Object instantiation with curly braces and : symbols

Question

Can you please explain the following object instantiation ? How it is called ?

Where can I find more information about this kind of object instantiation ?

#include <string>
#include <iostream>

using namespace std;

class Car {

public:
    string name;
    int wheels;
};

int main() {

    Car c{
        name: "vw",
        wheels: 4
    };

    return 0;
}

I am using GCC with the following options:

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 7.4.0-1ubuntu1~18.04.1' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-7 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 7.4.0 (Ubuntu 7.4.0-1ubuntu1~18.04.1

Edna

Do you have a compiler that compiles this? If so, what is it? – Heteroplasty

@Heteroplasty Both GCC and Clang: godbolt.org/z/DEucTZ (Though not with -pedantic-errors) – Flashing

Answer 1

This is called designated initializers, but it is using an obsolete syntax¹. This is a C feature that C++ partially implemented in C++20.

What

name: "vw"

means in this context is initialize the member name with the value "vw". The non obsolete version of the code would be

Car c{
    .name = "vw",
    .wheels = 4
};

GCC will also allow you to use the C version of designated initializers which is "more powerful" as an extension to the language.

¹ see: GCC 6.29 Designated Initializers

Deliquescence

The syntax in the question is however neither the C++20, nor the C syntax for designated initializers, is it? It seems to be some GNU specific syntax. – Flashing

I found this syntax has been obsolete since GCC 2.5. Looks like they still allow it, though. – Novocaine

@FredLarson GCC does not seem to warn about it without adding the -pedantic flag. Clang does however. See the godbolt I linked under the question. – Flashing

The name: value syntax is also a gcc extension, Standard C and Standard C++ only allow .name = value – Aenea

I have updated my question to include c++ compiler version. Thank you all ! – Edna