如何获取在 C++ 中使用 C 回调返回的结果

Question

我对在 C++ 中处理 C 回调比较陌生。我制作了一个 sqlite 包装器 c++ 类，它只调用 sqlite3_exec()。

static int callback(void *NotUsed, int argc, char **argv, char **azColName){
  SqliteAccessor* sqlite = static_cast<SqliteAccessor*> NotUsed; 
  if(argc > 0) { 
    sqlite->set_table_exists(true); 
  }
  return 0;
}

class SqliteAccessor{    
public:
bool has_table(const string dbName, const string tblName)
{
  string sql;
  sql = "SELECT " + quote_string(tblName) + "FROM " + quote_string(dbName)
           + "WHERE type = 'table' AND name = " + quote_string(tblName) + ";";
  char *zErrMsg = 0;
  int rc = sqlite3_exec(m_db, sql.c_str(), callback, (void*) this, &zErrMsg);
  if( rc != SQLITE_OK ){
    printf("SQL error: %s", zErrMsg);
    sqlite3_free(zErrMsg);
  }
  // anyway to return the result directly?
  // return hasTable;
}

// can I avoid the following methods and the member variable? 
void set_table_exists(bool isExist) { m_table_exist = isExist; }
bool get_table_exists() { return m_table_exist; }
private: 
  static bool m_table_exist; 
};

int caller(){
   SqliteAccessor sqlite;
   // to check if table exist
   if (sqlite->has_table()){
      // will above work or 
      // I should do with an extra call to query the changed state?        
   }
}

现在，我很困惑调用者如何从 sqlite 包装器中获取结果。我认为，调用者不能通过简单地调用has_table() 获得结果，因为结果是由set_table_exists() 从回调返回的。那么调用者是否应该通过另一个调用来获得结果，例如打电话给sqlite->get_table_exists()？

那么这意味着对于每一个回调，我需要在SqliteAccessor类中创建一个成员变量（也必须是static），以及一对set/get_state()，这样会很麻烦。

如何设计类以使其易于被调用者使用？ 不幸的是，我们的代码库不支持 c++11。

Answer 1

如果您使用 C++11，请考虑使用 lambda 而不是回调。

class SqliteAccessor{    
public:
bool has_table(const string dbName, const string tblName)
{
  bool hasTable = false;
  string sql;
  sql = "SELECT " + quote_string(tblName) + "FROM " + quote_string(dbName)
           + "WHERE type = 'table' AND name = " + quote_string(tblName) + ";";
  char *zErrMsg = 0;
  int rc = sqlite3_exec(m_db, sql.c_str(), [&](void *NotUsed, int argc, char **argv, char **azColName){
  SqliteAccessor* sqlite = static_cast<SqliteAccessor*> NotUsed; 
  if(argc > 0) { 
    hasTable = true; 
  }
 }
 , (void*) this, &zErrMsg);
  if( rc != SQLITE_OK ){
    printf("SQL error: %s", zErrMsg);
    sqlite3_free(zErrMsg);
  }
  return hasTable;
}
};

如果您无法访问 C++11，您始终可以手动编写仿函数。但是，您会失去一些简洁性和局部性。好的部分是函子可以保存您需要的状态。

struct callback{
  bool operator(void *NotUsed, int argc, char **argv, char **azColName)
  {
  SqliteAccessor* sqlite = static_cast<SqliteAccessor*> NotUsed; 
  if(argc > 0) { 
    hasTable = true; 
  }
  return false;
  }
  bool hasTable;
};


class SqliteAccessor{    
public:
bool has_table(const string dbName, const string tblName)
{
  bool hasTable = false;
  string sql;
  sql = "SELECT " + quote_string(tblName) + "FROM " + quote_string(dbName)
           + "WHERE type = 'table' AND name = " + quote_string(tblName) + ";";
  char *zErrMsg = 0;

  callback c;
  int rc = sqlite3_exec(m_db, sql.c_str(), c, (void*) this, &zErrMsg);
  if( rc != SQLITE_OK ){
    printf("SQL error: %s", zErrMsg);
    sqlite3_free(zErrMsg);
  }
  return c.hasTable;
}
};

Answer 2

我会这样做的方式是使回调成为私有静态成员函数，并基本上做你所做的。像这样：

class SqliteAccessor
{
public:
    bool has_table(const std::string dbName, const std::string tblName);

private:
    static int callback(void *NotUsed, int argc, char **argv, char **azColName);

    bool m_hasTable;
};

int SqliteAccessor::callback(void *NotUsed, int argc, char **argv, char **azColName)
{
     SqliteAccessor* sqlite = static_cast<SqliteAccessor*>(NotUsed);
     if(argc > 0) sqlite->m_hasTable = true;
     return 0;
}


bool SqliteAccessor::has_table(const std::string dbName, const std::string tblName)
{
    m_hasTable = false;
    string sql = "SELECT " + quote_string(tblName) + "FROM " + quote_string(dbName)
           + "WHERE type = 'table' AND name = " + quote_string(tblName) + ";";
    char *zErrMsg = 0;
    int rc = sqlite3_exec(m_db, sql.c_str(), callback, (void*) this, &zErrMsg);
    if( rc != SQLITE_OK )
    {
        printf("SQL error: %s", zErrMsg);
        sqlite3_free(zErrMsg);
    }

    return m_hasTable;
}

int caller()
{
   SqliteAccessor sqlite;
   // to check if table exist
   if (sqlite.has_table())
   {
      // do stuff :)
   }
}

如果 sqlite3_exec() 不适用于静态函数，您可以尝试使用这样的全局函数：

class SqliteAccessor
{
public:
    bool has_table(const std::string dbName, const std::string tblName);

private:    
    bool m_hasTable;

    friend int callback(void *NotUsed, int argc, char **argv, char **azColName);
};

int callback(void *NotUsed, int argc, char **argv, char **azColName)
{
     SqliteAccessor* sqlite = static_cast<SqliteAccessor*>(NotUsed);
     if(argc > 0) sqlite->m_hasTable = true;
     return 0;
}