Code compiling on gcc, but not on msvc - c++

I have a problem compiling a template using msvc-2010. It works perfectly using gcc 4.6.3.
I have boiled down the code to the essential (it doesn't make sense of course):
//Variant that works
template <typename T, T* Ptr>
void callFun()
{
}
//Traits class (type expands to the same type T* as above)
template <typename T>
class TraitsClass
{
public:
typedef T* type;
};
//Essentially the same as callFun2, only that the
//type of Ptr is expressed indirectly over a traits class
//The usage of this class is not possible, because of the error described below
template <typename T, typename TraitsClass<T>::type Ptr>
void callFun2()
{
}
//Provides a compile constant ptr for this example
void testFun()
{
}
int main()
{
//Works
callFun<void(), &testFun>();
//Fails
callFun2<void(), &testFun>();
//Works
callFun2<void(), 0>();
return 0;
}
The Error:
error C2975: 'Ptr' : invalid template argument for 'callFun2', expected compile-time constant expression
I find it interesting, that it only fails when the second type parameter is being used through a typedef in a Traits class.
g++ compiles this example correctly without warnings, even when using -Wall -Wextra -Werror -pedantic (Except for the unused parameters, of course)
Thank you very much.

Well, I think that the answer is that compilers are not written by gods. Programming standards in the compiler industry are extremely high, MS C++ is a good compiler, but it still contain bugs. I came across the following, that is somehow similar to what you are pointing at:
template <class item_struct>
struct THeapBasedArray
{
void Sort(int (__cdecl *compareFunction)(const item_struct *item1,
const item_struct *item2));
};
struct Item { int x; };
struct ItemPtrsArray : public THeapBasedArray<Item*>
{
static int __cdecl Compare1(const Item **pp1, const Item **pp2);
typedef Item *ItemPtr;
static int __cdecl Compare2(const ItemPtr *pp1, const ItemPtr *pp2);
};
int main()
{
ItemPtrsArray vect;
vect.Sort(ItemPtrsArray::Compare1);
vect.Sort(ItemPtrsArray::Compare2);
}
The first call to Sort fails with:
cpptest1.cxx(21) : error C2664: 'THeapBasedArray::Sort' : cannot convert parameter 1 from 'int (_cdecl *)(const Item **, const Item **)' to 'int (_cdecl *)(const item_struct *, const item_struct *)
while the second call compilers fine. For me this is a bug in a compiler. Sometimes this happens. I guess this is the answer.

Related

Taking Eigen::Vector as a parameter

I am trying to write a function that takes an Eigen::Vector<T, dim> as a parameter. However, the following example fails to compile:
#include <Eigen/Core>
template<class F, typename T, int dim>
void bar(F&& func, const Eigen::Vector<T, dim>& arg1) {
}
template<typename T, int dim>
void foo(const Eigen::Vector<T, dim>& a) {
return bar([] {}, a);
}
int main() {
Eigen::Vector<float, 3> v1{ 1.f,2.f,3.f };
foo(v1);
return 0;
}
This, under Visual Studio 2019, gives me the following error:
1>main.cpp(9,10): error C2672: 'bar': no matching overloaded function found
1>main.cpp(14): message : see reference to function template instantiation 'void foo<float,3>(const Eigen::Matrix<float,3,1,0,3,1> &)' being compiled
1>main.cpp(9,1): error C2784: 'void bar(F &&,const Eigen::Matrix<T,dim,1,|_Rows==&&?:&&_Rows!=?:,_Rows,1> &)': could not deduce template argument for 'const Eigen::Matrix<T,dim,1,|_Rows==&&?:&&_Rows!=?:,_Rows,1> &' from 'const Eigen::Matrix<float,3,1,0,3,1>'
1>main.cpp(4): message : see declaration of 'bar'
My questions:
What is this weird |_Rows==&&?:&&_Rows!=?: in the error message?
What can I do to make the above code compile?
The bar function should have T and dim availabe. I cannot just take const AnyType& arg1, because the actual implementation of bar depends on compile-time known values T and dim.
I have seen https://eigen.tuxfamily.org/dox/TopicFunctionTakingEigenTypes.html. I think I understand what they are saying, but I am not sure if it applies here. I am taking an actual Eigen::Vector as an argument, not an expression.
If there was an expression it would be fine for me, to have it materialized.
Nevertheless, if I try to follow their instruction and just use ArrayBase<Derived>, I lose the compile-time information about T and dim.
This indeed looks like an MSVC issue, it compiles fine with gcc >= 4.7, and clang >= 3.5: https://godbolt.org/z/kqoHyO
One possible workaround would be to explicitly write out what Eigen::Vector expands to:
template<class F, typename T, int dim>
void bar(F&& func, const Eigen::Matrix<T, dim, 1, 0, dim, 1>& arg1) {
}
https://godbolt.org/z/vlvSDP
The weird |_Rows==&&?:&&_Rows!=?: looks like MSVC mangled the default value of the Options template parameter:
AutoAlign |
( (_Rows==1 && _Cols!=1) ? Eigen::RowMajor
: (_Cols==1 && _Rows!=1) ? Eigen::ColMajor
: EIGEN_DEFAULT_MATRIX_STORAGE_ORDER_OPTION ),
If you want to get to the bottom of this, you should file a bug-report to the MSVC maintainers, maybe using a simplified example like this: https://godbolt.org/z/U_0Sh7 (probably it's possible to reduce this even more).

