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RE: C++ moving and returning objects part 2: move and copy (embedded friendly C++)

Jan Cumps — Tue, 13 Jan 2026 18:35:39 GMT

I used wikipedia's test case to check if C++ actually manages to allocate an object, created in a function, directly in its final memory location: yes

[embed:dc8ab71f-3b98-42d9-b0f6-e21e02a0f8e2:57e0d5ac-8dfb-48ce-add2-d3b2be1c8345:type=csharp&text=%23include%20%3Ciostream%3E%0D%0A%0D%0Astruct%20MyStruct%20%7B%0D%0A%20%20%20%20MyStruct%28%29%20%3D%20default%3B%0D%0A%20%20%20%20MyStruct%28%5B%5Bmaybe_unused%5D%5D%20const%20MyStruct%26%20x%29%20%7B%0D%0A%20%20%20%20%20%20%20%20std%3A%3Acout%20%3C%3C%20%28%22A%20copy%20was%20made.%5Cn%22%29%3B%20%7D%0D%0A%7D%3B%0D%0A%0D%0AMyStruct%20f%28%29%20%7B%0D%0A%20%20%20%20return%20MyStruct%28%29%3B%0D%0A%7D%0D%0A%0D%0AMyStruct%20f_named%28%29%20%7B%0D%0A%20%20%20%20MyStruct%20m%3B%0D%0A%20%20%20%20return%20m%3B%0D%0A%7D%0D%0A%0D%0Aint%20main%28%29%20%7B%0D%0A%20%20%20%20std%3A%3Acout%20%3C%3C%20%28%22Hello%20World%21%5Cn%22%29%3B%0D%0A%20%20%20%20MyStruct%20obj%20%3D%20f%28%29%3B%0D%0A%20%20%20%20MyStruct%20obj2%20%3D%20f_named%28%29%3B%0D%0A%7D]

obj and obj2 are 2 MyStruct objects. Even though they are created inside functions ( f() and f_named() ), there is no copying when these 2 functions exit.
They actually construct the object in the memory that's allocated in main directly.

C++ knows that the function return values are temporary and not used after the function exits.
So instead of creating a temporary object, and using a copy into obj or obj2 at return, it directly does the work in the destination object.

result: using a function to factor an object comes without any runtime cost.

RE: modern C++: write my own function that returns true / false, and a value, part 2 (embedded friendly)

Jan Cumps — Sat, 06 Dec 2025 19:03:58 GMT

code in action, for a Raspberry Pico GPS design:

19 satellites in view from my back window. The parser mechanism described above is used. stl algorithms split the found satellites into glonass and gps types.

RE: modern C++: write my own function that returns true / false, and a value, part 2 (embedded friendly)

DAB — Thu, 04 Dec 2025 23:33:33 GMT

Nice post Jan.

I like the idea of getting multiple types of information directly from functions.

RE: modern C++: write my own function that returns true / false, and a value, part 2 (embedded friendly)

Jan Cumps — Thu, 04 Dec 2025 19:42:10 GMT

I flagged this as embedded friendly, but there is a caveat for ARM controllers:

It needs C++ 15. If you are using a Raspberry Pico 2, in RISC-V mode, it will work. GCC15 is released for that.

But as of today, there is no published ARM GCC 15 toolchain. If the OpenArm initiative sticks to its schedule, a version 15 release is imminent...

RE: modern C++: initialise a static array inside a class

DAB — Fri, 28 Nov 2025 19:53:14 GMT

Nice update Jan.

RE: Generate Awesome ASCII ART with QAAA.py

colporteur — Wed, 26 Nov 2025 00:34:26 GMT

Shades of customer terminal windows. Cool!

RE: modern C++: initialise a static array inside a class

vmate — Mon, 24 Nov 2025 21:06:51 GMT

A notable exception is that with constexpr arrays, this can be done pre-c++17:
[embed:dc8ab71f-3b98-42d9-b0f6-e21e02a0f8e2:47de98d5-fed6-4b84-9725-142ddc6d2327:type=c_cpp&text=class%20myclass%20%7B%0Apublic%3A%0A%20%20%20%20static%20constexpr%20uint32_t%20myarray%5B4%5D%20%3D%20%7B1%2C%202%2C%203%2C%204%7D%3B%0A%7D%3B]

Also, in recent versions of C++, std::array should probably be used instead of 'regular' arrays:
[embed:dc8ab71f-3b98-42d9-b0f6-e21e02a0f8e2:b0b6e540-4942-478a-99eb-9bcccb49e685:type=c_cpp&text=class%20myclass%20%7B%0Aprivate%3A%0A%20%20%20%20inline%20static%20std%3A%3Aarray%3Cuint32_t%2C%204%3E%20myarray%20%3D%20%7B0%7D%3B%0A%7D]
Instead of myarray becoming a uint32_t pointer, and then the programmer is responsible for doing everything correctly with it, std::array actually behaves as a proper object, it knows its own size, can be assigned properly to another array, etc. It has a bunch of convenience features like .begin() and .end() too, and it is zero cost.

