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
    }
    else
    {
        fprintf(stderr, "Opened database successfully\n");
    }

    // execute SQL statement
    rc = sqlite3_exec(db, str, callback, 0, &zErrMsg);

    if( rc != SQLITE_OK )
    {
        fprintf(stderr, "SQL error: %s\n", zErrMsg);
        sqlite3_free(zErrMsg);
    }
    else
    {
        fprintf(stdout, "Records created successfully\n");
    }
    sqlite3_close(db);
}

static int callback(void *db, int argc, char **argv, char **azColName)
{
    cout << "Debug <in callback>" << endl;
    for(int i=0; i<argc; ++i)
    {
        //printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL");
        cout << ("%s", argv[i] ? argv[i] : "NULL") << endl;
    }
    tbl_exists = true;

    return 0;
}

Particularly this is my problem:

sql_query << "INSERT INTO Department1 (Date,Time,Accept,Factor1,Factor2,Factor3) VALUES ('" << col1 << "','" << col2 << "','" << col3 << "','" << col4 << "','" << col5 << "','" << col6 << "')";

As I don't understand if there is a proper way to mix in float, integer and string into const char * I'm not sure what's wrong, but like I said I have tried it for quite some time to properly assemble that string but have failed, as I just don't grasp pointers and strings in C++. Compiling fails due to following error

error: cannot convert ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ to ‘const char*’ for argument ‘2’ to ‘int sqlite3_exec(sqlite3*, const char*, int (*)(void*, int, char**, char**), void*, char**)’   rc = sqlite3_exec(db, str, callback, 0, &zErrMsg);

I also get the following warning

warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]

Answer 1

For your error, you're missing a .c_str() conversion of the std::string to a C-style string. sqlite3_exec takes C-style const char * strings, not C++ strings.

For your warning, you'll need to declare your parameters to create_entry() as const char * (or const char [] ) - because they're literals (constants that should not be modified). Declaring them non-const opens you up to accidentally modifying them.

Answer 2

Assembling the string becomes easier if you are not doing it at all:

void create_entry(const char col1[], const char col2[], const char col3[],
                  float col4, float col5, float col6)
{
    sqlite3 *db;
    int rc;
    const char *sql;
    static const char db_name[] = "db_test.db";
    sqlite3_stmt *stmt;

    sql = "INSERT INTO Department1 (Date,Time,Accept,Factor1,Factor2,Factor3)"
          " VALUES (?,?,?,?,?,?)";

    rc = sqlite3_open(db_name, &db);
    if( rc != SQLITE_OK )
    {
        fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
        return;
    }

    rc = sqlite3_prepare_v2(db, sql, -1, &stmt, NULL);
    if( rc != SQLITE_OK )
    {
        fprintf(stderr, "Can't prepare query: %s\n", sqlite3_errmsg(db));
        sqlite3_close(db);
        return;
    }

    sqlite3_bind_text(stmt, 1, col1, -1, SQLITE_STATIC);
    sqlite3_bind_text(stmt, 2, col2, -1, SQLITE_STATIC);
    sqlite3_bind_text(stmt, 3, col3, -1, SQLITE_STATIC);
    sqlite3_bind_double(stmt, 4, col4);
    sqlite3_bind_double(stmt, 5, col5);
    sqlite3_bind_double(stmt, 6, col6);

    rc = sqlite3_step(stmt);
    if( rc != SQLITE_DONE )
    {
        fprintf(stderr, "Can't execute query: %s\n", sqlite3_errmsg(db));
        sqlite3_finalize(stmt);
        sqlite3_close(db);
        return;
    }

    sqlite3_finalize(stmt);
    sqlite3_close(db);
}