什么是未声明的标识符错误？什么是常见原因以及如何解决它们？
示例错误文本:
error C2065: 'cout' : undeclared identifier
'cout' undeclared (first use in this function)
c++ compiler-errors declaration undeclared-identifier
c++ ×1
compiler-errors ×1
declaration ×1
undeclared-identifier ×1