[Solved] Proper use of memory with dynamic array lengths in c++

by


I’m not sure what you mean by “correct”; your code is not correct at least in the sense mentione by @SamVarshavchik, and which a compiler will tell you about:

a.cpp: In function ‘int getFifoData(unsigned char*)’:
a.cpp:20:29: warning: ISO C++ forbids variable length array ‘tBuff’ [-Wvla]
   uint8_t tBuff[txBuf.size()];

If you want to understand why C++ has no VLA’s read this SO question:

Why aren’t variable-length arrays part of the C++ standard?

Issues with the code

Here are some issues I believe you should consider.

Confusing names

  • The function readWrite() – does it read? does it write? does it do both? Who knows.
  • Don’t clip names. If you want to name a buffer, call it my_buffer, not my_buff. Similarly, diagnostic_output not diagOutput.
  • No impromptu initials. What’s a tBuffer? Is it a test buffer? A transaction buffer? A transmission buffer? A temporary buffer?
  • Not everyone knows what RX means.
  • getFifoData() – what does it even do? Is its parameter a “FIFO”? That’s not what the parameter name says. And if it is – where is that information we supposedly get? There’s no destination buffer that’s passed for use, nor a container that’s returned.

Chance of buffer overflow / invalid memory access

  • Why does getFifoData() take a buffer without also taking its length as well?
  • Better yet, why can’t it take a span?

Use of dynamically-allocated buffers

Both std::vector and the variable-length array are dynamically-allocated memory. VLAs have their own issues (see link above), but as for vectors – you would be performing a memory allocation call on each call of this function; and that might be expensive if it gets called a lot.

Logging

Printing to the console or to a file is slow. Well, sort of slow, anyway – this is all relative. Now, this happens within an “if” statement, but if you’ve configured your app to log things, you’ll be paying this price on every call to getFifoData().

Time it!

Finally, – if you’re worried about performance, time your function, or do it with a profiler. Then you can see how much time it actually takes and whether that’s a problem.

solved Proper use of memory with dynamic array lengths in c++