C99 designator member outside of aggregate initializer - c++

struct Foo {
char a[10];
int b;
};
static Foo foo = {.a="bla"};
Compiling the above code gives the following gcc error:
$ gcc -std=gnu++2a test.cpp
C99 designator ‘a’ outside aggregate initializer
I thought that c-string designators in initializer list like these are ok in C++20? What am I missing? I am using gcc version 10.

This is a known bug with GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55227
Unfortunately, you will have to either not use designated initializers or use a different initializer for the array:
static Foo foo = {"bla"};
static Foo foo = {.a={'b', 'l', 'a', 0}};

I got the same error and I dealt with mv my.cpp my.c
You also can find answer in this link:
https://pcbartists.com/firmware/esp32-firmware/designator-outside-aggregate-initializer-solved/
#ifdef __cplusplus
extern "C"
{
#endif
// C code goes here
#ifdef __cplusplus
}
#endif

I use strncpy (from <cstring>), for example:
strncpy(foo.str, "hello", sizeof(foo_t::str));
It seems to be optimized away, therefore generating the same assembly as normal approach in newer GCC (11.3^) and the same as using Artyer solution (the accepted one) of using char array.
Godbolt link: https://godbolt.org/z/9G7b6PT9b
However, the solution might cause warning: missing initializer for member 'foo_t::str' [-Wmissing-field-initializers] if you have the -Wextra warnings enable, but you can use -Wno-missing-field-initializers to exclude it.
By the way, with stucts like that you always have to remember the space is limited, and in many cases you might want leave trailing zero for string end. Using strncpy you can force that by adding:
foo.str[sizeof(foo_t::str) - 1] = 0;

