Use C99 extention in Qt Creator

tommytrojan

I try to use array designators like so:

const uchar background[49] = { [0 ... 48] = 236 };

But Qt Creator gives me warnings like warning: Array designators are a C99 extension and the project does not compile. So I try to add in CMakeLists.txt:

set(CMAKE_C_STANDARD 99)

or

set_target_properties(mytarget PROPERTIES
  C_STANDARD 99
)

But neither works. How do I use C99 standard?

Chris Kawa

What the warning says is that this is an extension to the C99 standard, not that the extension is part of the standard. So if you set your mode to standard C99 you're disabling extensions.

Try setting C_EXTENSIONS to true and see if that helps.

tommytrojan

Thank you for the reply!

I tried C_EXTENSIONS but it didn't work. I tried

set(CMAKE_C_EXTENSIONS TRUE)
set(CMAKE_CXX_EXTENSIONS TRUE)
set(CMAKE_C_EXTENSIONS ON)
set(CMAKE_CXX_EXTENSIONS ON)

or

set_target_properties(mytarget PROPERTIES
  C_EXTENSIONS ON
)

etc.... None of these works.

Chris Kawa

Is it a .c or .cpp file?
This is a GNU extension, so will only work with GCC. What compiler are you using?
If GCC then look at the command line output when you're compiling and see what -std flag is passed to the compiler.
Also, maybe it should be CMAKE_C_EXTENSIONS. I'm not that good with CMake.

kuzulis

set(CMAKE_C_STANDARD 99)
set(CMAKE_C_STANDARD_REQUIRED ON)

tommytrojan

@Chris-Kawa It is a .cpp file. I'm indeed using GCC. The -std flag shows -std=gnu++17. I'm quite confused... Is this a C extension, or a CPP extension? If it is a C extension, I can still use it in a cpp file, right?

I tried both CMAKE_C_EXTENSIONS and C_EXTENSIONS, neither works.

tommytrojan

@kuzulis Thank you for your reply! Unfortunately it does not work.

Chris Kawa

@tommytrojan Unless you specify the language via compiler parameter it will be determined by a file extension. See -x option. If you use .c it will compile as C. If you use .cpp it will compile as C++. You are compiling it as C++.

Is this a C extension, or a CPP extension?

The syntax you're trying to use is a C language extension. You are compiling this file as C++.

If it is a C extension, I can still use it in a cpp file, right?

I wouldn't assume that at all. Those are two different languages. C language extensions are, well, for C language.

tommytrojan

Thank you for your answer, I now understand it much better than before.

I tried almost all suggestions I can find on the web, nothing works. My intuition is to blame CMake for this. I have to give up on this syntax. Maybe it is good not to depend on GNU specific syntax.

It is a shame though because I want a const array of a specific value. Do you know another way to achieve it in c++?

Part of me cannot believe that it cannot be done in c++ in 2022 without GNU.

Chris Kawa

@tommytrojan said:

Maybe it is good not to depend on GNU specific syntax.

By using GNU language extensions you're tying yourself to a GNU compiler that supports them, which is not a great idea in general. Maybe in some cases, but I would say not if it's just for this one small thing.

It is a shame though because I want a const array of a specific value.

I don't think there's convenient way to do that in C++. Here's one inconvenient way. A helper that you can reuse. It's a compile time function in C++20.

template<typename T, int size>
constexpr auto make_array(T value)
{
    std::array<T, size> arr;
    arr.fill(value);
    return arr;
}


const auto background = make_array<unsigned char, 49>(236);

Btw. Out of curiosity - what do you need a constant array of same value for?

tommytrojan

Thank you! This template helper can be really useful!

@Chris-Kawa said in Use C99 extention in Qt Creator:

Btw. Out of curiosity - what do you need a constant array of same value for?

Indeed! In the past two hours I thought hard on this and decided to use a const int instead, and construct the needed array later on the fly.

Thank you for the follow up! I learned a lot from you.