C++报错解决：error: ‘int’ is not a class, struct, or union type typedef typename _Sequence::value_

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这篇博客详细记录了作者在使用C++编程时遇到的一个错误：尝试错误地typedef一个内置类型'int'，导致编译器报错。错误信息表明'int'不是类、结构体或联合类型。通过提供正确的typedef声明方式，作者解决了这个问题。

我在使用C++的时候，遇到了如下报错：

-> % g++ test.cpp
In file included from /usr/include/c++/7/queue:64:0,
                 from test.cpp:5:
/usr/include/c++/7/bits/stl_queue.h: In instantiation of ‘class std::queue<int, int>’:
test.cpp:33:21:   required from here
/usr/include/c++/7/bits/stl_queue.h:124:47: error: ‘int’ is not a class, struct, or union type
       typedef typename _Sequence::value_type  value_type;
                                               ^~~~~~~~~~
/usr/include/c++/7/bits/stl_queue.h:125:46: error: ‘int’ is not a class, struct, or union type
       typedef typename _Sequence::reference  reference;
                                              ^~~~~~~~~
/usr/include/c++/7/bits/stl_queue.h:126:51: error: ‘int’ is not a class, struct, or union type
       typedef typename _Sequence::const_reference const_reference;
                                                   ^~~~~~~~~~~~~~~
/usr/include/c++/7/bits/stl_queue.h:127:46: error: ‘int’ is not a class, struct, or union type
       typedef typename _Sequence::size_type  size_type;
                                              ^~~~~~~~~

使用clang编译的话报错更不容易懂：

-> % clang++ test.cpp 
In file included from test.cpp:5:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/queue:64:
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/stl_queue.h:124:24: error: type 'int' cannot be used
      prior to '::' because it has no members
      typedef typename  _Sequence::value_type           value_type;
                        ^
test.cpp:33:21: note: in instantiation of template class 'std::queue<int, int>' requested here
    queue<int, int> q;
                    ^
In file included from test.cpp:5:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/queue:64:
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/stl_queue.h:125:24: error: type 'int' cannot be used
      prior to '::' because it has no members
      typedef typename  _Sequence::reference            reference;
                        ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/stl_queue.h:126:24: error: type 'int' cannot be used
      prior to '::' because it has no members
      typedef typename  _Sequence::const_reference      const_reference;
                        ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/stl_queue.h:127:24: error: type 'int' cannot be used
      prior to '::' because it has no members
      typedef typename  _Sequence::size_type            size_type;
                        ^
4 errors generated.

报错是因为我错误地声明了：

queue<int, int> q;

正确的声明方法如下：

queue<int> q;
queue <pair<int, int> > q2;