MSVC pointer type attributes - c++

I recently ran into a compilation warning (promoted to error) using the 64-bit VS2013 compiler (update 3) making me aware of the existence of pointer type attributes.
The warning indicated a loss of precision as I was about to assign something of type 'Object *' to 'Object * __ptr32'.
I didn't hear about this '__ptr32' suffix yet, so I did some googling and found the following link: http://blog.aaronballman.com/2013/05/msvc-pointer-type-attributes/ .
After reading this, it became clear that I shouldn't simply ignore this warning as I might get ugly bugs in my 64-bit application...
Trying to pinpoint the problem further, I wrote following small program:
#include <type_traits>
struct O { };
struct OD { using V = O *; };
template <typename X, typename Y> struct expect_equal { static_assert(std::is_same<X, Y>::value, "Types do not correspond."); };
template <class D> using VD = typename D::V;
template <class D> struct Exp { template <template <class> class T> using DT = T<D>; };
int main(void)
{
expect_equal<Exp<OD>::DT<VD>, O *> e;
return 0;
}
While it would compile using clang, I got the following error/assertion failure using my MSVC 64-bit compiler (note: the 32-bit compiler did not yield compilation errors):
Microsoft (R) C/C++ Optimizing Compiler Version 18.00.30723 for x64
Copyright (C) Microsoft Corporation. All rights reserved.
test.cpp
test.cpp(6) : error C2338: Types do not correspond.
test.cpp(13) : see reference to class template instantiation 'expect_equal<O * __ptr32,O *>' being compiled
I was wondering whether someone could indicate what I am doing wrong here: I don't use an explicit pointer type attribute anywhere. This also might be a an unspecified behavior or a bug of either the STL implementation or the compiler, but most likely I am missing some important point here... Thanks in advance!

Related

Why does the compiler issue a template recursion error?

I'm currently trying to deduce a std::tuple type from several std::vectors that are passed as parameters. My code works fine using gcc, but the compilation fails with Visual Studio Professional 2019 with the message "fatal error C1202: recursive type or function dependency context too complex".
It has been mentioned for my previous post (C++ "fatal error C1202: recursive type or function dependency context too complex" in visual studio, but gcc compiles) that the problem is caused by template recursion as explained in C++ template compilation error - recursive type or function dependency. However, I don't see where the recursion occurs.
So my questions are:
Why is there an (infinite) recursion?
How can it be resolved?
Here is the code (I'm bound to C++11):
#include <tuple>
#include <vector>
template <typename TT,typename Add>
auto addTupBase(TT t,std::vector<Add> a) ->decltype (std::tuple_cat(t,std::make_tuple(a[0])))
{
return std::tuple_cat(t,std::make_tuple(a[0])) ;
}
template <typename TT,typename Add,typename... Args>
auto addTupBase(TT t,std::vector<Add> a,Args... args)-> decltype(addTupBase(addTupBase(t,a),args...))
{
return addTupBase(addTupBase(t,a),args...);
}
template <typename T,typename... Args>
auto addTup(std::vector<T> in,Args... args) ->decltype(addTupBase(std::make_tuple(in[0]),args...))
{
return addTupBase(std::make_tuple(in[0]),args...);
}
int main()
{
using TupleType = decltype(addTup(std::vector<char>{2},std::vector<int>{5},std::vector<double>{32423}));
TupleType t;
std::get<2>(t) = 342.2;
return 0;
}
MSVC has some conformance issues when running in /permissive mode and "The Microsoft C++ compiler doesn't currently support binding nondependent names when initially parsing a template. This doesn't conform to section 14.6.3 of the C++ 11 ISO specification. This can cause overloads declared after the template (but before the template is instantiated) to be seen.".
If you instead use /permissive- "The compiler [...] implements more of the requirements for two-phase name look-up" and will compile your program as-is.
Also note that you aren't actually using C++11. MSVC 19.24 supports C++14 and up.

Eigen 3.3.4 compilation error on MIPS Compiler (C++98)

I compile Eigen 3.3.4 under Multi-2000 (Green Hill) MIPS C++ compiler.
After I solved some align macro issue in Macros.h, I met a problem again for some template expression.
Find the Error Details as below. I know it should be some issue for my old C++ compiler, but anyone whom can propose an work around it will be greatly appreciated.
"C:\Eigen/Eigen/src/Core/util/Meta.h", line 389
: error:
expression must have pointer-to-class type
template <typename C> static meta_yes testFunctor(C const *,typename enable_if<(sizeof(return_ptr<C>()->operator()())>0)>::type * =
0);
^
"C:\Eigen/Eigen/src/Core/util/Meta.h", line 389
: error:
incomplete type is not allowed
template <typename C> static meta_yes testFunctor(C const *,typename enable_if<(sizeof(return_ptr<C>()->operator()())>0)>::type * =
0);
^
So The Problem must be inside expression as below. But I really can not figure it out.
template <typename C> static meta_yes testFunctor(C const *,typename enable_if<(sizeof(return_ptr<C>()->operator()())>0)>::type * = 0);
Update 8/25-> whether "typename enable_if" cause this?

