C+中sqlite3回调函数的正确使用+； 我有以下C++代码，用于与SqLITE3一起进行测试。 它是一个名为customer的类，声明了一个回调函数。只要sqlite3\u exec（）从SQLite数据库返回结果（记录），就会调用此回调函数
我不喜欢这种构造，因为处理结果的源代码位于类外的回调函数中，而不是由调用C+中sqlite3回调函数的正确使用+； 我有以下C++代码，用于与SqLITE3一起进行测试。 它是一个名为customer的类，声明了一个回调函数。只要sqlite3\u exec（）从SQLite数据库返回结果（记录），就会调用此回调函数,c++,sqlite,C++,Sqlite,我不喜欢这种构造，因为处理结果的源代码位于类外的回调函数中，而不是由调用sqlite3_exec（）的类方法处理的结果 我可以使用全局变量，在回调函数从SQL查询结果中提取完值后，这些变量将在类方法中使用。但是，如果有多条记录，并且回调函数被调用多次，该怎么办呢。然后，我需要使用数组，除非我确保我将只得到一个结果 我是否需要忘记回调函数，而深入研究SQLite API的调用 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 "
sqlite3_exec（）// 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
//客户
#包括“Customer\Customer.h”
//##开始模块%50E6CCB50119。附加声明=是
静态int回调（void*NotUsed，int argc，char**argv，char**azColName）
{
int i；
字符*列名称；
char*columnValueString；
short int columnValueShortInt=0；
int columnValueInt=0；
cout在这种情况下，人们通常会利用void*
（您称之为NotUsed
）回调参数——当你安装回调时定义的参数。对于C++，你通常会将参数设置为<代码>这个指向你感兴趣对象的指针，并且你会在你的类中（如有必要）在你的类中做出回调（A<代码>外部）C++“函数”，< <代码>朋友>代码>。
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)
{
...
}
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);
}
class客户
{
...
公众：
int回调（int argc，char**argv，char**azColName）；
};
静态int c_回调（void*param，int argc，char**argv，char**azColName）
{
客户*cust=重新解释铸件（参数）；
返回客户->回调（argc、argv、azColName）；
}
char*customer:：getCustomer（int-id）
{
...
rc=sqlite3_exec（db、sql、c_回调、this和errMsg）；
...
}
int customer:：回调（int argc，char**argv，char**azColName）
{
...
}
使用的缺点是，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（这可能导致读取大型表时出现问题）。
此外，回调始终是一个单独的函数（即使它在同一个类中）
对于您的示例查询，使用//API如下所示：
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)
{
...
}
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);
}
（此代码通过错误代码发送<代码>字符串来处理错误。）/P>这是我当前的C++知识，而不是经验，必须对这一点进行处理。如果我通过这个指针，调用调用函数就可以填充类实例（对象）的属性。然后在sqlite3_exec调用完成后处理方法本身中的值？是的。您需要将传入的
void*
转换为对象指针类型，可能使用类似于MyClass*object=（MyClass*）的内容NotUsed；
。从那时起，您可以访问有关对象的所有公共内容；如果您想访问私有（或受保护）内容，则需要（在类中）声明回调（）
是一个friend函数。您可以在google搜索中轻松找到friend函数的示例。谢谢，我将尝试实现您的建议，我已经研究了上面给出的示例。因此，看起来c_回调函数正在将sql查询的结果传递给类方法回调，然后可以在其中提取查询结果e place，即填充this pointer对象属性。然后在getCustomer中可以进行进一步的处理，对吗？通常，您会对每个sqlite3_exec
调用使用一个单独的回调。这真的是最小的代码示例吗？不太可能，我想，我是这个论坛的新手，下次会尝试最小化代码，谢谢您的提示.人们给出如此有用和详细的解决方案的速度给我留下了深刻的印象，非常感谢。