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Michael
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 Posted: Sat Apr 17, 2004 6:26 am Post subject: Compilation problem for friend binary operator in template |
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Hi all
Any idea whats wrong here?
// forward declarations for non-member friend
template <typename T>
class Vec;
template <typename T>
std::valarray<T>& operator - ( const Vec<T>& v1, const Vec<T>& v2 );
// class itself
template <typename T>
class Vec {
public:
friend std::valarray<T>& operator - <>( const Vec& v1, const Vec& v2 );
};
// declaration
template <typename T>
std::valarray<T>& operator - ( const Vec<T>& v1, const Vec<T>& v2 )
{
return v1.vref() - v2.vref();
}
....to give some context, v1.vref() and v2.vref() return references to
valarray<float>s within Vec<float>s, and valarray<> has the binary '-'
operator defined in the standard library.
Compiles OK on it's own but when I try to use it like this:
Vec<float> a(5), b(5), c(5);
a.vref() = b - c;
I get the following...
-----------------------------------------------------
/Users/mwh/ACS/Coding/HR C++ code/cpp/hr_vec.h: In function
`std::valarray<_Tp>& HR::operator-(const HR::Vec<T>&, const HR::Vec<T>&)
[with T = float]':
/Users/mwh/ACS/Coding/HR C++ code/cpp/hr_vec.h:250: error: could not convert
`std::operator-(const std::valarray<_Tp>&, const std::valarray<_Tp>&) [with
_Tp = float]((+(+v2)->HR::Vec<T>::vref() const [with T = float]()))' to
`std::valarray<float>&'
..-----------------------------------------------------
....but if I just use raw valarray<float>'s instead it compiles & runs fine.
GCC 3.3.
TIA
Michael
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sashan
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| Posted: Sun Apr 18, 2004 12:40 am Post subject: Re: Compilation problem for friend binary operator in templa |
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Michael wrote:
| Quote: | Hi all
Any idea whats wrong here?
// forward declarations for non-member friend
template <typename T
class Vec;
template
std::valarray
// class itself
template <typename T
class Vec {
public:
friend std::valarray
};
// declaration
template <typename T
std::valarray
{
return v1.vref() - v2.vref();
}
...to give some context, v1.vref() and v2.vref() return references to
valarray<float>s within Vec<float>s, and valarray<> has the binary '-'
operator defined in the standard library.
Compiles OK on it's own but when I try to use it like this:
Vec<float> a(5), b(5), c(5);
a.vref() = b - c;
I get the following...
-----------------------------------------------------
/Users/mwh/ACS/Coding/HR C++ code/cpp/hr_vec.h: In function
`std::valarray<_Tp>& HR::operator-(const HR::Vec<T>&, const HR::Vec<T>&)
[with T = float]':
/Users/mwh/ACS/Coding/HR C++ code/cpp/hr_vec.h:250: error: could not convert
`std::operator-(const std::valarray<_Tp>&, const std::valarray<_Tp>&) [with
_Tp = float]((+(+v2)->HR::Vec<T>::vref() const [with T = float]()))' to
`std::valarray<float>&'
.-----------------------------------------------------
...but if I just use raw valarray<float>'s instead it compiles & runs fine.
GCC 3.3.
|
The error message could be better but it's complaining that it can't
convert from a const reference to a reference. Your friend function
should be returning a const reference like this:
friend const std::valarray<T>& operator - (const Vec<T>& v1, const
Vec<T>& v2 );
You probably have to add a const member function vref to your Vec class.
--
sashan
http://www.cs.auckland.ac.nz/~sgov008
Public Key: search www.keyserver.net for key id 0x63B4EBF5
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llewelly
Guest
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| Posted: Sun Apr 18, 2004 12:41 am Post subject: Re: Compilation problem for friend binary operator in templa |
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Michael <mwh_acs (AT) hotmail (DOT) com> writes:
| Quote: | Hi all
Any idea whats wrong here?
|
I think you snipped too much out of your code fragments.
In general, the fragment you post should be the smallest example
which can reproduce the problem. Your example can't reproduce
the problem you report.
| Quote: |
// forward declarations for non-member friend
template <typename T
class Vec;
template
std::valarray
// class itself
template <typename T
class Vec {
public:
friend std::valarray
};
// declaration
template <typename T
std::valarray
{
return v1.vref() - v2.vref();
|
Here you are returning a reference to a temporary. Returning a
reference to a temporary and then using said reference has
undefined behavior. (gcc 3.3.3 warned me about this.)
| Quote: | }
...to give some context, v1.vref() and v2.vref() return references to
valarray<float>s within Vec<float>s, and valarray<> has the binary '-'
operator defined in the standard library.
Compiles OK on it's own but when I try to use it like this:
Vec<float> a(5), b(5), c(5);
a.vref() = b - c;
|
What does vref() do? What does it return?
| Quote: |
I get the following...
-----------------------------------------------------
/Users/mwh/ACS/Coding/HR C++ code/cpp/hr_vec.h: In function
`std::valarray<_Tp>& HR::operator-(const HR::Vec<T>&, const HR::Vec<T>&)
[with T = float]':
/Users/mwh/ACS/Coding/HR C++ code/cpp/hr_vec.h:250: error: could not convert
`std::operator-(const std::valarray<_Tp>&, const std::valarray<_Tp>&) [with
_Tp = float]((+(+v2)->HR::Vec<T>::vref() const [with T = float]()))' to
`std::valarray<float>&'
.-----------------------------------------------------
|
I tried a few variations, but I could not reproduce either of these
error messages on gcc 3.3.3
Eventually I ended up with this:
#include<valarray>
// forward declarations for non-member friend
template <typename T>
class Vec;
template <typename T>
std::valarray<T> operator - ( const Vec<T>& v1, const Vec<T>& v2 );
// class itself
template <typename T>
class Vec {
public:
Vec(int i):a(i){}
friend std::valarray<T> operator - <>( const Vec& v1, const Vec& v2 );
std::valarray<T> a;
std::valarray<T> & vref(){return a;}
std::valarray<T> const& vref()const{return a;}
};
// declaration
template <typename T>
std::valarray<T> operator - ( const Vec<T>& v1, const Vec<T>& v2 )
{
return v1.vref() - v2.vref();
}
int main()
{
Vec<float> a(5), b(5), c(5);
a.vref() = b - c;
}
Is it anything like what you wanted?
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Dhruv Matani
Guest
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| Posted: Sun Apr 18, 2004 10:01 am Post subject: Re: Compilation problem for friend binary operator in templa |
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On Sat, 17 Apr 2004 02:26:20 -0400, Michael wrote:
| Quote: | ...but if I just use raw valarray<float>'s instead it compiles & runs fine.
GCC 3.3.
|
Cutting a long story short, change the function operator-() to return a
valarray<> by value instead of reference. Ideally, you should be returning
the class for which you are defining the operator-(), but then again, it
is totally your decision as to what behaviour you desire.
For more information on why to return by value, reffer Scott Meyer's book:
Effective C++ and More Effective C++. Your question is answered in the
first one.
--
Regards,
-Dhruv.
Proud to be a Vegetarian.
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