Workaround for debug symbol error with auto member function?

There seems to be an issue with debug symbols and auto.
I have an auto function in a class:
#include <cstddef>
template <typename T>
struct binary_expr {
auto operator()(std::size_t i){
return 1;
}
};
int main(){
binary_expr<double> b;
return 0;
}
When I compile with G++ (4.8.2) and -g, I have this error:
g++ -g -std=c++1y auto.cpp
auto.cpp: In instantiation of ‘struct binary_expr<double>’:
auto.cpp:11:25: required from here
auto.cpp:4:8: internal compiler error: in gen_type_die_with_usage, at dwarf2out.c:19484
struct binary_expr {
^
Please submit a full bug report,
with preprocessed source if appropriate.
See <https://bugs.gentoo.org/> for instructions.
With clang++ (3.4) and -g, I have this:
clang++ -g -std=c++1y auto.cpp
error: debug information for auto is not yet supported
1 error generated.
If I remove the -g or set the type explicitly, it works perfectly.
Isn't clang++ supposed to be C++14 feature complete ?
Is there a workaround for these limitations or I'm screwed ?
This now seems to works on Clang 3.5 SVN. Live Example. It seems that the culprit was a commit from May 2013, see this message on the Clang mailing list.
PR16091: Error when attempting to emit debug info for undeduced auto
return types
Perhaps we should just suppress this, rather than erroring, but since
we have the infrastructure for it I figured I'd use it - if this is
determined to be not the right thing we should probably remove that
infrastructure entirely. I guess it's lying around from the early days
of implementing debug info support.
// RUN: %clang_cc1 -emit-llvm-only -std=c++1y -g %s 2>&1 | FileCheck %s
2
3 struct foo {
4 auto func(); // CHECK: error: debug information for auto is not yet supported
5 };
6
7 foo f;
However, I cannot find the commit that removed this info, perhaps there was an improvement that now prevents that this behavior is being triggered.
Even after some time, the only workaround I have found is to make the function template, pretty stupid workaround... Apparently, clang has no problem with auto functions that are template. I don't know if this works in every case, but until now it had worked for me.
#include <cstddef>
template <typename T>
struct binary_expr {
template<typename E = void>
auto operator()(std::size_t i){
return 1;
}
};
int main(){
binary_expr<double> b;
return 0;
}

C++11 Variadic Template Function -- where's the error?

The following code gives compilation errors on both MSVC++ 2012 CTP (with C+11 support) and Intel C++ XE 13.0:
template <typename F, typename... Args>
void apply(F f, std::tuple<Args...>& args) {
// doesn't do much yet
}
bool f1(char c) {
return c == 'c';
}
int main(int argc, char* argv[]) {
auto t = std::make_tuple('c');
apply(f1, t);
return 0;
}
VS2012 error is:
error C2243: 'type cast' :
conversion from 'std::tuple<char,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil> *'
to 'std::tuple<std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil> &' exists,
but is inaccessible
Error on Intel C++ XE 13.0 is:
error : no instance of function template "apply" matches the argument list
What am I missing? Are there really errors here, or do I just have two bad compilers?
UPDATE: Same (or similar) results when I use boost::tuple instead of std::tuple on both compilers.
ADDENDUM: Thanks for all the cross-checks in the comments. I've sent bug reports to both of these fine corporations.
That is likely a VC11 bug in the implementation of the Standard Library. Although the CTP supports variadic templates, to the best of my knowledge the Standard Library was not rewritten to use them and adopts some machinery to simulate variadic templates. This is likely at the origin of your problem.

Same code blocks about function template compiled ok under g++ but error under VC6, why?

I am reading the chapter "function templates" of C++ Primer 3rd edition, and when I tried to follow the example, I found the code almost the same as the book has encountered an error during compilation under VC6 but everything is ok under g++. I don't know why?
Here is the code:
#include <iostream>
using namespace std;
template<typename T1, typename T2, typename T3>
T1 my_min(T2 a, T3 b)
{
return a>b?b:a;
}
int main()
{
int (*fp)(int, int) = &my_min<int>;
cout<<fp(3,5)<<endl;
return 0;
}
The error happened under VC6 appears like:
error C2440: 'initializing' : cannot convert from '' to 'int (__cdecl *)(int,int)'
None of the functions with this name in scope match the target type
VC6 is an ancient compiler whose support for templates is woefully lacking, so it cannot cope with legal code in many circumstances. You should ditch it and download VS 2010 Express instead.