C++中sqlite3回调函数的正确使用
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Proper use of callback function of sqlite3 in C++
提问by Michel de Man
I have the following C++ code for testing purposes in conjunction with SQLite3.
It's a class called customerwith a callback function declared. This callback function is called whenever sqlite3_exec()returns results (records) from the SQLite database.
我有以下 C++ 代码用于与 SQLite3 结合进行测试。它是一个customer声明了回调函数的类。每当sqlite3_exec()从 SQLite 数据库返回结果（记录）时，都会调用此回调函数。
What I don't like about this construction is that source code to process the results is located in a call back function outside of the class rather than the results being processed by the class method from which sqlite3_exec()is called.
我不喜欢这种构造的是，处理结果的源代码位于类外部的回调函数中，而不是由sqlite3_exec()调用的类方法处理的结果。
I could use global variables that will be used in the class method after the callback function has finished extracting the values from the SQL query results. But what if there is more than one record and the call back function is called several times. Then I need to work with arrays unless I make sure that I will only have single results.
在回调函数完成从 SQL 查询结果中提取值后，我可以使用将在类方法中使用的全局变量。但是如果有不止一条记录并且回调函数被调用了几次呢？然后我需要使用数组，除非我确保我只会有一个结果。
Do I need to forget about the callback function and go into deeper calls of the SQLite API?
我是否需要忘记回调函数并深入调用 SQLite API？
Or do I need to go to a C++ wrapper, I suppose that there is no call back mechanism there and the results being passed back to the class method itself?
或者我需要去一个 C++ 包装器，我想那里没有回调机制，结果被传递回类方法本身？
// customer
#include "Customer\customer.h"
//## begin module%50E6CCB50119.additionalDeclarations preserve=yes
static int callback(void *NotUsed, int argc, char **argv, char **azColName)
{
int i;
char* columnName;
char* columnValueString;
short int columnValueShortInt = 0;
int columnValueInt = 0;
cout << "begin of callback function\n";
for(i=0; i<argc; i++)
{
columnName = azColName[i];
if (strcmp(columnName, "FirstName")==0 || strcmp(columnName, "LastName")==0)
{
columnValueString = argv[i];
cout << "columnName = " << columnName << "; value = " << columnValueString <<"\n";
}
else
{
if(strcmp(columnName, "Age")==0)
{
stringstream(argv[i]) >> columnValueShortInt;
cout << "columnName = " << columnName << "; value = " << columnValueShortInt <<"\n";
}
else // strcmp(columnName, "Id")==0)
{
stringstream(argv[i]) >> columnValueInt;
cout << "columnName = " << columnName << "; value = " << columnValueInt <<"\n";
}
}
}
cout << "end of call back function \n";
return 0;
}
//## end module%50E6CCB50119.additionalDeclarations
// Class customer
customer::customer ()
//## begin customer::customer%50F969EE01E4.hasinit preserve=no
//## end customer::customer%50F969EE01E4.hasinit
//## begin customer::customer%50F969EE01E4.initialization preserve=yes
//## end customer::customer%50F969EE01E4.initialization
{
//## begin customer::customer%50F969EE01E4.body preserve=yes
customerId = 0;
zErrMsg = 0;
customerDataBaseRc = sqlite3_open("customerdb", &customerDataBase);
if(customerDataBaseRc)
{
fprintf(stderr, "Can't open database %s\n", sqlite3_errmsg(customerDataBase));
sqlite3_close(customerDataBase);
}
const char * pSQL[6];
const char * sqlStatement;
pSQL[0] = "create table customerTable (Id int, FirstName varchar(30), LastName varchar(30), Age smallint)";
// execute all the sql statements
for(int i = 0; i < 1; i++)
{
customerDataBaseRc = sqlite3_exec(customerDataBase, pSQL[i], callback, 0, &zErrMsg);
if( customerDataBaseRc !=SQLITE_OK )
{
fprintf(stderr, "SQL error: %s\n", zErrMsg);
sqlite3_free(zErrMsg);
break; // break the loop if error occur
}
}
//## end customer::customer%50F969EE01E4.body
}
customer::~customer ()
{
//## begin customer::~customer%50F93279003E.body preserve=yes
const char *pSQL[6];
// Remove all data in customerTable
pSQL[0] = "delete from customerTable";
// Drop the table from database
pSQL[1] = "drop table customerTable";
// execute all the sql statements
for(int i = 0; i < 2; i++)
{
customerDataBaseRc = sqlite3_exec(customerDataBase, pSQL[i], callback, 0, &zErrMsg);
if( customerDataBaseRc !=SQLITE_OK )
{
fprintf(stderr, "SQL error: %s\n", zErrMsg);
sqlite3_free(zErrMsg);
break; // break the loop if error occur
}
}
cout << "destructor";
//## end customer::~customer%50F93279003E.body
}
//## Other Operations (implementation)
unsigned int customer::createCustomer (char iCustomerFirstName[20], char iCustomerLastName[20], unsigned short iCustomerAge)
{
