C ++ populate向量从sqlite3回调函数？ [英] C++ populate vector from sqlite3 callback function?

问题描述

我有一个sqlite3表地址像这样：

  + ---- + + -------- + 
 | ID |名称|数字| 
 + ---- + ------ + -------- + 
 | 1 | John | 413434 | 
 + ---- + ------ + -------- + 
  

而我想填充全局向量，我将能够使用别的地方，并与其他一些数据连接。
到目前为止，我有这个：

  ... 
 #include< sqlite3.h> 
 
 using namespace std; 
 
 static int callbackDB（void * NotUsed，int argc，char ** argv，char ** szColName）
 {
 for（int i = 0; i  cout<< szColName [i]< =<< argv [i]<< endl; 
 
 return 0; 
} 
 
 int main（int argc，char * argv []）{
 vector< vector< string& >表; 
 for（int i = 0; i <2; i ++）
 table.push_back（std :: vector< std :: string>（））; 
 
 
 sqlite3 * db; 
 char * szErrMsg = 0; 
 
 //打开数据库
 int rc = sqlite3_open（database.db，& db）; 
 
 const char * query; 
 query =SELECT * FROM addresses; 
 
 rc = sqlite3_exec（db，query，callbackDB，0，& szErrMsg）; 
 
 return 0; 
} 
  

如何获得矢量或其他方式的结果，

解决方案

sqlite3_exec的第四个参数可以用来传递信息到回调。
在您的情况下，指向表的指针将非常有用：

  typedef vector< vector< string> > table_type; 
 static int callbackDB（void * ptr，int argc，char * argv []，char * cols []）
 {
 table_type * table = static_cast< table_type *>（ptr） 
 vector< string>行; 
 for（int i = 0; i  row.push_back（argv [i]？argv [i]：（NULL） 
 table-> push_back（row）; 
 return 0; 
} 
 ... 
 rc = sqlite3_exec（db，query，callbackDB，& table，& errmsg）;但是，使用像这样的回调并不是非常有用，因为你只是一串裸体字符串。 
您应该直接使用prepare / step / finalize接口，以便可以使用正确的数据类型：
 地址{
 public：
 Address（int id，const string& name，const string& number）; 
 ... 
} 
 
 ... 
矢量<地址>地址; 
 sqlite3_stmt * stmt; 
 rc = sqlite3_prepare_v2（db，SELECT id，name，number FROM addresses，
 -1，& stmt，NULL）; 
 if（rc！= SQLITE_OK）{
 cerr<< SELECT failed：<< sqlite3_errmsg（db）< endl; 
 return ...; //或throw 
} 
 while（（rc = sqlite3_step（stmt））== SQLITE_ROW）{
 int id = sqlite3_column_int（stmt，0）; 
 const char * name = reinterpret_cast< const char *>（sqlite3_column_text（stmt，1））; 
 const char * number = reinterpret_cast< const char *>（sqlite3_column_text（stmt，2））; 
 //让我们假设number可以为NULL：
 addresses.push_back（Address（id，name，number？number：））; 
} 
 if（rc！= SQLITE_DONE）{
 cerr<< SELECT failed：<< sqlite3_errmsg（db）< endl; 
 //如果你返回/抛出这里，不要忘记finalize 
} 
 sqlite3_finalize（stmt）; 
  
 
I have a sqlite3 table "addresses" like this:
+----+------+--------+
| ID | name | number |
+----+------+--------+
| 1  | John | 413434 |
+----+------+--------+
And i want to populate global vector, which i would be able to use somewhere else and join it with some other data.
So far, i have this:
...
#include <sqlite3.h>

using namespace std;

static int callbackDB(void *NotUsed, int argc, char **argv, char **szColName)
{
    for(int i = 0; i < argc; i++)
        cout << szColName[i] << " = " << argv[i] << endl;

    return 0;
}

int main(int argc, char* argv[]) {
    vector<vector<string> > table;
    for( int i = 0; i < 2; i++ )
        table.push_back(std::vector< std::string >());


    sqlite3 *db;
    char *szErrMsg = 0;

    // open database
    int rc = sqlite3_open("database.db", &db);

    const char *query;
    query = "SELECT * FROM addresses";

    rc = sqlite3_exec(db, query, callbackDB, 0, &szErrMsg);

    return 0;
}
How could i get results in vector or in some other way so that i'll be able to use it easier?
解决方案
The fourth parameter of sqlite3_exec can be used to pass information to the callback.
In your case, a pointer to the table would be useful:
typedef vector<vector<string> > table_type;
static int callbackDB(void *ptr, int argc, char* argv[], char* cols[])
{
    table_type* table = static_cast<table_type*>(ptr);
    vector<string> row;
    for (int i = 0; i < argc; i++)
        row.push_back(argv[i] ? argv[i] : "(NULL)");
    table->push_back(row);
    return 0;
}
...
    rc = sqlite3_exec(db, query, callbackDB, &table, &errmsg);
However, using callbacks like this is not very useful because you get just a bunch of naked strings.
You should rather use the prepare/step/finalize interface directly so that you can use proper data types:
class Address {
public:
    Address(int id, const string& name, const string& number);
    ...
}

    ...
    vector<Address> addresses;
    sqlite3_stmt *stmt;
    rc = sqlite3_prepare_v2(db, "SELECT id, name, number FROM addresses",
                            -1, &stmt, NULL);
    if (rc != SQLITE_OK) {
        cerr << "SELECT failed: " << sqlite3_errmsg(db) << endl;
        return ...; // or throw
    }
    while ((rc = sqlite3_step(stmt)) == SQLITE_ROW) {
        int id = sqlite3_column_int(stmt, 0);
        const char* name = reinterpret_cast<const char*>(sqlite3_column_text(stmt, 1));
        const char* number = reinterpret_cast<const char*>(sqlite3_column_text(stmt, 2));
        // let's assume number can be NULL:
        addresses.push_back(Address(id, name, number ? number : ""));
    }
    if (rc != SQLITE_DONE) {
        cerr << "SELECT failed: " << sqlite3_errmsg(db) << endl;
        // if you return/throw here, don't forget the finalize
    }
    sqlite3_finalize(stmt);


                        
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