[SOLVED] C++: which type of cast is best for audio callbacks?

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[SOLVED] C++: which type of cast is best for audio callbacks?

Postby CrocoDuck » Sat Dec 03, 2016 3:55 pm

Hi there, I am new to C++ programming, but thanks to this website I made it to the point I can understand how this example works. I see that Harry used C-style casts, for example:

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float* inputBuffer = (float*)jack_port_get_buffer ( inputPort , nframes);


However, after this section, I got used to static_cast and I would write that line as follows:

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float *inputBuffer = static_cast<float*>(jack_port_get_buffer(inputPort, nframes));


Seems to me that static_cast does not make run time checks, so I have the feeling it should be faster?

What do you think guys? Is there any reason to prefer a particular kind of cast in a callback?
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Re: C++: which type of cast is best for audio callbacks?

Postby sadko4u » Sat Dec 03, 2016 5:22 pm

I would like to say, for C++ best cast is reinterpret_cast in this case.
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Re: C++: which type of cast is best for audio callbacks?

Postby folderol » Sat Dec 03, 2016 5:26 pm

Static cast seems reasonable, but I'll watch to see who comes along with more explanation.

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Re: C++: which type of cast is best for audio callbacks?

Postby sadko4u » Sat Dec 03, 2016 5:30 pm

folderol wrote:Static cast seems reasonable, but I'll watch to see who comes along with more explanation.


Here's some explanation:

Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. It is purely a compiler directive which instructs the compiler to treat the sequence of bits (object representation) of expression as if it had the type new_type.


So by using reinterpret_cast, you say to compiler: "This pointer really points to beginning of array of float".

http://en.cppreference.com/w/cpp/langua ... rpret_cast
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Re: C++: which type of cast is best for audio callbacks?

Postby CrocoDuck » Sat Dec 03, 2016 6:37 pm

Very interesting. Thanks!
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Re: C++: which type of cast is best for audio callbacks?

Postby tramp » Sat Dec 03, 2016 6:50 pm

sadko4u wrote:So by using reinterpret_cast, you say to compiler: "This pointer really points to beginning of array of float".

A reinterpret_cast is a cast that represents an unsafe conversion that might reinterpret the bits of one value as the bits of another value. For example, casting an int* to a double* is legal with a reinterpret_cast, though the result is unspecified. Similarly, casting an int to a void* is perfectly legal with reinterpret_cast, though it's unsafe. Only use reinterpret_cast if what you're doing really is changing the interpretation of some bits in the machine.
In general, you should always prefer static_cast for casts that should be safe. If you accidentally try doing a cast that isn't well-defined, then the compiler will report an error.

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Re: C++: which type of cast is best for audio callbacks?

Postby sadko4u » Sat Dec 03, 2016 8:59 pm

tramp wrote:Only use reinterpret_cast if what you're doing really is changing the interpretation of some bits in the machine.

Sure, that's why I think that reinterpret_cast is more usable to assume chunk of memory to be array of floats rather than static_cast.
In general, you should always prefer static_cast for casts that should be safe. If you accidentally try doing a cast that isn't well-defined, then the compiler will report an error.

That's right and true for some objects with some hierarchy but not for low-level programming where each chunk of memory should be straightforward interpreted as some collection of objects of some data type.
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Re: C++: which type of cast is best for audio callbacks?

Postby folderol » Sat Dec 03, 2016 10:47 pm

Sounds potentially rather dangerous.

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Re: C++: which type of cast is best for audio callbacks?

Postby tramp » Sun Dec 04, 2016 4:06 am

sadko4u wrote:
tramp wrote:Only use reinterpret_cast if what you're doing really is changing the interpretation of some bits in the machine.

Sure, that's why I think that reinterpret_cast is more usable to assume chunk of memory to be array of floats rather than static_cast.
In general, you should always prefer static_cast for casts that should be safe. If you accidentally try doing a cast that isn't well-defined, then the compiler will report an error.

That's right and true for some objects with some hierarchy but not for low-level programming where each chunk of memory should be straightforward interpreted as some collection of objects of some data type.


sadko4u wrote:Here's some explanation:

Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. It is purely a compiler directive which instructs the compiler to treat the sequence of bits (object representation) of expression as if it had the type new_type.



Well, let's check this explanation:

Let's take a little, basic test program:

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int main()
{
    float buf[10] = {1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,9.0,10.0};
    float f = 0;
   
    float *ptr = static_cast<float*>(buf);
   
    //float *ptr = reinterpret_cast<float*>(buf);
   
    for (int i = 0; i<10; ++i) {
         f = ptr[i] ;
    }
   
   return 0;
}

and let's build verbose assembler code from it:

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 c++ -S -fverbose-asm -g -O2 cast_test.cpp -o static_cast_test.s

now comment out the static_cast, UN-commend the reinterpret_cast and build the assembly:

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c++ -S -fverbose-asm -g -O2 cast_test.cpp -o reinterpret_cast_test.s

Now, let's check the diff:

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 diff -Nur static_cast_test.s reinterpret_cast_test.s

turns out:

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--- static_cast_test.s   2016-12-04 04:33:06.850996404 +0100
+++ reinterpret_cast_test.s   2016-12-04 04:33:34.378996375 +0100
@@ -3,8 +3,8 @@
 #   compiled by GNU C version 4.9.2, GMP version 6.0.0, MPFR version 3.1.2-p3, MPC version 1.0.2
 # GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
 # options passed:  -imultiarch x86_64-linux-gnu -D_GNU_SOURCE cast_test.cpp
-# -mtune=generic -march=x86-64 -auxbase-strip static_cast_test.s -g -O2
-# -fverbose-asm
+# -mtune=generic -march=x86-64 -auxbase-strip reinterpret_cast_test.s -g
+# -O2 -fverbose-asm
 # options enabled:  -faggressive-loop-optimizations
 # -fasynchronous-unwind-tables -fauto-inc-dec -fbranch-count-reg
 # -fcaller-saves -fcombine-stack-adjustments -fcommon -fcompare-elim


As you can see, the only difference here is the name of the file produced, as we produce verbose assembler code. Lets check the same again with plain assembler.