Related

Clang: error: invalid use of non-static data member

Is this gcc being overly nice and doing what the dev thinks it will do or is clang being overly fussy about something. Am I missing some subtle rule in the standard where clang is actually correct in complaining about this
Or should I use the second bit of code which is basically the how offsetof works
[adrian#localhost ~]$ g++ -Wall -pedantic -ansi a.cc
[adrian#localhost ~]$ a.out
50
[adrian#localhost ~]$ cat a.cc
#include <iostream>
struct Foo
{
char name[50];
};
int main(int argc, char *argv[])
{
std::cout << sizeof(Foo::name) << std::endl;
return 0;
}
[adrian#localhost ~]$ clang++ a.cc
a.cc:10:29: error: invalid use of non-static data member 'name'
std::cout << sizeof(Foo::name) << std::endl;
~~~~~^~~~
1 error generated.
[adrian#localhost ~]$ g++ -Wall -pedantic -ansi b.cc
[adrian#localhost ~]$ a.out
50
[adrian#localhost ~]$ cat b.cc
#include <iostream>
struct Foo
{
char name[50];
};
int main(int argc, char *argv[])
{
std::cout << sizeof(static_cast<Foo*>(0)->name) << std::endl;
return 0;
}
[adrian#localhost ~]$ clang++ b.cc
[adrian#localhost ~]$ a.out
50
I found adding -std=c++11 stops it complaining. GCC is fine
with it in either version.
Modern GCC versions allow this even in -std=c++98 mode. However, older versions, like GCC 3.3.6 of mine, do complain and refuse to compile.
So now I wonder which part of C++98 I am violating with this code.
Wikipedia explicitly states that such a feature was added in C++11, and refers to N2253, which says that the syntax was not considered invalid by the C++98 standard initially, but then intentionally clarified to disallow this (I have no idea how non-static member fields are any different from other variables with regard to their data type). Some time later they decided to make this syntax valid, but not until C++11.
The very same document mentions an ugly workaround, which can also be seen throughout the web:
sizeof(((Class*) 0)->Field)
It looks like simply using 0, NULL or nullptr may trigger compiler warnings for possible dereference of a null pointer (despite the fact that sizeof never evaluates its argument), so an arbitrary non-zero value might be used instead, although it will look like a counter-intuitive “magic constant”. Therefore, in my C++ graceful degradation layer I use:
#if __cplusplus >= 201103L
#define CXX_MODERN 2011
#else
#define CXX_LEGACY 1998
#endif
#ifdef CXX_MODERN
#define CXX_FEATURE_SIZEOF_NONSTATIC
#define CxxSizeOf(TYPE, FIELD) (sizeof TYPE::FIELD)
#else
// Use of `nullptr` may trigger warnings.
#define CxxSizeOf(TYPE, FIELD) (sizeof (reinterpret_cast<const TYPE*>(1234)->FIELD))
#endif
Usage examples:
// On block level:
class SomeHeader {
public:
uint16_t Flags;
static CxxConstExpr size_t FixedSize =
#ifdef CXX_FEATURE_SIZEOF_NONSTATIC
(sizeof Flags)
#else
sizeof(uint16_t)
#endif
;
}; // end class SomeHeader
// Inside a function:
void Foo(void) {
size_t nSize = CxxSizeOf(SomeHeader, Flags);
} // end function Foo(void)
By the way, note the syntax difference for sizeof(Type) and sizeof Expression, as they are formally not the same, even if sizeof(Expression) works — as long as sizeof (Expression) is valid. So, the most correct and portable form would be sizeof(decltype(Expression)), but unfortunately it was made available only in C++11; some compliers have provided typeof(Expression) for a long time, but this never was a standard extension.

unused-variable warning different for auto variables

Using gcc (4.7.2 here) I get warnings about unused auto variables, but not about other variables:
// cvars.h
#ifndef CVARS_H_
#define CVARS_H_
const auto const_auto = "const_auto";
const char const_char_array[] = "const_char_array";
const char * const_char_star = "const_char_star";
const char use_me = 'u';
#endif // CVARS_H_
//---
//comp_unit.cpp
#include "cvars.h"
void somef()
{
//const_auto // commented out - unused
use_me; // not using any of the others either
}
// compile with $ g++ -std=c++11 -Wunused-variable -c comp_unit.cpp
// gcc outputs warning: ‘cvars::const_auto’ defined but not used [-Wunused-variable]
// but does not complain about the other variables
Is this an inconsistency in GCC?
1.1 If so, what should happen in all cases, warning or no warning?
1.2 If not, what is the reason for the difference in behavior?
Note: Concerning 1.1, I imagine no warning should be printed in this case (this is what clang does). Otherwise, any compilation unit including a constant-defining header but not using all the constants within would contain lots of warnings.
These warnings are entirely up to the implementation, so there is no "should". But, yes, I agree: constants would ideally not generate these warnings even when declared using auto.
Since I can reproduce your observation in GCC 4.7 and GCC 4.8.0, but not in GCC 4.8.1 or 4.9, I'd say the guys over at GNU would agree too. In fact, I believe you're seeing bug 57183.

Why does -Wunused-variable in GCC produce an error even on static const?

I have a header, core/types.hh, used by several different build targets. It has the following declaration:
core/types.hh
typedef std::size_t Size;
static const Size SZ_MAX = std::numeric_limits<Size>::max();
...
Some of the targets use this constant, some don't. So I get:
error: 'core::SZ_MAX' defined but not used"
I use scons with GCC 4.7.3 on Linux. I have -Wall set and want to keep it that way.
As far as I understand from the GCC documentation, this shouldn't give a warning:
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This warning is enabled by -Wall.
So I don't see why I get a warning (which turns into an error).
On other answers, people were advised to make the declaration extern and to do the assignment in the file that uses the constant. This file is used by many other files, so it would loose its constant-ness if I do that. Furthermore, this file has header guards, so I think this should mean that the constant is actually created only once.
I'd appreciate any help!
Yuval
Possible duplicates:
How to use typed constants with “unused variable” warnings?
c++ static array declared in h file gives warning 'defined but not used'
It seems that this was not the error that halted compilation.
Rather, if GCC find another error, it would still report on this too.
I actually had another unused variable, and that's what caused this error in the first place.
For example, when creating the following files:
file1.cc
#include "head1.hh"
int main() {