How can I see the type deduced for a template type parameter?

Is there an easy way to force compilers to show me the type deduced for a template parameter? For example, given
template<typename T>
void f(T&& parameter);
const volatile int * const pInt = nullptr;
f(pInt);
I might want to see what type is deduced for T in the call to f. (I think it's const volatile int *&, but I'm not sure.) Or given
template<typename T>
void f(T parameter);
int numbers[] = { 5, 4, 3, 2, 1 };
f(numbers);
I might want to find out if my guess that T is deduced to be int* in the call to f is correct.
If there's a third-party library solution (e.g., from Boost), I'd be interested to know about it, but I'd also like to know if there's an easy way to force a compilation diagnostic that would include the deduced type.
Link time solution:
On my platform (OS X), I can get the linker to give me this information by simply making a short program that is complete, minus the definition of the function I'm curious about:
template<typename T>
void f(T&& parameter); // purposefully not defined
int
main()
{
const volatile int * const pInt = nullptr;
f(pInt);
}
Undefined symbols for architecture x86_64:
"void f<int const volatile* const&>(int const volatile* const&&&)", referenced from:
_main in test-9ncEvm.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Admittedly I get the "triple reference", which should be interpreted as an lvalue reference (due to reference collapsing), and is a demangling bug (perhaps I can get that fixed).
Run time solution:
I keep a type_name<T>() function handy for this type of thing. A completely portable one is possible, but sub-optimal for me. Here it is:
#include <type_traits>
#include <typeinfo>
#include <string>
template <typename T>
std::string
type_name()
{
typedef typename std::remove_reference<T>::type TR;
std::string r = typeid(TR).name();
if (std::is_const<TR>::value)
r += " const";
if (std::is_volatile<TR>::value)
r += " volatile";
if (std::is_lvalue_reference<T>::value)
r += "&";
else if (std::is_rvalue_reference<T>::value)
r += "&&";
return r;
}
I can use it like:
#include <iostream>
template<typename T>
void f(T&& parameter)
{
std::cout << type_name<T>() << '\n';
}
int
main()
{
const volatile int * const pInt = nullptr;
f(pInt);
}
which for me prints out:
PVKi const&
That's not terribly friendly output. Your experience may be better. My platform ABI is based on the Itanium ABI. And this ABI includes this function:
namespace abi
{
extern "C"
char*
__cxa_demangle(const char* mangled_name, char* buf, size_t* n, int* status);
}
I can use this to demangle C++ symbols into a human readable form. An updated type_name<T>() to take advantage of this is:
#include <type_traits>
#include <typeinfo>
#include <string>
#include <memory>
#include <cstdlib>
#include <cxxabi.h>
template <typename T>
std::string
type_name()
{
typedef typename std::remove_reference<T>::type TR;
std::unique_ptr<char, void(*)(void*)> own
(
abi::__cxa_demangle(typeid(TR).name(), nullptr, nullptr, nullptr),
std::free
);
std::string r = own != nullptr ? own.get() : typeid(TR).name();
if (std::is_const<TR>::value)
r += " const";
if (std::is_volatile<TR>::value)
r += " volatile";
if (std::is_lvalue_reference<T>::value)
r += "&";
else if (std::is_rvalue_reference<T>::value)
r += "&&";
return r;
}
And now the previous main() prints out:
int const volatile* const&
I have tried the followings with g++ 4.7.2 and clang++ 3.4 (trunk 184647); they both give
a compile-time error and the error message is containing the deduced type.
I have no access to MSVC 12, please check what happens and provide feedback.
#include <string>
template <typename T>
struct deduced_type;
template<typename T>
void f(T&& ) {
deduced_type<T>::show;
}
int main() {
f(std::string()); // rvalue string
std::string lvalue;
f(lvalue);
const volatile int * const pInt = nullptr;
f(pInt);
}
The error messages: g++ 4.7.2
error: incomplete type deduced_type<std::basic_string<char> > used in nested name specifier
error: incomplete type deduced_type<std::basic_string<char>&> used in nested name specifier
error: incomplete type deduced_type<const volatile int* const&> used in nested name specifier
and clang++
error: implicit instantiation of undefined template deduced_type<std::basic_string<char> >
error: implicit instantiation of undefined template deduced_type<std::basic_string<char> &>
error: implicit instantiation of undefined template deduced_type<const volatile int *const &>
The note / info messages also contain the type of f with both compilers, for example
In instantiation of void f(T&&) [with T = std::basic_string<char>]
It's butt-ugly but works.
To get the compiler to show you the type of a variable (perhaps in a round about way);
T parameter;
....
void f(int x);
...
f(parameter);
compiler should complain that "T" cannot be converted to int, assuming that it actually can't.
A slightly more succinct way to trigger the compiler diagnostic from Ali's answer is with a deleted function.
template <typename T>
void f(T&&) = delete;
int main() {
const volatile int * const pInt = nullptr;
f(pInt);
return 0;
}
With GCC 8.1.0, you get:
error: use of deleted function 'void f(T&&) [with T = const volatile int* const&]

