Compiler still not using C++20

[lefebvresam]

When you use typical C++20 features like:

typedef std::map<const char*, Widget*, decltype([](const char* str1, const char* str2){ return std::strcmp(str1, str2) < 0; }) > widget_level_map;

Then you get:

[build] /home/sam/git/hmipanel/source/widgets/widget.h:30:49: error: lambda-expression in unevaluated context only available with '-std=c++20' or '-std=gnu++20'

You have to do the 'old' way:

struct StrCompare : public std::less<bool(const char*, const char*)> {
    constexpr bool operator() (const char* str1, const char* str2) const {
        return std::strcmp(str1, str2) < 0;
    }
};
class Widget;
typedef std::map<const char*, Widget*, StrCompare> widget_level_map; // for each name a widget pointer

Another option is to use string objects instead of string literals, which compare the content of the string instead of the pointer address (which points to the same literal when used more than once in your code as a optimalization but does not allow dynamic update). However my feeling is that it works less efficiënt.

typedef std::map<std::string, Widget*> widget_level_map;

[JohnK1987]

Your probably need to look into profiles settings https://github.com/mbed-ce/mbed-os/blob/2deef5a6817a9bdf2db597c5f4e61db3f15474e9/tools/profiles/debug.json#L2-L17

However, the compiler will probably report many issues because MbedOS code base is compatible with older version i think.

[lefebvresam]

I think more issues will popup if I do the switch. As long as the whole project is not C++20 compliant, users are not encouraged to switch to the latstest standard because of these potential issues. I don't know if there are 'programming rules' in the CE to align programming efforts into a specific C++ revision?

[JojoS62]

In Mbed-os, the C++ standard was long time only C++11 and also not always using the current gcc. It was handled conservative because changes caused a lot of testing to ensure the correct functions. And Mbed-os had to maintain compatibility for GCC, IAR and Keil compilers, and not all where always at the same standard level. Also the C++ runtime libs differ and new features are usually later available in Cortex-M than in X86/64 architectures. With Mbed-CE it is a little bit easier as it support is limited to GCC, so I would like also to have C++20 using the current gcc version. Newer GCC were also more picky about warnings, the current Mbed code base produces a lot and some clean up is also needed.

[multiplemonomials]

Hmm... I am a bit hesitant to change all of Mbed to C++20, because the Arduino IDE is using a very old GCC version that does not support it, so it would break Arduino support.

That said, you can enable C++20 for your own code, I believe, by doing something like

set_property(TARGET MyExecutable PROPERTY CXX_STANDARD 20)

That will enable C++20 support for a specific executable target (MyExecutable). Let me know if that's not working!

[JohnK1987]

Let me know if that's not working!

@lefebvresam any feedback?

[lefebvresam]

I did not switched to C++20 because of all potential issues and warnings. I used the 'old way' practices. However this could be a discussion point for the whole community how and when to switch to a newer cpp version to make it feature proof. I don't know if there is already a topic created for it. I prefer using the 'out of the box' practices as much as possible.