using keyword

The using keyword is used to alias names. It was introduced in C++11.

using namespace std;

Use the using keyword to alias a longer type name, for example with Google Test:

using T = Testing::T;

You can also do something like this (from gxf code)

template <typename T>
using Expected = Expected<T, gxf_result_t>;

using ll = long long;

• There is also typedef which does the same thing (typedef long long ll;)

Visibility

Visibility of using inside a class in same as accessing member variables. That’s why a lot of std things like std::is_same and std::conditional are implemented with a struct to have default public visibility.

struct ExampleStruct {
    using MyType = int;  // Public by default
};
 
class ExampleClass {
    using MyType = int;  // Private by default
};

If you try to use it afterwards,

ExampleStruct::MyType a = 5;  // ✅ Works (public)
ExampleClass::MyType b = 5;  ❌ Error (private)

Typedef vs. Using

typedef vs. using??

Seems like they both do the same thing, but you can leverage using for templates.

See https://stackoverflow.com/questions/10747810/what-is-the-difference-between-typedef-and-using.

Example

template <typename T>
typedef std::vector<T> Vec; // ❌ Error!
 
template <typename T>
using Vec = std::vector<T>;  // ✅ Works perfectly
 
// Afterwards...
Vec<int> v; 

You had to use a workaround before

template <typename T>
struct Vec {
    typedef std::vector<T> type;
};

Related

• typedef
• typename keyword