C++ GCC Why this sfinae code can be compiled with GCC 4.7, but not with 4.8?

I like to use local classes in template classes to perform constructions like "static if". But I've faced with the problem that gcc 4.8 does not want to compile my code. However 4.7 does.
This sample:
#include <type_traits>
#include <iostream>
#include <string>
using namespace std;
struct A {
void printA() {
cout << "I am A" << endl;
}
};
struct B {
void printB() {
cout << "I am B" << endl;
}
};
template <typename T>
struct Test {
void print() {
struct IfA {
constexpr IfA(T &value) : value(value) {
}
T &value;
void print() {
value.printA();
}
};
struct IfB {
constexpr IfB(T &value) : value(value) {
}
T &value;
void print() {
value.printB();
}
};
struct Else {
constexpr Else(...) {}
void print() {
}
};
typename conditional<is_same<T, A>::value, IfA, Else>::type(value).print();
typename conditional<is_same<T, B>::value, IfB, Else>::type(value).print();
}
T value;
};
int main() {
Test<A>().print();
Test<B>().print();
}
Options:
g++ --std=c++11 main.cc -o local-sfinae
Task:
Given classes A and B with different interfaces for printing.
Write a generic class Test that can print both A and B.
Do not pollute either any namespace or class scope.
Description of the code:
This is only a clean example.
I use an approach like this, because I want to generalize the construction "static if". See, that I pass the arguments to IfA and IfB classes via their fields, not directly to the print() function.
I use such constructions a lot.
I've found that these constructions should not be in (pollute) class scope. I mean they should be placed in a method scope.
So the question.
This code can not be compiled with GCC 4.8. Because it checks ALL classes, even if they are never used. But it has not instantiate them in binary (I've commented the lines that cause errors and compiled it with gcc 4.8). Proof:
$ nm local-sfinae |c++filt |grep "::If.*print"
0000000000400724 W Test<A>::print()::IfA::print()
00000000004007fe W Test<B>::print()::IfB::print()
See, there is no Test::print()::IfB::print(). (See later: 'void Test::print()::IfB::print() [with T = A]')
The errors if I compile aforementioned code with gcc 4.8:
g++ --std=c++11 main.cc -o local-sfinae
main.cc: In instantiation of 'void Test<T>::print()::IfB::print() [with T = A]':
main.cc:36:9: required from 'void Test<T>::print() [with T = A]'
main.cc:49:21: required from here
main.cc:34:17: error: 'struct A' has no member named 'printB'
value.printB();
^
main.cc: In instantiation of 'void Test<T>::print()::IfA::print() [with T = B]':
main.cc:28:9: required from 'void Test<T>::print() [with T = B]'
main.cc:50:21: required from here
main.cc:26:17: error: 'struct B' has no member named 'printA'
value.printA();
^
Is it a GCC 4.8 bug?
Or is it GCC 4.7 bug? Maybe the code should not be compiled.
Or it is a my bug, and I should not rely on the compiler behavior/should not use such approach to implement "static if".
Additional info:
This simple code compiles on 4.7, but not on 4.8. I shortened it.
struct A {
void exist() {
}
};
template <typename T>
struct Test {
void print() {
struct LocalClass {
constexpr LocalClass(T &value) : value(value) {
}
T &value;
void print() {
value.notExist();
}
};
}
T value;
};
int main() {
Test<A>().print();
}
Errors:
main.cc: In instantiation of 'void Test<T>::print()::LocalClass::print() [with T = A]':
main.cc:16:9: required from 'void Test<T>::print() [with T = A]'
main.cc:22:21: required from here
main.cc:14:17: error: 'struct A' has no member named 'notExist'
value.notExist();
^
Have tested two GCC 4.8 versions: 2012.10 and 2013.02. Hope it is GCC 4.8 bug and it can be fixed.
LocalClass is not a template. The "not instantiated if not used" rule is only applicable to member functions of class templates.
That is, when Test::print() is instantiated, everything that is inside is brought to life, including the unused member of its local class.
There is no SFINAE in your code.
SFINAE applies during template argument deduction and argument substitution (the 'S' in SFINAE stands for substitution) but the only substitution in your program happens when substituting A for T in the template parameter list of Test, which doesn't fail.
You then call print() which instantiates Test<A>::print(), which doesn't involve any substitution, and you get an error because value.notExist(); is not valid.
SFINAE has to be used in substitution contexts, such as template argument deduction caused by a function call or when deducing template parameters with default arguments.