RE: C++ write your own stream class - part 1: output stream initial design (embedded friendly C++)

Jan Cumps — Sat, 15 Nov 2025 15:00:58 GMT

a real world example. Implementation for a Raspberry Pico. Using its UART api:

Code:

[embed:dc8ab71f-3b98-42d9-b0f6-e21e02a0f8e2:a562dd8b-d276-4ce2-9069-2f6f482f7585:type=csharp&text=class%20uartostream%20%7B%0Apublic%3A%0A%20%20%20%20uartostream%28%29%20%7B%7D%0A%0A%20%20%20%20uartostream%26%20operator%20%3C%3C%20%28const%20char%2A%20msg%29%20%7B%0A%20%20%20%20%20%20%20%20uart_puts%28UART_ID%2C%20msg%29%3B%0A%20%20%20%20%20%20%20%20return%20%2Athis%3B%0A%20%20%20%20%7D%0A%0A%20%20%20%20uartostream%26%20operator%20%3C%3C%20%28const%20std%3A%3Astring%26%20msg%29%20%7B%0A%20%20%20%20%20%20%20%20uart_puts%28UART_ID%2C%20msg.c_str%28%29%29%3B%0A%20%20%20%20%20%20%20%20return%20%2Athis%3B%0A%20%20%20%20%7D%0A%7D%3B]

RE: C++ write your own stream class - part 1a: the cost of objects can be zero (embedded friendly C++)

DAB — Thu, 13 Nov 2025 20:38:25 GMT

Nice update Jan.

RE: C++ write your own stream class - part 1a: the cost of objects can be zero (embedded friendly C++)

Jan Cumps — Thu, 13 Nov 2025 17:46:04 GMT

In case you want to embark on a RISC vs ARM tangent: this post is not about that.
It's a comparison about C++ and C designs, and a check: are C++ constructs too expensive for microcontrollers (any)?

I'd be interested into a GCC vs other toolchains battle though ...

RE: C++ write your own stream class - part 1a: the cost of objects can be zero (embedded friendly C++)

Jan Cumps — Thu, 13 Nov 2025 16:38:29 GMT

I added the RISC-V assembler:

[embed:dc8ab71f-3b98-42d9-b0f6-e21e02a0f8e2:2ff237af-4343-4f39-b075-b5fadb28a4fe:type=text&text=main%3A%0A%20%20%20%20%20%20%20%20addi%20%20%20%20sp%2Csp%2C-16%0A%20%20%20%20%20%20%20%20li%20%20%20%20%20%20a5%2C53%0A%20%20%20%20%20%20%20%20sb%20%20%20%20%20%20a5%2C15%28sp%29%0A%20%20%20%20%20%20%20%20li%20%20%20%20%20%20a0%2C0%0A%20%20%20%20%20%20%20%20addi%20%20%20%20sp%2Csp%2C16%0A%20%20%20%20%20%20%20%20jr%20%20%20%20%20%20ra]

RE: C++ write your own stream class - part 1: output stream initial design (embedded friendly C++)

Jan Cumps — Wed, 12 Nov 2025 17:14:31 GMT

Is this expensive? The facts

When I use a subclass uartiostream and its << operator:

This compiles to

main:
        mov     r3, #53
        mov     r0, #0
        sub     sp, sp, #8
        strb    r3, [sp, #7]
        add     sp, sp, #8
        bx      lr
u:
        .space  1

when I use plain C to do the same:

main:

        mov     r3, #53
        mov     r0, #0
        sub     sp, sp, #8
        strb    r3, [sp, #7]
        add     sp, sp, #8
        bx      lr

The only difference is a single address location (32 bits) to hold the object u

0 cpu cycles

That's virtually cost-free

RE: C++ write your own stream class - part 1: output stream initial design (embedded friendly C++)

DAB — Tue, 11 Nov 2025 20:37:34 GMT

Nice post Jan.

RE: C++ write your own stream class - part 1: output stream initial design (embedded friendly C++)

Jan Cumps — Tue, 11 Nov 2025 20:26:39 GMT

Defining an input streamer has the same complexity as the output streamer.

Once the in- and out-streamers are written, writing a bidirectional one isn't that difficult:

[embed:dc8ab71f-3b98-42d9-b0f6-e21e02a0f8e2:cc081195-6eb2-4329-af2e-fb1d07d62d54:type=csharp&text=class%20uartistream%20%7B%0D%0Apublic%3A%0D%0A%20%20uartistream%28%29%20%7B%7D%0D%0A%20%20%0D%0A%20%20uartistream%26%20operator%20%3E%3E%20%28std%3A%3Astring%26%20msg%29%20%7B%0D%0A%20%20%20%20%2F%2F%20read%20from%20UART%0D%0A%20%20%20%20return%20%2Athis%3B%0D%0A%20%20%7D%20%20%0D%0A%20%20uartistream%26%20operator%20%3E%3E%20%28char%2A%20msg%29%20%7B%0D%0A%20%20%20%20%20%20%2F%2F%20read%20from%20UART%0D%0A%20%20%20%20return%20%2Athis%3B%0D%0A%20%20%7D%0D%0A%7D%3B%0D%0A%0D%0Aclass%20uartiostream%3A%20public%20uartistream%2C%20public%20uartostream%20%7B%0D%0A%7D%3B]

Inheriting form both base classes doesn't cost anything - because I didn't define any virtual functions.

The difficult work will be at the lower level: write the implementation of the << and >> operators using device dependent UART APIs.

RE: modern C++ for loop, but you still want an index

Stuart_S — Fri, 07 Nov 2025 02:58:06 GMT

Is the 'auto' keyword valid in basic C, I might give it a shot on the old DevCPP