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c++opencvnamespaces
Bill Grates
Bill Grates

Reputation: 906

How does C++ know where to look for the namespace specified using "using namespace ..."?

For eg., when using OpenCV, we specify using namespace cv; But where does C++ look down to know where it is defined?

Upvotes: 1

Views: 892

Answers (2)

Teivaz
Teivaz

Reputation: 5665

using namespace will not make everything declared in that namespace visible. It will expose only what translation unit "sees". Consider following code

One.h

#pragma once
namespace ns
{
  void function1();
}

Two.h

#pramga once
namespace ns
{
  void function2();
}

main.cpp

#include "Two.h" // <-- included only Two.h
using namespace ns;

int main()
{
  function2(); // <-- is a ns::function2() located in Two.h
  function1(); // <-- error compiler does not know where to search for the function
  return 0;
}

What happened here is the compiler created translation unit with all preprocessor directives resolved. It will look something like this:

namespace ns
{
  void function2();
}
using namespace ns;

int main()
{
  function2(); // <-- OK. Declaration is visible
  function1(); // <-- Error. No declaration
  return 0;
}

Upvotes: 3

R Sahu
R Sahu

Reputation: 206597

How does C++ know where to look for the namespace specified using using namespace …?

It doesn't.

When you use

using namespace cv;

the scope of search for names (of classes, functions, variables, enums, etc) is expanded. The names are searched in the cv namespace in addition to other scopes in which they are normally searched.

Upvotes: 1

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