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# namespace

#include "common.hpp"

// ERROR: same naming rules as vars

//namespace 2D{}

// BAD: by convention, namespaces start with lower case

namespace D2{}

int i;

void f() {}

void prototype();

namespace namea {

int in_namea_only = 0;

class C {

public:

C() {}

};

namespace nameaa {

int i;

void f() {}

class C {

public:

C() {}

};

}

namespace nameab {

int i;

void f() {

::i = 0;

// namea::nameab::i

i = 0;

// namea::nameaa::i

nameaa::i = 0;

// Only affects this function. see C::C() at the f() call.

using namespace nameaa;

}

class C :

public namea::C

//public C

// ERROR: refers to current incomplete C already

//not existem namea::C

{

C() {

// No ambiguity because using inside f() only afects the function.

f();

}

};

}

int i;

void f() {

::i = 0;

i = 0; //namea::i

namea::i = 0;

nameaa::i = 0; //namea::nameaa::i

}

namespace namea {

// Can add new members

int j;

void newFunc(){}

class B{};

// ERROR: redefinition

//int i;

// Implementation of namea::prototype.

//void prototype(){}

}

// ERROR: must be declared/defined inside.

//namea::i = 0;

//void namea::prototype(){}

//int namea::j;

//void namea::g(){}

//class namea::B{};

// ERROR.

//template<class T> namespace t {}

// ADL

namespace adl0 {

struct s {};

int adl(struct s s){

return 0;

}

int i;

int adlNoType(int i){

return 0;

}

int adlMultiArg(int i, struct s s, int j){

return 0;

}

namespace adl1 {

struct s {};

int adl(struct s s){

return 1;

}

int i;

int adlNoType(int i){

return 1;

}

int adl0FromAdl1(struct adl0::s s) {

return 1;

}

int adl0And1FromAdl1(struct adl0::s s0, struct s s1) {

return 1;

}

float adl01(struct adl0::s s, struct s s1){

return 0.5;

}

namespace adl0 {

float adl01(struct s s, struct adl1::s s1){

return 0.5;

}

int main() {

// variables

{

int i;

i = 0; //inner i

::i = 0; //global i

namea::i = 0; //namea i

i++;

assert(i == 1);

assert(::i == 0);

assert(namea::i == 0);

f();

namea::f();

namea::nameaa::f();

}

# using

Be very careful with `using`, because there is no way to unuse afterwards.

In particuar, *never* use `using namespace X` on the toplevel a header file,

or you shall confuse includers to tears.

{

using namespace namea;

// ERROR ambiguous

//f();

//::f

//namea::f

::f();

namea::f();

namea::nameaa::f();

}

// Brackets limit the using namespace scope.

// It is obligatory to specify unused namespaces.

//in_namea_only = 1;

// ERROR: no namespace inside funcs

//namespace main{}

// Namespace chaining

{

using namespace namea;

using namespace nameaa;

// ERROR ambiguous

// ::f

// namea::f

// namea::nameaa:f

//f();

::f();

namea::f();

namea::nameaa::f();

}

// Namespace alias

namespace newNamea = namea;

{

using namespace newNamea;

// ERROR: ambiguous.

// ::f

// namea::f

//f();

}

// Subimport

{

// Imports only name::f.

using namea::f;

f();

// OK: overwrides global f().

//namea::f

// ERROR: only f was imported.

//C c;

};

# ADL

Argument dependent name lookup.

<http://en.wikipedia.org/wiki/Argument-dependent_name_lookup>

Allows for functions without namespace qualification:

- to be found

- to haves ambiguities resolved

based on the namespace in which the types of their arguments are defined.

Explains why operator `<<` does not need the `std::` qualifier,

even though *must* be implemented as a non-member function!!

(see info on operator overload for why)

ADL for operators is a major use case, because specifying namespaces

for operators completely destroys their eyecandy appeal.

{

// ADL allows both to be found and differentiated!

{

struct adl0::s s;

assert(adl(s) == 0);

}

{

struct adl1::s s;

assert(adl(s) == 1);

}

// Only works if the type is defined on the same namespace as the function.

{

struct adl0::s s;

// ERROR: not declared on this scope

//assert(adl0FromAdl1(s) == 1);

}

// Works if at least one of the argument types is in the namespace.

{

struct adl0::s s;

assert(adlMultiArg(0, s, 1) == 0);

}

// Lookup works even if types from both namespaces are used.

{

struct adl0::s s0;

struct adl1::s s1;

assert(adl0And1FromAdl1(s0, s1) == 1);

}

// Of course, calls can still be ambiguous.

{

struct adl0::s s0;

struct adl1::s s1;

// ERROR: ambiguous call

//assert(adl01(s0, s1) == 0.5);

}

// Only works for *types* defined in the namespaces, not values.

{

//assert(adlNoType(adl0::i) == 0);

// ERROR: adlNoType not found on this scope

//assert(adlNoType(adl1::i) == 0);

}

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