写了10几年C++，居然看不懂一个new语句。求大神帮解释

o_yale_o 2017-02-21 09:47:16

ObPacket* spacket = new(session+1)ObPacket(); 这个new后面的（）里是什么意思？在指针session+1上new？

注：这是阿里开源数据库oceanbase源代码，贴上相关源代码：（函数体内第10行）

    int ObClientManager::create_session(const ObServer& server,

                                        const int32_t pcode, const int32_t version,

                                        const int64_t timeout, const ObDataBuffer& in_buffer,

                                        int64_t size, easy_session_t *& session) const

    {

      int ret = OB_SUCCESS;

      easy_addr_t addr = convert_addr_from_server(&server);

      session = easy_session_create(static_cast<int>(size));

      if (NULL == session)

      {

        TBSYS_LOG(WARN, "create session failed");

        ret = OB_LIBEASY_ERROR;

      }

      if (OB_SUCCESS == ret)

      {

        ObPacket* spacket = new(session+1)ObPacket();

        //从线程局部中拿出该请求所对应的trace id

        TraceId *trace_id = GET_TSI_MULT(TraceId, TSI_COMMON_PACKET_TRACE_ID_1);

        if (NULL == trace_id)

        {

          ret = OB_ALLOCATE_MEMORY_FAILED;

          TBSYS_LOG(ERROR, "no memory, alloc trace id failed, ret=%d", ret);

        }

        else

        {

          spacket->set_req_sign(ObPacket::tsi_req_sign());

          ObPacket::tsi_req_sign() = 0;

          /*

          if (trace_id->is_invalid())

          {

            (trace_id->id).seq_ = atomic_inc(&(ObPacket::seq_generator_));

            uint16_t *local_ip_suffix = GET_TSI_MULT(uint16_t, TSI_COMMON_IP_SUFFIX_1);

            (trace_id->id).ip_ = *local_ip_suffix;

            uint16_t *port = GET_TSI_MULT(uint16_t, TSI_COMMON_PORT_1);

            (trace_id->id).port_ = *port;

          }

          */

          //else

          //{

            spacket->set_trace_id(trace_id->uval_);

          //}

          spacket->set_packet_code(pcode);

          //为这个新的packet分配一个channel id

          uint32_t new_chid = atomic_inc(&ObPacket::global_chid);

          spacket->set_channel_id(new_chid);

          spacket->set_api_version(version);

          spacket->set_source_timeout(timeout);

          spacket->set_data(in_buffer);

          ObDataBuffer buffer(reinterpret_cast<char*>(spacket + 1), size - sizeof(ObPacket));

          spacket->serialize(&buffer);

          spacket->set_packet_buffer(buffer.get_data(), buffer.get_position());

          spacket->get_inner_buffer()->get_position() = buffer.get_position();

          session->status = EASY_CONNECT_SEND;//auto connect if there are no connection to addr

          session->r.opacket = spacket;

          session->addr = addr;

          session->thread_ptr = handler_;

          easy_session_set_timeout(session, static_cast<ev_tstamp>(timeout)/1000);

        }

      }

      return ret;

    }

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flying_music 2017-02-21

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引用 7 楼 o_yale_o 的回复:

[quote=引用 2 楼 cyfcsd 的回复:] 定位new表达式，它不分配内存，而是在一个已经分配的内存空间中初始化一个对象

VC里的new支持这功能吗？[/quote]

#include <iostream>

using namespace std;

int main()
{
    int *p = (int*)malloc(sizeof(int));
    int *q = new(p)int(5);
    cout<<*q<<endl;
    free(p);
    p = NULL;
    system("pause");
    return 0;
}

o_yale_o 2017-02-21