To use or , I need to use “using namespace std;”. What is the meaning of it? When I have multiple .h and .cpp files, where should I put it: inside .h file or .cpp file? Or both wherever I have or ?

Remember that almost all the classes, functions, constants, typdefs and objects of the Standard Library are not declared in the global namespace anymore, but rather they are declared in namespace std. Therefore, when you refer to these identifiers, you have to use their qualified names or use an appropriate using-declaration or using-directive. The “using namespace std;” statement is called a using-directive. It instructs the compiler to resolve all references to identifiers in the program that don’t have qualified names as if they did appear in their qualified form: std::xxx, where xxx is the identifier name as it appears in the program. For example, the unqualified names string and cin are construed as std::string and std::cin, respectively. You can reads more about namespaces here:


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