C++ MFC SQLite sqlite3_exec 回调

Question

所以我正在尝试在 Visual Studio 2010 中将 SQLite 与 MFC 对话一起使用。我不清楚如何使用回调函数来保存查询结果，该查询试图计算表的数量在我的数据库中，进入变量 m_Results。有没有办法做到这一点，或者有没有办法让我访问 nTables 变量？

static int callback(void *data, int argc, char **argv, char **azColName){
    int i;
    fprintf(stderr, "%s: ", (const char*)data);
    data = argv[0];
    return 0;
}

BOOL CDBpracticeDlg::OnInitDialog(){
    ...

    // TODO: Add extra initialization here
    sqlite3 *db;
    char *zErrMsg = 0;
    int rc;
    char *sql;
    const char* data = "Callback function called";

    rc = sqlite3_open("structInfo_Test.db", &db);

    if( rc ){
        fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
        return(0);
    }else{
        fprintf(stderr, "Opened database successfully\n");
    }

    /* Create SQL statement */
    sql = "Select Count(*) as nTables FROM sqlite_master where type='table';";

    /* Execute SQL statement */
    rc = sqlite3_exec(db, sql, callback, &m_Results, &zErrMsg);
    if( rc != SQLITE_OK ){
        char error[200];
        strcpy(error,"SQL error: ");
        strcat(error,zErrMsg);
        m_Results = error;
        fprintf(stderr, "SQL error: %s\n", zErrMsg);
        sqlite3_free(zErrMsg);
    }else{
        fprintf(stdout, "Operation done successfully\n");
    }
    UpdateData(FALSE);
    sqlite3_close(db);

    return TRUE;  // return TRUE  unless you set the focus to a control
}

Answer 1

sqlite3_exec 的回调API 被广泛记录在案。据称它具有误导性的参数名称这一事实也无济于事。提供更自然的形式参数名称大有帮助:

static int callback(void* context,  // user-provided object (4th param to sqlite3_exec)
                    int columnCount,      // number of columns
                    char** columnValues,  // array of column values as C-style strings
                    char** columnName)    // array of column names as C-style strings

要从 回调 更新 m_Results 对象，您只需将 context 转换为正确的类型，然后使用它:

static int callback(void* context, int columnCount,
                    char** columnValues, char** columnNames) {
    CMyType& results = *static_cast<CMyType*>(context);
    // Use 'results' which is a reference to 'm_Results'.
    for (int index = 0; index < columnCount; ++index) {
        // Assuming CMyType has operator+=(const char*) (like a CString)
        results += columnNames[index];
        results += ": ";
        results += columnValues[index];
    }
    // Return 0 to continue invoking the callback for the remaining rows in the result
    // set. Returning non-zero will terminate the callbacks, and sqlite3_exec()
    // returns SQLITE_ABORT.
    return 0;

作为替代方案，您可以将 this 代替 &m_Results 传递给 sqlite3_exec()，并从回调中调用公共(public)类成员:

static int callback(void* context, int columnCount,
                    char** columnValues, char** columnNames) {
    CDBpracticeDlg* obj = static_cast<CDBpracticeDlg*>(context);
    // Delegate callback to class member implementation
    return obj->MyCallback(columnCount, columnValues, columnNames);
}

...

rc = sqlite3_exec(db, sql, callback, this, nullptr);