C++ SQLite3 查询问题未解决

Question

我对 SQL 和 C++ 有点陌生。我确实相信我提出的问题会产生一个简单的解决方案。

我必须创建一个 sql 查询以将一些值插入现有表 (const char *sql_query)，其中包含混合字符串和浮点值。我的问题是确定字符串汇编操作，而不会出现编译器错误。

这是我的尝试：

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;
}

尤其是我的问题：

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

我不明白是否有适当的方法将浮点数、整数和字符串混合到 const char * 我不确定出了什么问题，但是就像我说的那样，我已经尝试了很长时间来正确组装该字符串但失败了，因为我只是不掌握 C++ 中的指针和字符串。由于以下错误，编译失败

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);

我也收到以下警告

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

Answer 1

对于您的错误，您缺少将 std::string 转换为 C 样式字符串的 .c_str() 转换。 sqlite3_exec 采用 C 风格的 const char * 字符串，而不是 C++ 字符串。

对于您的警告，您需要将create_entry() 的参数声明为const char *（或const char []） - 因为它们是文字（不应修改的常量）。将它们声明为非 const 会使您意外修改它们。

Answer 2

如果你根本不做，组装字符串会变得更容易：

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);
}