- Using std::string by reference - 3 Updates
- operator == on vectors - 13 Updates
- Telemetry in "hello world" MS C++ 2015 - 1 Update
- Cross-platform C++ OpenGL-based GUI library coming soon - 1 Update
| James Moe <jimoeDESPAM@sohnen-moe.com>: Jun 12 12:47PM -0700 gcc 4.7.3 std::string UtilsSetPathSep2 (std::string & str, const char * path_sep_char_new) { ... } When passing the argument "str" by reference, "std::string &", this error is emitted: pmmsend.c: In function 'pmmsend_return_code_t program_args(int, char**, prog_args_t&)': pmmsend.c:449:63: error: invalid initialization of reference of type 'std::string& {aka std::basic_string<char>&}' from expression of type 'char*' pmmsend.c:287:8: error: in passing argument 1 of 'std::string UtilsSetPathSep2(std::string&, const char*)' When "std::string &" is changed to "std::string", no error(s) result. Is it not possible to use std::string by reference? -- James Moe jmm-list at sohnen-moe dot com |
| Ian Collins <ian-news@hotmail.com>: Jun 13 07:57AM +1200 On 06/13/16 07:47 AM, James Moe wrote: > UtilsSetPathSep2(std::string&, const char*)' > When "std::string &" is changed to "std::string", no error(s) result. > Is it not possible to use std::string by reference? Yes, but in this case you are passing a char* which will create a temporary string. You can't bind a temporary object to a reference, so you will have to use const std::string&, std::string or std::string&& as the parameter type in this case. -- Ian |
| "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Jun 13 01:29AM +0200 On 12.06.2016 21:47, James Moe wrote: > pmmsend.c:449:63: error: invalid initialization of reference of type > 'std::string& {aka std::basic_string<char>&}' from expression of type > 'char*' This means that you have tried to use a `char*` or a `char` array as actual argument for the `std::string&` formal argument. Since `std::string&` is a reference-to-non-`const`, where the function can /modify/ the actual argument object, you need an actual argument of the exact same type or a class derived from that type (in the latter case the reference is bound to the `std::string` base class sub-object). The function can't very well modify an instance of some other type as if it were a `std::string`. > pmmsend.c:287:8: error: in passing argument 1 of 'std::string > UtilsSetPathSep2(std::string&, const char*)' > When "std::string &" is changed to "std::string", no error(s) result. Because now the argument is passed by value, which means that the actual argument is copied or converted, or the behavior is /as if/ there is a copying or conversion. And `std::string` provides a constructor that accepts a `char const*`. I.e. you get an automatic conversion. The function's code now operates on a copy or an instance resulting from a conversion, and so it can't affect the original argument, in your specific case the `char` array. > Is it not possible to use std::string by reference? That's possible. Since the function /returns/ a `std::string` you probably wanted `std::string const&` anyway. :-) Cheers & hth., - Alf |
| Paul <pepstein5@gmail.com>: Jun 12 04:02AM -0700 I don't seem to understand operators on vectors. I thought that you could write someVec == {} as a statement saying that someVec is empty. However, this doesn't compile and I don't know why. My error message is "expected primary-expression before '{' token". I'd be grateful if someone could enlighten me. The complete code is below. Thanks again for your help. Paul #include <vector> int main() { std::vector<int> z; if(z == {}) ; // empty statement. } |
| Victor Bazarov <v.bazarov@comcast.invalid>: Jun 12 08:34AM -0400 On 6/12/2016 7:02 AM, Paul wrote: > if(z == {}) > ; // empty statement. > } There is no operator== declared for std::vector. Equality operators are not compiler-defined for user-defined classes. The library classes like containers, streams, algorithms, etc. are user-defined from the language point of view. Some of them (like std::string) do define operators like == (as members or stand-alone functions). Most do not. You can find more information on the available functionality if you just RTFM. V -- I do not respond to top-posted replies, please don't ask |
| Bonita Montero <Bonita.Montero@gmail.com>: Jun 12 03:06PM +0200 Am 12.06.2016 um 14:34 schrieb Victor Bazarov: > There is no operator== declared for std::vector. ... http://www.cplusplus.com/reference/vector/vector/operators/ -- http://facebook.com/bonita.montero/ |
