为什么会出现“无法将'type *'转换为'type * {aka struct}”的错误？

Question

我正在尝试为应用程序编写框架（用于数据库连接池），但也希望能够将其编译为单独的模块。 我所拥有的：

#include <iostream>
#define OK      0
#define ERROR   -1
//--- DUMMY DECLARATIONS BELOW TO MAKE IT COMPILE ---//
#define LOG_NS_ERROR std::cout
class test{
    public:
        int get_child(std::string x);};
test *_global;
typedef struct test_struct{} _db_transact;
class db_transact{
    public: 
        db_transact(int,int&,int&);
};
int _ns;
int _log_id;
//--- DUMMY DECLARATIONS ABOVE TO MAKE IT COMPILE ---//
class db_c_hndlr {
    public: 
        db_c_hndlr(unsigned int poolsz);
        ~db_c_hndlr(void);
        _db_transact *db_conn_get(void);
    private:
        int pool_alloc(unsigned int poolsz);
        _db_transact **_db_pool;
        unsigned int _used;
        unsigned int _poolsz;
};
//---------------------------------------------------------

db_c_hndlr::db_c_hndlr(unsigned int poolsz) {
    if (!poolsz){
        LOG_NS_ERROR << "A valid poolsize >0 must be specified";
        return;
    }
    _poolsz = poolsz;
    if(pool_alloc(poolsz)==OK)
        _used = 0;
    else
       LOG_NS_ERROR << "db_c_hndlr::db_c_hndlr couldn't allocate poolsize " << poolsz;
}
//---------------------------------------------------------

db_c_hndlr::~db_c_hndlr() {
    int i = 0;

    for (i=0; i<_poolsz; i++) {
        if (_db_pool[i])
            if (nullptr != _db_pool[i]) delete _db_pool[i];
    }
}
//---------------------------------------------------------

int db_c_hndlr::pool_alloc(unsigned int poolsz) {
    *_db_pool=(_db_transact*)calloc(poolsz, sizeof(_db_transact*));
    if(_db_pool[poolsz-1])
        return OK;
    else
        return ERROR;

}
//---------------------------------------------------------

_db_transact *db_c_hndlr::db_conn_get(void) {
    if (_used < _poolsz)
        _db_pool[_used] = new db_transact(_global->get_child("db_config"), _ns, _log_id);
    else
        return NULL;
    return _db_pool[_used++];
}

但是当我尝试编译它时，我得到了：

$ gcc --std=c++11 pseudo_code.cpp 
pseudo_code.cpp: In member function ‘_db_transact* db_c_hndlr::db_conn_get()’:
pseudo_code.cpp:66:25: error: cannot convert ‘db_transact*’ to ‘_db_transact* {aka test_struct*}’ in assignment
         _db_pool[_used] = new db_transact(_global->get_child("db_config"), _ns,
                         ^

为什么我想知道这个错误？