C++ Namespaces Inline namespace
Example
C++11
inline namespace includes the content of the inlined namespace in the enclosing namespace, so
namespace Outer
{
inline namespace Inner
{
void foo();
}
}
is mostly equivalent to
namespace Outer
{
namespace Inner
{
void foo();
}
using Inner::foo;
}
but element from Outer::Inner:: and those associated into Outer:: are identical.
So following is equivalent
Outer::foo();
Outer::Inner::foo();
The alternative using namespace Inner; would not be equivalent for some tricky parts as template specialization:
For
#include <outer.h> // See below
class MyCustomType;
namespace Outer
{
template <>
void foo<MyCustomType>() { std::cout << "Specialization"; }
}
-
The inline namespace allows the specialization of
Outer::foo// outer.h // include guard omitted for simplification namespace Outer { inline namespace Inner { template <typename T> void foo() { std::cout << "Generic"; } } } -
Whereas the
using namespacedoesn't allow the specialization ofOuter::foo// outer.h // include guard omitted for simplification namespace Outer { namespace Inner { template <typename T> void foo() { std::cout << "Generic"; } } using namespace Inner; // Specialization of `Outer::foo` is not possible // it should be `Outer::Inner::foo`. }
Inline namespace is a way to allow several version to cohabit and defaulting to the inline one
namespace MyNamespace
{
// Inline the last version
inline namespace Version2
{
void foo(); // New version
void bar();
}
namespace Version1 // The old one
{
void foo();
}
}
And with usage
MyNamespace::Version1::foo(); // old version
MyNamespace::Version2::foo(); // new version
MyNamespace::foo(); // default version : MyNamespace::Version1::foo();