Functor to variadic template function

I want to make functor to generic function, but I get compiler error.
Here is the code:
template <class T>
struct Creator
{
template <typename...Ts>
static std::shared_ptr<T> create(Ts&&... vs)
{
std::shared_ptr<T> t(new T(std::forward<Ts>(vs)...));
return t;
}
};
class Car:
public Creator<Car>
{
private:
friend class Creator<Car>;
Car()
{
}
};
int main()
{
auto car=Car::create();
std::function< std::shared_ptr<Car> () > createFn=&Car::create;
return 0;
}
I get the following error in GCC 4.6.3 on the second statement(the first is OK):
error: conversion from β€˜<unresolved overloaded function type>’
to non-scalar type β€˜std::function<std::shared_ptr<Car>()>’ requested
Any hint appreciated.
If the pointer of a template function is needed, the template must be instantiated first.
std::function<std::shared_ptr<Car>()> createFn = &Car::create<>;
This will make it compile on clang++ 3.1, but g++ 4.8 still refuses to compile, which I believe is a bug.
You could provide a lambda function instead:
std::function<std::shared_ptr<Car>()> createFn = []{ return Car::create(); };

Compilation error with inheritance of templates accessed through pointer of pointers

Template compilation error using pointer of pointer: (What causes the code not to compile ??)
template <int DIM> class Interface { };
template <int DIM> class Implementation : public Interface<DIM> { };
template <int DIM> class Modifier {
public: void modify(const Interface<DIM>** elem) { } // only one * compiles !!
};
template <int DIM> class Caller {
Modifier<DIM> modifier;
Implementation<DIM>** impl; // only one * compiles !!
public: void call() { modifier.modify(impl); }
};
void main() {
Caller<-13> caller;
caller.call(); // also compiles with this line commented
}
Gives this compilation error (on Visual Studio 1988 professional):
imlgeometry.cpp(-10) : error C2664: 'Modifier<DIM>::modify' : cannot convert parameter 1 from 'Implementation<DIM> **' to 'const Interface<DIM> **'
with
[
DIM=-23
]
Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
imlgeometry.cpp(-16) : while compiling class template member function 'void Caller<DIM>::call(void)'
with
[
DIM=-29
]
imlgeometry.cpp(-17) : see reference to class template instantiation 'Caller<DIM>' being compiled
with
[
DIM=-34
]
The problem is that in C++ it is not legal (or safe) to convert a T** to a const T**. The reason is that if you could do this, you would end up being able to subvert const. For example:
const T value;
T* mutablePtr;
const T** doublePtr = &mutablePtr; // Illegal, you'll see why.
*doublePtr = &value; // Legal, both sides have type const int*.
// However, mutablePtr now points at value!
*mutablePtr = 0; // Just indirectly modified a const value!
To fix this, you'll need to update your code so that you aren't trying to do this conversion. For example, you might change the parameter type of modify to be
const Interface<DIM> * const *
Since it is legal to convert T** to const T* const*.
Hope this helps!