| "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Jun 12 03:24PM +0200 * On 12.06.2016 14:34, Victor Bazarov wrote: > containers, streams, algorithms, etc. are user-defined from the language > point of view. Some of them (like std::string) do define operators like > == (as members or stand-alone functions). Most do not. Adding to that... With all standard containers (unless I'm mistaken, in which case, with most all standard containers) you can use the misleadingly named member function `empty` to check whether it's empty. Contrary to what the name indicates, it does not empty the collection. It just checks. std::bitset is not technically a container and has a differently named empty checker, a method called `none`. To get more readable code you can define an `is_empty` function for vectors, like this: template< class Item > auto is_empty( std::vector<Item> const& v ) -> bool { return v.empty(); } and then you can write more clearly if( is_empty( z ) ) { ... Defining such a wrapper for each container type of interest is some work though. But defining a single general more smart wrapper that determines if there is a boolean `empty` method and if so, uses it, is even more work. I'm not posting my code for that, it would simply be too long, but if you're interested you can look at it ¹at GitHub. Cheers & hth., - Alf Links: ¹ https://github.com/alf-p-steinbach/cppx/blob/master/source_code/cppx/core_language_support/sizes_and_lengths.hpp |
| "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Jun 12 03:34PM +0200 On 12.06.2016 15:06, Bonita Montero wrote: > Am 12.06.2016 um 14:34 schrieb Victor Bazarov: >> There is no operator== declared for std::vector. ... > http://www.cplusplus.com/reference/vector/vector/operators/ Victor jumped to the wrong conclusion, but it was so reasonable that I posted a follow-up to that just assuming that he was right. The actual issue is, I believe, purely syntactical: that the `{}` is only supported syntactically in a (syntactically) function call. For you /can/ write this: if( std::operator==( z, {} ) ) ; // empty statement. Cheers & hth., - Alf |
| Paavo Helde <myfirstname@osa.pri.ee>: Jun 12 04:54PM +0300 On 12.06.2016 14:02, Paul wrote: > if(z == {}) > ; // empty statement. > } std::operator== is not a member of std::vector, it is a standalone function which has many overloads. When you pass it z and {} it does not know which overload because types are not the same and there is no exact match. There are a couple of workarounds: a) cast/convert the operand into correct type manually: if (z == std::vector<int>({})) b) use a function which is not overloaded: bool AreEqual(const std::vector<int>& a, const std::vector<int>& b) { return a==b; } ... if (AreEqual(z, {})) Of course, for the problem at hand you could rather use if (z.empty()) instead, as suggested by Alf. hth Paavo |
| Paavo Helde <myfirstname@osa.pri.ee>: Jun 12 05:15PM +0300 On 12.06.2016 16:34, Alf P. Steinbach wrote: > posted a follow-up to that just assuming that he was right. > The actual issue is, I believe, purely syntactical: that the `{}` is > only supported syntactically in a (syntactically) function call. It is supported also in initialization std::vector<int> z = {}; std::vector<int> z2 {}; > For you /can/ write this: > if( std::operator==( z, {} ) ) > ; // empty statement. Sorry, this does not work: 1>main.cpp(26): error C2665: 'std::operator ==' : none of the 5 overloads could convert all the argument types 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\system_error(410): could be 'bool std::operator ==(const std::error_condition &,const std::error_code &) throw()' 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\system_error(402): or 'bool std::operator ==(const std::error_code &,const std::error_condition &) throw()' 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\exception(507): or 'bool std::operator ==(const std::exception_ptr &,std::nullptr_t)' 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\exception(502): or 'bool std::operator ==(std::nullptr_t,const std::exception_ptr &)' 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\exception(497): or 'bool std::operator ==(const std::exception_ptr &,const std::exception_ptr &)' 1> while trying to match the argument list '(std::vector<int,std::allocator<_Ty>>, initializer-list)' 1> with 1> [ 1> _Ty=int 1> ] |
