What is the use of "using namespace std"?
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std namespace (where features of the C++ Standard Library, such as string or vector , are declared). After you write this instruction, if the compiler sees string it will know that you may be referring to std::string , and if it sees vector , it will know that you may be referring to std::vector . (Provided that you have included in your compilation unit the header files where they are defined, of course.)
If you don't write it, when the compiler sees string or vector it will not know what you are refering to. You will need to explicitly tell it std::string or std::vector , and if you don't, you will get a compile error.
When you make a call to using namespace <some_namespace>; all symbols in that namespace will become visible without adding the namespace prefix. A symbol may be for instance a function, class or a variable.
Eg if you add using namespace std; you can write just cout instead of std::cout when calling the operator cout defined in the namespace std .
This is somewhat dangerous because namespaces are meant to be used to avoid name collisions and by writing using namespace you spare some code, but loose this advantage. A better alternative is to use just specific symbols thus making them visible without the namespace prefix. Eg:
#include <iostream>
using std::cout;
int main() {
cout << "Hello world!";
return 0;
}
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