Convenient C++ struct initialisation

Since style A is not allowed in C++ and you don't want style B then how about using style BX:

FooBar fb = { /*.foo=*/ 12, /*.bar=*/ 3.4 };  // :)

At least help at some extent.

You could use a lambda:

const FooBar fb = [&] {
    FooBar fb;
    fb.foo = 12;
    fb.bar = 3.4;
    return fb;
}();

More information on this idiom can be found on Herb Sutter's blog.

Designated initializes will be supported in c++2a, but you don't have to wait, because they are officialy supported by GCC, Clang and MSVC.

#include <iostream>
#include <filesystem>

struct hello_world {
    const char* hello;
    const char* world;
};

int main () 
{
    hello_world hw = {
        .hello = "hello, ",
        .world = "world!"
    };
    
    std::cout << hw.hello << hw.world << std::endl;
    return 0;
}

GCC Demo MSVC Demo

Update 2021

As @Code Doggo noted, anyone who is using Visual Studio 2019 will need to set /std:c++latest  for the "C++ Language Standard" field contained under Configuration Properties -> C/C++ -> Language.