//## begin customer::createCustomer%50EBFFA3036B.body preserve=yes
const char *sqlStatement;
string result; // string which will contain the result
ostringstream convert; // stream used for the conversion
convert << "insert into customerTable (Id, FirstName, LastName, Age) values (" << customerId << ", '" << iCustomerFirstName << "', '" << iCustomerLastName << "', " << iCustomerAge << ")";
result = convert.str(); // set 'Result' to the contents of the stream
sqlStatement = result.c_str();
// Execute sql statement
customerDataBaseRc = sqlite3_exec(customerDataBase, sqlStatement, callback, 0, &zErrMsg);
// Check for errors
if(customerDataBaseRc !=SQLITE_OK )
{
fprintf(stderr, "SQL error: %s\n", zErrMsg);
sqlite3_free(zErrMsg);
}
return customerId++;
//## end customer::createCustomer%50EBFFA3036B.body
}
char * customer::getCustomer (unsigned int iCustomerId)
{
//## begin customer::getCustomer%50ED3D700186.body preserve=yes
const char *sqlStatement;
char *tmp ="blabla";
string result; // string which will contain the result
ostringstream convert; // stream used for the conversion
convert << "select * from customerTable where Id = " << iCustomerId;
result = convert.str(); // set 'Result' to the contents of the stream
sqlStatement = result.c_str();
// Execute the sql statement
customerDataBaseRc = sqlite3_exec(customerDataBase, sqlStatement, callback, 0, &zErrMsg);
// Check for errors
if(customerDataBaseRc !=SQLITE_OK )
{
fprintf(stderr, "SQL error: %s\n", zErrMsg);
sqlite3_free(zErrMsg);
}
return tmp;
//## end customer::getCustomer%50ED3D700186.body
}
// Additional Declarations
//## begin customer%50E6CCB50119.declarations preserve=yes
//## end customer%50E6CCB50119.declarations
//## begin module%50E6CCB50119.epilog preserve=yes
//## end module%50E6CCB50119.epilog
回答by mah
What one typically does in this case is take advantage of the void *(which you call NotUsed) parameter of the callback -- a parameter youdefine when you install the callback. For C++, you would typically set that parameter to the thispointer to your interested object, and you would make the callback (an extern "C"function in a c++ source file) a friendmethod to your class (if necessary).
在这种情况下，人们通常做的是利用回调的void *（您称之为NotUsed）参数——您在安装回调时定义的参数。对于 C++，您通常将该参数设置为this指向您感兴趣的对象的指针，并且您将使回调（extern "C"c++ 源文件中的函数）成为friend类的方法（如有必要）。
This would look like this:
这看起来像这样：
class customer
{
...
public:
int callback(int argc, char **argv, char **azColName);
};
static int c_callback(void *param, int argc, char **argv, char **azColName)
{
customer* cust = reinterpret_cast<customer*>(param);
return cust->callback(argc, argv, azColName);
}
char* customer::getCustomer(int id)
{
...
rc = sqlite3_exec(db, sql, c_callback, this, &errMsg);
...
}
int customer::callback(int argc, char **argv, char **azColName)
{
...
}
回答by CL.
Using sqlite3_exechas the disadvantages that you have to convert some values back from a string to a number, and that it needs to allocate memory for all result records (which can lead to problems when reading large tables).
Furthermore, the callback always is a separate function (even if it's in the same class).
使用sqlite3_exec的缺点是您必须将某些值从字符串转换回数字，并且需要为所有结果记录分配内存（这可能会导致读取大表时出现问题）。此外，回调始终是一个单独的函数（即使它在同一个类中）。
For your example query, using the sqlite3_prepare/sqlite3_step/sqlite3_finalizeAPI would look like this:
为了您的示例查询，使用sqlite3_prepare/ sqlite3_step/ sqlite3_finalizeAPI是这样的：
void one_customer::readFromDB(sqlite3* db, int id)
{
sqlite3_stmt *stmt;
int rc = sqlite3_prepare_v2(db, "SELECT FirstName, LastName, Age"
" FROM customerTable"
" WHERE Id = ?", -1, &stmt, NULL);
if (rc != SQLITE_OK)
throw string(sqlite3_errmsg(db));
rc = sqlite3_bind_int(stmt, 1, id); // Using parameters ("?") is not
if (rc != SQLITE_OK) { // really necessary, but recommended
string errmsg(sqlite3_errmsg(db)); // (especially for strings) to avoid
sqlite3_finalize(stmt); // formatting problems and SQL
throw errmsg; // injection attacks.
}
rc = sqlite3_step(stmt);
if (rc != SQLITE_ROW && rc != SQLITE_DONE) {
string errmsg(sqlite3_errmsg(db));
sqlite3_finalize(stmt);
throw errmsg;
}
if (rc == SQLITE_DONE) {
sqlite3_finalize(stmt);
throw string("customer not found");
}
this->id = id;
this->first_name = string(sqlite3_column_text(stmt, 0));
this->last_name = string(sqlite3_column_text(stmt, 1));
this->age = sqlite3_column_int(stmt, 2);
sqlite3_finalize(stmt);
}
(This code handles errors by just throwing a stringwith the error message.)
（此代码通过抛出string带有错误消息的a 来处理错误。）
