If we allow taking the address than the lifetime needs to be longer than the lifetime of the pointer we're assigning it to, that's all.

-----Original Message-----
From: Jens Maurer <Jens.Maurer_at_gmx.net>
Sent: Wednesday, May 12, 2021 1:20 PM
To: Charlie Barto <Charles.Barto_at_[hidden]osoft.com>; liaison_at_lists.isocpp.org; Zhihao Yuan <zy_at_miator.net>
Subject: Re: [wg14/wg21 liaison] P2174 Status (and a tenuous offer to help)

On 12/05/2021 22.11, Charlie Barto wrote:
> That's true, but for that distinction to be useful the behavior would have to differ depending on if the _outer_ struct is a temporary, right?

I don't understand this comment.

In C++, T{x,y} is an rvalue, so you can't take its address.
If we make (T){x,y} the same as T[x,y}
(which it practically is under some current compilers, I gather), your example is one which highlights the other difference of C vs. hypothetical-C++, beyond lifetime.

This is independent of outer vs. inner struct.

Jens


> -----Original Message-----
> From: Jens Maurer <Jens.Maurer_at_[hidden]>
> Sent: Wednesday, May 12, 2021 1:06 PM
> To: liaison_at_[hidden]rg; Zhihao Yuan <zy_at_miator.net>
> Cc: Charlie Barto <Charles.Barto_at_microsoft.com>
> Subject: Re: [wg14/wg21 liaison] P2174 Status (and a tenuous offer to
> help)
>
> On 12/05/2021 21.51, Charlie Barto via Liaison wrote:
>> But in any case the before/after comparison
>>
>> extern “C” VkResult vkCreateInstance(const VkInstanceCreateInfo*,
>> const VKAllocationCallbacks*, VkInstance*);
>>
>> in c++:
>>
>> VkApplicationInfo appinfo {
>> .sType = VK_STRUCTURE_TYPE_APPLICATION_INFO,
>> .pApplicationName = “Hello World”,
>>                 .applicationVersion = VK_MAKE_VERSION(0, 1, 0),
>> .pEngineName = “No Engine”,
>> .apiVersion = VK_API_VERSION_1_2 };
>> VkInstanceCreateInfo createInfo {
>> .sType = VK_STRUCTURE_TYPE_INSTANCE_CREATE_INFO,
>>                 .pApplicationInfo = &appinfo, }; VkInstance instance
>> = 0; auto result = vkCreateInstance(&createInfo, nullptr, &instance);
>>
>> in C:
>> VkInstance instance = 0;
>> VkResult result = vkCreateInstance(&(VkInstanceCreateInfo){
>>   .sType = VK_STRUCTURE_TYPE_INSTANCE_CREATE_INFO,
>>                 .pApplicationInfo = &(VkApplicationInfo) {
>> .sType =
>> VK_STRUCTURE_TYPE_APPLICATION_INFO,
>> .pApplicationName = “Hello World”,
>>                                 .applicationVersion =
>> VK_MAKE_VERSION(0, 1, 0),
>> .pEngineName = “No Engine”,
>> .apiVersion = VK_API_VERSION_1_2
>> }}, NULL, &instance);
>
> I'd like to point out that this example is not about lifetime (full-expression vs. block), but about the fact that in C, compound literals are lvalues, so you can take their address.
>
> Jens
>
>