| "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Jun 12 04:22PM +0200 On 12.06.2016 16:15, Paavo Helde wrote: > It is supported also in initialization > std::vector<int> z = {}; > std::vector<int> z2 {}; Yep, thanks! > overloads could convert all the argument types > 1> C:\Program Files (x86)\Microsoft Visual Studio > 12.0\VC\include\system_error(410): It's a Good Idea to compile code for C++11 and later, with a compiler that supports the relevant feature(s). You should upgrade from VS 2012 to VS 2015. The latest update is 3, but I've only gotten around to 2. {example} [C:\my\forums\so\156] > g++ --version | find "++" g++ (tdm64-1) 5.1.0 [C:\my\forums\so\156] > cl /nologo- 2>&1 | find "++" Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23725 for x86 [C:\my\forums\so\156] > type foo.cpp #include <vector> int main() { std::vector<int> z; if( std::operator==( z, {} ) ) ; // empty statement. } [C:\my\forums\so\156] > g++ foo.cpp foo.cpp: In function 'int main()': foo.cpp:7:7: warning: suggest braces around empty body in an 'if' statement [-Wempty-body] ; // empty statement. ^ [C:\my\forums\so\156] > cl foo.cpp /Feb foo.cpp foo.cpp(7): warning C4390: ';': empty controlled statement found; is this the intent? [C:\my\forums\so\156] > _ {/example} Which exemplifies that I'm not always so pedantically correct in where I place the small example code snippets. Hrmf. Cheers & hth., - Alf |
| "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Jun 12 04:27PM +0200 On 12.06.2016 15:54, Paavo Helde wrote: > function which has many overloads. When you pass it z and {} it does not > know which overload because types are not the same and there is no exact > match. No no, passing `z` and `{}` /to the function/ is supported and works nicely with modern compilers. It's the operator notation that doesn't support `{}`. Else-thread you remarked that it's not only supported syntactically for function calls, but also for initializers. And I can add to that, also for `return` statements. Which however can be regarded as initialization of the returned object. [snip] Cheers & hth., - Alf |
| Paavo Helde <myfirstname@osa.pri.ee>: Jun 12 05:29PM +0300 On 12.06.2016 17:15, Paavo Helde wrote: >> if( std::operator==( z, {} ) ) >> ; // empty statement. > Sorry, this does not work: Correction: it seems this might accidentally work if one avoids including most standard headers (the errors in my previous post appeared because of #include <iostream>). However, this is not guaranteed because standard headers like <vector> can freely include other standard headers. |
| "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Jun 12 04:30PM +0200 On 12.06.2016 16:29, Paavo Helde wrote: > including most standard headers (the errors in my previous post appeared > because of #include <iostream>). However, this is not guaranteed because > standard headers like <vector> can freely include other standard headers. I can report that it works niceley with `<iostream>` included, with g++ 5.1 and MSVC 2015. And there's no accident about it. :) Cheers & hth., - Alf |
| Paavo Helde <myfirstname@osa.pri.ee>: Jun 12 06:53PM +0300 On 12.06.2016 17:30, Alf P. Steinbach wrote: > I can report that it works niceley with `<iostream>` included, with g++ > 5.1 and MSVC 2015. > And there's no accident about it. :) I see. That's nice to hear. It seems I need to upgrade indeed. I wonder if there have been any changes in this area in C++ standards or is it just another MSVC bug? Cheers Paavo |
| Vir Campestris <vir.campestris@invalid.invalid>: Jun 12 06:15PM +0100 On 12/06/2016 15:22, Alf P. Steinbach wrote: > You should upgrade from VS 2012 to VS 2015. > The latest update is 3, but I've only gotten around to 2. I take it you heard about the telemetry thing? Andy |
| Alain Ketterlin <alain@universite-de-strasbourg.fr.invalid>: Jun 12 11:46AM +0200 >>> https://yro.slashdot.org/story/16/06/10/1350245/visual-studio-2015-c-compiler-secretly-inserts-telemetry-code-into-binaries [...] > But this brings up a very important point (one which I saw back in the > 90's but hasn't been followed up in much). We all trust our compilers > and libraries not to add extra code. But is this trust misplaced? One of the comments on the slashdot page refers to "the Ken Thompson attack", described at http://c2.com/cgi/wiki?TheKenThompsonHack Ken Thompson notes ("Reflections on Trusting Trust" are available from http://dl.acm.org/citation.cfm?id=358210 (or http://www.ece.cmu.edu/~ganger/712.fall02/papers/p761-thompson.pdf) -- Alain. |
| Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk>: Jun 12 03:51AM +0100 Hi all, Line edits, spin boxes, sliders and modal dialogs implemented in my upcoming Cross-platform C++ GUI/game library "neogfx": http://neogfx.org /Flibble |
| You received this digest because you're subscribed to updates for this group. You can change your settings on the group membership page. To unsubscribe from this group and stop receiving emails from it send an email to comp.lang.c+++unsubscribe@googlegroups.com. |
No comments:
Post a Comment