I have a gut-feeling VS2012 is wrong on this one, but I'm not certain.
After looking this question , I felt like trying to implement something similar.
My version works fine on Visual Studio 2012, but does not even compile on Ideone .
Here is my main interface:
#include <iostream>
#include <string>
template <class In, class Out>
struct Pipe
{
typedef In in_type ;
typedef Out out_type ;
In in_val ;
Pipe (const in_type &in_val = in_type()) : in_val (in_val)
{
}
virtual auto operator () () const -> out_type
{
return out_type () ;
}
};
template <class In, class Out, class Out2>
auto operator>> (const Pipe <In, Out> &lhs, Pipe <Out, Out2> &rhs) -> Pipe <Out, Out2>&
{
rhs = lhs () ;
return rhs ;
}
template <class In, class Out>
auto operator>> (const Pipe <In, Out> &lhs, Out &rhs) -> Out&
{
rhs = lhs () ;
return rhs ;
}
Here are a few test classes:
struct StringToInt : public Pipe <std::string, int>
{
StringToInt (const std::string &s = "") : Pipe <in_type, out_type> (s)
{
}
auto operator () () const -> out_type
{
return std::stoi (in_val) ;
}
};
struct IntSquare : public Pipe <int, int>
{
IntSquare (int n = 0) : Pipe <in_type, out_type> (n)
{
}
auto operator () () const -> out_type
{
return in_val * in_val ;
}
};
struct DivideBy42F : public Pipe <int, float>
{
DivideBy42F (int n = 0) : Pipe <in_type, out_type> (n)
{
}
auto operator () () const -> out_type
{
return static_cast <float> (in_val) / 42.0f ;
}
};
And here's the driver:
int main ()
{
float out = 0 ;
StringToInt ("42") >> IntSquare () >> DivideBy42F () >> out ;
std::cout << out << "\n" ;
return 0 ;
}
Ideone is complaining about template deductions and its unable to find the correct operator>>
candidate function:
prog.cpp: In function ‘int main()’:
prog.cpp:75:21: error: no match for ‘operator>>’ (operand types are ‘StringToInt’ and ‘IntSquare’)
StringToInt ("42") >> IntSquare () >> DivideBy42F () >> out ;
^
prog.cpp:75:21: note: candidates are:
prog.cpp:23:6: note: Pipe<Out, Out2>& operator>>(const Pipe<In, Out>&, Pipe<Out, Out2>&) [with In = std::basic_string<char>; Out = int; Out2 = int]
auto operator>> (const Pipe <In, Out> &lhs, Pipe <Out, Out2> &rhs) -> Pipe <Out, Out2>&
^
prog.cpp:23:6: note: no known conversion for argument 2 from ‘IntSquare’ to ‘Pipe<int, int>&’
prog.cpp:30:6: note: template<class In, class Out> Out& operator>>(const Pipe<In, Out>&, Out&)
auto operator>> (const Pipe <In, Out> &lhs, Out &rhs) -> Out&
^
prog.cpp:30:6: note: template argument deduction/substitution failed:
prog.cpp:75:35: note: deduced conflicting types for parameter ‘Out’ (‘int’ and ‘IntSquare’)
StringToInt ("42") >> IntSquare () >> DivideBy42F () >> out ;
Which compiler is correct? If Ideone is correct, is there any easy fix to this code?
The first template basically fails because you can't bind a prvalue temporary - IntSquare ()
- to a non-const lvalue reference.
no known conversion for argument 2 from ‘IntSquare’ to ‘Pipe<int, int>&’
This says that you can't initialize Pipe<int, int>&
with a prvalue of type IntSquare
. The value category is unfortunately not explicitly mentioned in the error message. Although this is a standard rule VC++ ignores it to ease (or troublesome) a C++-programmers daily life.
The second template
template <class In, class Out>
auto operator>> (const Pipe <In, Out> &lhs, Out &rhs) -> Out&
{
rhs = lhs () ;
return rhs ;
}
fails because for two different deductions of Out
two different types were deduced - the first being int
(for lhs
) and the second being IntSquare
.
Ideone (actually, GCC) is correct here. In Visual Studio, it compiles because of an infamous extension which allows temporaries to bind to non-const lvalue references (the standard forbids that).
I see several possible ways to solve this in standard C++:
One, don't use temporaries for the pipeline stages:
int main ()
{
float out = 0 ;
StringToInt stage1("42");
IntSquare stage2;
DivideBy24F stage3;
stage1 >> stage2 >> stage3 >> out ;
std::cout << out << "\n" ;
return 0 ;
}
Two, create a "stay" function (opposite of std::move
), and use that:
template <class T>
T& stay(T &&x) { return x; }
int main ()
{
float out = 0 ;
stay(StringToInt ("42")) >> stay(IntSquare ()) >> stay(DivideBy42F ()) >> out ;
std::cout << out << "\n" ;
return 0 ;
}
Three, provide an overload of operator >>
taking an r-value reference:
template <class In, class Out, class Out2>
auto operator>> (const Pipe <In, Out> &&lhs, Pipe <Out, Out2> &&rhs) -> Pipe <Out, Out2>&
{
return lhs >> rhs; // Notice that lhs and rhs are lvalues!
}
Of course, ideally you'd provide mixed &, &&
and &&, &
overloads as well.
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