What's the Key Difference Between `typedef` and `using` for Type Aliases in C ?

Understanding the Nuances of 'typedef' and 'using' in C

The 'typedef' and 'using' keywords in C both facilitate the creation of type aliases. While they serve similar purposes, subtle differences exist in their implementation.

In C 11 and onwards, the 'using' syntax emerged as a concise way to define type aliases. For example:

using MyInt = int;

This is semantically equivalent to the traditional 'typedef':

typedef int MyInt;

However, the 'using' syntax gained prevalence due to its ability to express "template typedefs":

template<class T> using MyType = AnotherType<T, MyAllocatorType>;

Now, let's explore potential differences between 'typedef' and 'using' regarding alias strength. 'typedef' is known for its "weak" aliasing behavior, meaning it doesn't create a new type but merely establishes a new name. Conversions between aliased types and their original types are implicit.

The question arises: does 'using' behave similarly or does it result in the creation of a new type? The answer, as per the C Standard (7.1.3.2), is that they are equivalent in semantics:

"It [using] has the same semantics as if it were introduced by the typedef specifier. In particular, it does not define a new type and it shall not appear in the type-id."

Therefore, both 'typedef' and 'using' create weak aliases without generating new types, resulting in implicit conversions between aliased and original types.

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