‘cout’ was not declared in this scope

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当遇到'cout' was not declared in this scope的错误，通常是因为忘记包含<iostream>头文件或没有使用std命名空间。正确的做法是同时包含头文件并使用命名空间。

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You can get the error

 ‘cout’ was not declared in this scope
if you forget to include iostream or to use the std namespace.

WRONG

 #include <iostream>  // no using namespace std;
 int main(void) 
 {
   cout << "A";
   return 0;
 }

ALSO WRONG

 using namespace std;   // no #include iostream
 int main(void) 
 {
   cout << "A";
   return 0;
 }
RIGHT

 #include <iostream>   // one of two
 using namespace std;    // two of two, yay!
 int main(void) 
 {
   cout << "A";
   return 0;
 }

‘cout’ was not declared in this scope

http://cplusplus.syntaxerrors.info/index.php?title=%E2%80%98cout%E2%80%99_was_not_declared_in_this_scope