about Namespace and global variables

[SP1D3R]

`Use variables in a named namespace instead of using external global variables.
`Use variables in an unnamed namespace instead of using static global variables.

when I am learning c++, i read these two sentences above, I cannot understand it well, so I come for asking help.
What does the two mean?

[vlad from moscow]

Any name declared outside any function is being declared in some namespace. If the namespace is not specified explicitly then it means that the name is declared in the global namespace. For example

#include <iostream>

int x = 10;  // x declared in the global namespace

namespace A 
{
   int x = 20; // x declared in namespace A
}

int main()
{
   int x = 30; // local x

   std::cout << "x = " << x << std::endl;
   std::cout << "A::x = " << A::x << std::endl;
   std::cout << "::x = " << ::x << std::endl;
}

With rare exceptions names declared in namespaces have external linkages.

According to the C++ 11 Standard names declared in unnamed namespaces have internal linkage that is they can be accessed only inside the translation unit where they are declared.

[SP1D3R]

oic, thx alot :D
and how can I use the external namespace variable in other files?
global is using "extern int x", and what about x in namespace A.

[vlad from moscow]

extern int A::x;
or

namespace A
{
   extern int x;
}

[MiiNiPaa]

Unnamed namespaces declared as

namespace
{
    int x; //x can be accessed in this file only
}

Unnamed namespaces are preferred because
a) Static keyword is deprecated when used to declare file scope
b) You can use them to define a class which can be used in curent file only (to hide implementation details for example) functions and whatever you want

vlad from moscow told about global variables in named namespaces already.

[Peter87]

MiiNiPaa wrote:

a) Static keyword is deprecated when used to declare file scope

This has been undeprecated in C++11.

[SP1D3R]

vlad, i have tried your way "extern int A::x;", but I get "namespace A has no member x". Do I need to put the "int x" into the header files? Now it's in another .cpp file, not .h file.

cc.cpp

#include <iostream>

int main()
{
	hello::ok;
}

cc2.cpp

#include <iostream>

namespace hello
{
	int ok;
}

[MiiNiPaa]

Thanks for the info. I found this proposal where stated that

The use of static in namespace scope should not be deprecated.

As it makes little sense to allow some use and dissalow other they just un-deprecated it everywhere.
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3296.html#FI6

I find funny that no deprecated feature was ever deleted (except export which wasn't fully supported anyway) or changed meaning (aside from auto which nobody used) because of backward compatibility

[vlad from moscow]

@N8cc

vlad, i have tried your way "extern int A::x;", but I get "namespace A has no member x".

Use the record

namespace A
{
   extren int x;
}

[Peter87]

MiiNiPaa wrote:

I find funny that no deprecated feature was ever deleted (except export which wasn't fully supported anyway) or changed meaning (aside from auto which nobody used) because of backward compatibility

I think they deserve an applaud for being so brave to remove the implicit conversion to non-const char* for string literals.

[vlad from moscow]

Try the code

cc.cpp

#include <iostream>

namespace hello
{
   extern int ok;
}

int main()
{
	hello::ok;
} 

cc2.cpp

#include <iostream>

namespace hello
{
	int ok;  // or int ok = 10;
}