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How to make std::format work with your own data types(C++20)?

+3 votes
asked Jan 4 by Areeb Sherjil (1,920 points)
I want to be able to write the following:

std::format("My new box, Box({:.2},{:.2},{:.2})", box.getLength(), box.getWidth(), box.getHeight());

to

std::format("My new box, Box({:.2},{:.2},{:.2})", box.getLength(), box.getWidth(), box.getHeight());

This is what I've done so far:

template <>

class std::formatter<Box> :public std::formatter<double>

{

public:

    using std::formatter<double>::format;

    auto format(const Box& box, auto& context)

    {

        auto iter = format(box.getLength(), context);

        context.advance(iter);

        out = format(box.getWidth(), context);

        context.advance(iter);

        return format(box.getHeight(), context);

    

    }

}; The box class definition is inside another header file. As you can see this code above does not work.

1 Answer

0 votes
answered Jul 6 by AADI JAIN (340 points)

i would recommend not to use std:: and write using namespace std instead or for your own data type 

first make a header file and save it with   (.H)  extention and then include it in your file  to use your own data type without using  std::

commented Jul 6 by Peter Minarik (55,660 points)
edited Jul 7 by Peter Minarik
You just gave terrible advice there suggesting not to use the fully qualified name of an identifier, it is quite a bad idea.

The recommendation is to always use fully qualified names to prevent future problems or even malicious attacks.

Please, read more here (https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice), why you should always qualify the full name and not rely on "using namespace" in real-world applications.
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