What is the C++ equivalent of python's import as?

Question

I've tried several variations of this with no joy...

auto& hrctp = std::chrono::high_resolution_clock::time_point;
auto& hrcn = std::chrono::high_resolution_clock::now;

I know I can use...

use namespace std::chrono::high_resolution_clock;

And I understand one shouldn't try too hard to replicate the paradigms of one language in another but I'm just curious. Is there an equivalent?

Answer 1

It's simple. Short answer... It's different for types and functions.

// Alias the type...
using hr_time_point = std::chrono::high_resolution_clock::time_point;

// Create reference (like an alias) for the function
auto &hr_now = std::chrono::high_resolution_clock::now;

The compiler will undoubtedly optimize away the reference, and call the referent directly.

This would work equally well:

inline auto hr_now() { return std::chrono::high_resolution_clock::now(); }

Again, the optimizer will optimize out the indirection.

Answer 2

This is more complex than it looks. As Cheers and hth says, aliasing is different for types and functions and namespaces.

For a simple type like std::chrono::high_resolution_clock::time_point, you can either use typedef or using:

using hrctp = std::chrono::high_resolution_clock::time_point;

or

typedef std::chrono::high_resolution_clock::time_point hrctp;

The advantage of using is that you can use it for template classes too.

For a static member function or a stand-alone function embedded in a namespace, you can just use a pointer to the function:

const auto hrcn = std::chrono::high_resolution_clock::now;

You can't do this for non-static member functions (a pointer-to-member-function is a completely different beast), but fortunately you don't need to (because you invoke non-static member functions on an object of the appropriate type).

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The options for time_point are purely done at compile time. However the function alias may impose a run-time penalty (because you are calling the function through a pointer, rather than jumping there directly). However, write your code for clarity first, and speed second. (OTOH, the C++ way would probably be:

using hrc =std::chrono::high_resolution_clock;

and then use hrc::time_point and hrc::now.

Answer 3

If you want to define an alias, use a using directive. That means this would work:

using hrctp = std::chrono::high_resolution_clock::time_point;

For the function, you could use something like this:

const auto hrcn = std::chrono::high_resolution_clock::now;

This creates a function pointer to the static function.