int bad_unused_variable;
return my_ns::JUST_ANOTHER_CONST;
}
head1.hh
#ifndef HEAD1
#define HEAD1
#include <stdint.h>
#include <cstddef>
#include <limits>
namespace my_ns {
typedef std::size_t Size;
static const Size SZ_MAX = std::numeric_limits<Size>::max();
static const Size JUST_ANOTHER_CONST = 8;
}
#endif
You get:
> g++ -Wall -Werror file1.cc -O2 -std=c++98 -o file1
file1.cc: In function 'int main()':
file1.cc:4:6: error: unused variable 'bad_unused_variable' [-Werror=unused-variable]
In file included from file1.cc:1:0:
head1.hh: At global scope:
head1.hh:10:20: error: 'my_ns::SZ_MAX' defined but not used [-Werror=unused-variable]
cc1plus: all warnings being treated as errors
EDIT
This also seems to have been answered here: gcc warnings: defined but not used vs unused variable - there they mention the subtle differences between the two warning messages (unused variable vs defined but not used). Still, it doesn't really answer as to why GCC behaves this way...

How to trace out why gcc and g++ produces different code

Is it possible to see what is going on behind gcc and g++ compilation process?
I have the following program:
#include <stdio.h>
#include <unistd.h>
size_t sym1 = 100;
size_t *addr = &sym1;
size_t *arr = (size_t*)((size_t)&arr + (size_t)&addr);
int main (int argc, char **argv)
{
(void) argc;
(void) argv;
printf("libtest: addr of main(): %p\n", &main);
printf("libtest: addr of arr: %p\n", &arr);
while(1);
return 0;
}
Why is it possible to produce the binary without error with g++ while there is an error using gcc?
I'm looking for a method to trace what makes them behave differently.
# gcc test.c -o test_app
test.c:7:1: error: initializer element is not constant
# g++ test.c -o test_app
I think the reason can be in fact that gcc uses cc1 as a compiler and g++ uses cc1plus.
Is there a way to make more precise output of what actually has been done?
I've tried to use -v flag but the output is quite similar. Are there different flags passed to linker?
What is the easiest way to compare two compilation procedures and find the difference in them?
In this case, gcc produces nothing because your program is not valid C. As the compiler explains, the initializer element (expression used to initialize the global variable arr) is not constant.
C requires initialization expressions to be compile-time constants, so that the contents of local variables can be placed in the data segment of the executable. This cannot be done for arr because the addresses of variables involved are not known until link time and their sum cannot be trivially filled in by the dynamic linker, as is the case for addr1. C++ allows this, so g++ generates initialization code that evaluates the non-constant expressions and stores them in global variables. This code is executed before invocation of main().
Executables cc1 and cc1plus are internal details of the implementation of the compiler, and as such irrelevant to the observed behavior. The relevant fact is that gcc expects valid C code as its input, and g++ expects valid C++ code. The code you provided is valid C++, but not valid C, which is why g++ compiles it and gcc doesn't.
There is a slightly more interesting question lurking here. Consider the following test cases:
#include <stdint.h>
#if TEST==1
void *p=(void *)(unsigned short)&p;
#elif TEST==2
void *p=(void *)(uintptr_t)&p;
#elif TEST==3
void *p=(void *)(1*(uintptr_t)&p);
#elif TEST==4
void *p=(void *)(2*(uintptr_t)&p);
#endif
gcc (even with the very conservative flags -ansi -pedantic-errors) rejects test 1 but accepts test 2, and accepts test 3 but rejects test 4.
From this I conclude that some operations that are easily optimized away (like casting to an object of the same size, or multiplying by 1) get eliminated before the check for whether the initializer is a constant expression.
So gcc might be accepting a few things that it should reject according to the C standard. But when you make them slightly more complicated (like adding the result of a cast to the result of another cast - what useful value can possibly result from adding two addresses anyway?) it notices the problem and rejects the expression.

Are variable length arrays an extension in Clang too?

I understand that VLAs are not part of C++11 and I have seen this slip by GCC. It is part of the reason I switched to Clang. But now I am seeing it Clang too. I am using clang 3.2 (one behind the latest) and I am compiling with
-pedantic and -std=c++11
I expect my test to NOT compile yet it compiles and runs.
int myArray[ func_returning_random_int_definitely_not_constexpr( ) ];
Is this a compiler bug or an I missing something?
In response to the comment here is the random_int_function()
#include <random>
int random_int_function(int i)
{
std::default_random_engine generator;
std::uniform_int_distribution<int> distribution(1,100);
int random_int = distribution(generator);
return i + random_int;
}
Yes, variable length arrays are supported in clang 3.2/3.3 contrary to
the C++11 Standard (§ 8.3.4/1).
So as you say, a program such as:
#include <random>
int random_int_function(int i)
{
std::default_random_engine generator;
std::uniform_int_distribution<int> distribution(1,100);
int random_int = distribution(generator);
return i + random_int;
}
int main() {
int myArray[ random_int_function( 0 ) ];
(void)myArray;
return 0;
}
compiles and runs. However, with the options -pedantic; -std=c++11 that
you say you passed, clang 3.2/3,3 diagnoses:
warning: variable length arrays are a C99 feature [-Wvla]
The behaviour matches that of gcc (4.7.2/4.8.1), which warns more emphatically:
warning: ISO C++ forbids variable length array ‘myArray’ [-Wvla]
To make the diagnostic be an error, for either compiler, pass -Werror=vla.
Simply plugging the snippets you posted into IDEone, without putting the array declaration into a function, I get
prog.cpp:12:39: error: array bound is not an integer constant before ‘]’ token
Adding a main() function around it results in success, as you observed.
Since C++11 doesn't allow for array declarations that are legal in main but not namespace scope, and that is a property of VLAs, it is reasonable to conclude that is what you are seeing.
Update: courtesy Coliru.org, the message from Clang is
main.cpp:12:9: error: variable length array declaration not allowed at file scope
So that's fairly definite.
Use these options:
-Wvla to warning vla uses
-Werror=vla to consider vla an error.
This works in both the clang and gcc compilers.