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 c++ -S  cast_test.cpp -o static_cast_test.s

switch comment and

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c++ -S  cast_test.cpp -o reinterpret_cast_test.s

do the diff

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diff -Nur static_cast_test.s reinterpret_cast_test.s

outputs:


Conclusion , the only "advantage" you've when using reinterpret_cast in this case, is, ta, ta,tata, right, you've turn of the compiler type check, there is no difference in runtime behave. As the static_cast is, same as the reinterpret_cast, a pure compiler directive.
The using of a reinterpret_cast on a foreign function call, is in general no good idea, as you cant grant the result for the future.

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Re: C++: which type of cast is best for audio callbacks?

Postby CrocoDuck » Sun Dec 04, 2016 11:15 am

Thank you for your detailed explanation guys, it is really helping me understanding more about casting. I think I will perhaps stick with static_cast for the time being.
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Re: C++: which type of cast is best for audio callbacks?

Postby sadko4u » Sun Dec 04, 2016 11:23 am

tramp wrote:Conclusion , the only "advantage" you've when using reinterpret_cast in this case, is, ta, ta,tata, right, you've turn of the compiler type check, there is no difference in runtime behave. As the static_cast is, same as the reinterpret_cast, a pure compiler directive.
The using of a reinterpret_cast on a foreign function call, is in general no good idea, as you cant grant the result for the future.

I don't want to make holy wars around C++ because I consider C++ a very dangerous language with complicated design. You're right that static_cast does nothing. But I prefer to use reinterpret_cast in these cases just to tell to the compiler "even don't try to do something with this". Also, reinterpret_cast is the most similar form of the C-style cast (type *)x.
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Re: C++: which type of cast is best for audio callbacks?

Postby tramp » Sun Dec 04, 2016 12:03 pm

sadko4u wrote:I don't want to make holy wars around C++ because I consider C++ a very dangerous language with complicated design. You're right that static_cast does nothing. But I prefer to use reinterpret_cast in these cases just to tell to the compiler "even don't try to do something with this". Also, reinterpret_cast is the most similar form of the C-style cast (type *)x.


sadko4u, you can use what ever you wont, you can prefer what ever you wont, in your project. I prefer compiler warnings/errors during compile time,when something is wrong, instead receive a crash during run. Therefor I switch on in principle any compiler checks I can get…
Also, keep in mind that the use of reinterpret_cast is restricted in any bigger C++ project I'm aware of, for the reasons I've given here. Patches or commits, which makes use of it unfounded, will be refused. It is considered as "bad codding style",
CrocoDuck wrote:Hi there, I am new to C++ programming,

and I don't think it is a good idea to advice it here for no reason.

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Re: C++: which type of cast is best for audio callbacks?

Postby CrocoDuck » Sun Dec 04, 2016 12:43 pm

tramp wrote:and I don't think it is a good idea to advice it here for no reason.


The very first sticky thread here is called Developers Introduction & Tutorials. I apologize if I misused this communication channel, but that made me think that this was a good place as any to ask for advice.

sadko4u wrote:I consider C++ a very dangerous language with complicated design.


That's exactly the reason why people in my position need to ask questions, as the language itself has many pitfalls and, especially in the audio realm, the documentation is sparse and hard to read.

Anyway, if there is a more appropriate location, on this forum or somewhere else, to ask for advice let me know. I guess I will retain from posting ever again in this section.

Thank you very much for your time and help :wink: , you answered my question well and I am gonna mark this as Solved.
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Re: [SOLVED] C++: which type of cast is best for audio callbacks?

Postby tramp » Sun Dec 04, 2016 12:51 pm

CrocoDuck wrote:The very first sticky thread here is called Developers Introduction & Tutorials. I apologize if I misused this communication channel, but that made me think that it was a good place as any to ask for advice.


Don't get me wrong, CrocoDuck.
You are in the right place here.
I just pointed out, that the advice sadko4u gives, is wrong by assumptions and dangerous by doing.
My commend goes to sadko4u, not to you.
CrocoDuck wrote:That's exactly the reason why people in my position need to ask questions, as the language itself has many pitfalls and, especially in the audio realm, the documentation is sparse and hard to read.


:) , you are welcome

EDIT:// CrocoDuck, I've just re-read my post before, and now I see why you thing my comment goes to your stance, no, it didn't, it is just the follow-up from the previous sentence. Sorry for the confusion about that.

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Re: [SOLVED] C++: which type of cast is best for audio callbacks?

Postby CrocoDuck » Sun Dec 04, 2016 1:00 pm

tramp wrote:You are in the right place here.


Uh oh, sorry, I misinterpreted your comment... oops :oops:

tramp wrote:and I don't think it is a good idea to advice it here for no reason.


For some whatever reason I read it as "and I don't think it as a good idea to ask for advice here for no reason". I still have to wake up properly...
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