In C++ functions can be declared inside namespaces. Namespaces are just what its name infer, a collection of names.
To access a name (let’s say the function pow()) that is declared inside the namespace std, you can access it in 2 ways: using the namespace access prefix std:: (i.e. std::pow()), or you can declare using namespace std. However as @Yksisarvinen mentioned, you should avoid the use of using namespace.
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solved Why when I include