函数中定义的结构/类的编译错误

以下代码有什么问题。因为如果我在 main 函数中定义类，编译就会失败，我不明白编译器错误。

从这里测试代码

注释 drift_f 的第一个定义（main() 外部）并取消注释 drif_t 的内部定义（在 main() 函数内），编译器将收到以下错误消息：

prog.cpp: In function ‘int main()’:
prog.cpp:27:24: error: template argument for ‘template<class> class std::allocator’ uses local type ‘main()::drift_t’
      std::deque<drift_t> drift; drift.push_back(drift_t(0,0));drift.push_back(drift_t(-50,-50));
                        ^
prog.cpp:27:24: error:   trying to instantiate ‘template<class> class std::allocator’
prog.cpp:27:24: error: template argument 2 is invalid
prog.cpp:27:31: error: invalid type in declaration before ‘;’ token
      std::deque<drift_t> drift; drift.push_back(drift_t(0,0));drift.push_back(drift_t(-50,-50));
                               ^
prog.cpp:27:39: error: request for member ‘push_back’ in ‘drift’, which is of non-class type ‘int’
      std::deque<drift_t> drift; drift.push_back(drift_t(0,0));drift.push_back(drift_t(-50,-50));
                                       ^
prog.cpp:27:69: error: request for member ‘push_back’ in ‘drift’, which is of non-class type ‘int’
      std::deque<drift_t> drift; drift.push_back(drift_t(0,0));drift.push_back(drift_t(-50,-50));
#include <iostream>
#include <deque>
using namespace std;
    class drift_t{ //It works
      public:
        int _drift;
        int _immediateDrift;
        drift_t() : _drift(0), _immediateDrift(0) {}
        drift_t(int d, int expected) : _drift(d), _immediateDrift(expected) {}
        drift_t(const drift_t& ro) : _drift(ro._drift), _immediateDrift(ro._immediateDrift) {}
        drift_t& operator = (const drift_t& ro) { this->_drift = ro._drift; this->_immediateDrift = ro._immediateDrift; return *this; }
      } ;//*/
int main() {
    /*class drift_t{ //It doesn't works
      public:
        int _drift;
        int _immediateDrift;
        drift_t() : _drift(0), _immediateDrift(0) {}
        drift_t(int d, int expected) : _drift(d), _immediateDrift(expected) {}
        drift_t(const drift_t& ro) : _drift(ro._drift), _immediateDrift(ro._immediateDrift) {}
        drift_t& operator = (const drift_t& ro) { this->_drift = ro._drift; this->_immediateDrift = ro._immediateDrift; return *this; }
      } ;//*/
     std::deque<drift_t> drift; drift.push_back(drift_t(0,0));drift.push_back(drift_t(-50,-50));
    return 0;
}