| "Rick C. Hodgin" <rick.c.hodgin@gmail.com>: Jul 09 08:31PM -0700 I have something like this and I need to assign a value to the second member of the union: struct SAbc { int discriminator; union { int value; char* ptr; }; }; When I use it in source code, it's an example like this: SAbc abc[] = { { 1, 1 }, // Initializes discriminator, value { 2, "test" }, // Should initialize discriminator, ptr }; I remember from GCC a syntax which was something like this: { 2, {.ptr = "test"} }, But I can't figure out how to get it to work in Microsoft's C++ compiler, and I don't know what to search for to find it via Google. Best regards, Rick C. Hodgin |
| Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk>: Jul 10 05:06AM +0100 On 10/07/2016 04:31, Rick C. Hodgin wrote: > { 2, {.ptr = "test"} }, > But I can't figure out how to get it to work in Microsoft's C++ compiler, > and I don't know what to search for to find it via Google. Use boost.variant and then when it becomes available use std::variant. /Flibble |
| "Öö Tiib" <ootiib@hot.ee>: Jul 10 04:39AM -0700 On Sunday, 10 July 2016 06:31:56 UTC+3, Rick C. Hodgin wrote: > { 2, {.ptr = "test"} }, > But I can't figure out how to get it to work in Microsoft's C++ compiler, > and I don't know what to search for to find it via Google. I am making three assumptions (assuming "yes" to three questions below). 1) PODs and aggregates are used to ensure that minimal run-time calls are made? 2) 'char*' initialized from pointer to string literal won't be ever used as pointer to mutable char? 3) discriminator is always 1 on case of int and always 2 on case of string literal? If I'm wrong, use 'boost::variant' instead of 'union' ... otherwise here is my attempt: // the class struct SAbc { int discriminator; union { int value; char const* ptr; }; constexpr explicit SAbc(int v) : discriminator(1) , value(v) {} constexpr explicit SAbc(char const* p) : discriminator(2) , ptr(p) {} }; // the array constexpr SAbc abc[] = { SAbc(1), SAbc("test"), }; |
| Frank Tetzel <s1445051@mail.zih.tu-dresden.de>: Jul 10 03:29PM +0200 > { 2, {.ptr = "test"} }, > But I can't figure out how to get it to work in Microsoft's C++ > compiler, and I don't know what to search for to find it via Google. This is called designated initializers[1] and is part of the C99 standard, not in any C++ standard as far as i know. GCC allows it in C++ code nonetheless as a GNU extension, similar to variable-length-arrays. Clang supports it too but as Visual Studio only has a recent C++ compiler it mostly likely does not. Just define two overloaded ctors for the structure and use normal list initialization. struct SAbc { int discriminator; union { int value; const char *ptr; }; SAbc(int discriminator, int value) : discriminator(discriminator), value(value) {} SAbc(int discriminator, const char *ptr) : discriminator(discriminator), ptr(ptr) {} }; [1] https://gcc.gnu.org/onlinedocs/gcc-6.1.0/gcc/Designated-Inits.html Regards, Frank |
| woodbrian77@gmail.com: Jul 10 09:54AM -0700 On Sunday, July 10, 2016 at 6:40:27 AM UTC-5, Öö Tiib wrote: > SAbc(1), > SAbc("test"), > }; I prefer your approach to Frank's, but I'd encourage the OP to reconsider the use of the union. He may not need it and at the least he should improve the name of the struct -- "SAbc" doesn't cut it. Brian Ebenezer Enterprises - In G-d we trust. http://webEbenezer.net |
| "Rick C. Hodgin" <rick.c.hodgin@gmail.com>: Jul 10 12:26PM -0700 Brian Wood wrote: > ... at the least he should improve the name of the struct > -- "SAbc" doesn't cut it. It was an example. The actual name is appropriately descriptive. I appreciate everyone's response. I like the solutions offered. I have a need in this project to use unions, so I need the syntax to allow the content to be populated. If it's not possible in MSVC++, I'll use GCC for this part and link it in. I have already done this for another need which GCC allowed, but MSVC++ did not. It allowed constant text to be assigned to a pointer, but in readwrite memory, rather than readonly. Best regards, Rick C. Hodgin |
| Kalle Olavi Niemitalo <kon@iki.fi>: Jul 10 11:02PM +0300 > for another need which GCC allowed, but MSVC++ did not. > It allowed constant text to be assigned to a pointer, but in > readwrite memory, rather than readonly. Huh? Can't you do that portably: static char readwrite[] = "some text you want"; char *pointer = readwrite; Then assign pointer[2] = 'l' as you like. |
| "Rick C. Hodgin" <rick.c.hodgin@gmail.com>: Jul 10 02:52PM -0700 On Sunday, July 10, 2016 at 4:02:44 PM UTC-4, Kalle Olavi Niemitalo wrote: > static char readwrite[] = "some text you want"; > char *pointer = readwrite; > Then assign pointer[2] = 'l' as you like. It refers to this form: http://compgroups.net/comp.lang.c++/link-object-files-from-vc++-and-gcc/2080401 By using MinGW and compiling the data and then linking to the MSVC++ project, the constant data by pointer is in readwrite memory. Best regards, Rick C. Hodgin |
| "Rick C. Hodgin" <rick.c.hodgin@gmail.com>: Jul 10 02:54PM -0700 On Sunday, July 10, 2016 at 4:02:44 PM UTC-4, Kalle Olavi Niemitalo wrote: > static char readwrite[] = "some text you want"; > char *pointer = readwrite; > Then assign pointer[2] = 'l' as you like. Here are the details I created from that reference: https://groups.google.com/d/msg/comp.lang.c/UXGdxAzPgrg/h9NIIMAPf4kJ Best regards, Rick C. Hodgin |
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