subclassing std::valarray

[Jan Callewaert]

Hi,

since I want to specify an extra function for a std::valarray<float>, I want
to subclass it:

class FVector : public std::valarray<float> {
public:
FVector() : std::valarray<float>() {}
FVector(size_t size) : std::valarray<float>(size) {}
FVector(const FVector & other) : std::valarray<float>(other) {}
FVector(const float& f, size_t size) : std::valarray<float>(f, size
{}
FVector(const float * f, size_t size) : std::valarray<float>(f,
size) {}

float getDistance(const FVector & other) const;
};

I used to use a typedef for my FVector, but now I need the getDistance
method.

If I compile this however with gcc-3.4.4, I get the following error:

g++ -O2 -g -fno-inline -Wall -pipe -fPIC -Ibuild -Isrc -Isrc -c -o
build/Matrix.os src/Matrix.cpp
g++ -O2 -g -fno-inline -Wall -pipe -fPIC -Ibuild -Isrc -Isrc -c -o
build/NeuralNetwork.os src/NeuralNetwork.cpp
src/NeuralNetwork.cpp: In member function `FVector
NeuralNetwork::evalNetwork(const FVector&) const':
src/NeuralNetwork.cpp:168: error: conversion from
`std::_Expr<std::_BinClos<std::__plus, std::_ValArray, std::_ValArray,
float, float>, float>' to non-scalar type `FVector' requested
src/NeuralNetwork.cpp:170: error: conversion from
`std::_Expr<std::_ValFunClos<std::_ValArray, float>, float>' to non-scalar
type `FVector' requested
src/NeuralNetwork.cpp:172: error: conversion from
`std::_Expr<std::_BinClos<std::__plus, std::_ValArray, std::_ValArray,
float, float>, float>' to non-scalar type `FVector' requested
src/NeuralNetwork.cpp:174: error: conversion from
`std::_Expr<std::_ValFunClos<std::_ValArray, float>, float>' to non-scalar
type `FVector' requested
scons: *** [build/NeuralNetwork.os] Error 1
scons: building terminated because of errors.

As you see it complains on the operators:

168 FVector hidInput = getInp2Hid() * input +
getHiddenBiasWeights();
169 // Calculate the output of the hidden layer.
170 FVector hidOutput = hidInput.apply(intervalMapFunction);
171 // Calculate the input in the output layer.
172 FVector outpInput = getHid2Outp() * hidOutput +
getOutputBiasWeights();
173 // Calculate the output of the output layer.
174 FVector outpOutput = outpInput.apply(intervalMapFunction);
Do the operators not get inherited properly and should I redefine them all,
just like with the constructors (*sigh*)?

Regards,

Jan

[Victor Bazarov]

Jan Callewaert wrote:

since I want to specify an extra function for a std::valarray<float>, I want
to subclass it:

class FVector : public std::valarray<float> {
public:
float getDistance(const FVector & other) const;
};

I used to use a typedef for my FVector, but now I need the getDistance
method.
Actually, it might be better to have 'getDistance' a non-member function
that takes two arguments... It has no business of being a member, it
does not change its object.

If I compile this however with gcc-3.4.4, I get the following error:

[...]
src/NeuralNetwork.cpp:172: error: conversion from
`std::_Expr<std::_BinClos<std::__plus, std::_ValArray, std::_ValArray,
float, float>, float>' to non-scalar type `FVector' requested
[..]
As you see it complains on the operators:
[..]
172 FVector outpInput = getHid2Outp() * hidOutput +
getOutputBiasWeights();
[...]
Do the operators not get inherited properly and should I redefine them all,
just like with the constructors (*sigh*)?

No, but they return 'valarray', not FVector. What you need is
a constructor for FVector _from_ a std::valarray<float>:

FVector(std::valarray<float> const& a) : std::valarray<float>(a) {}

but you definitely have no justification to create 'FVector' in the first
place. A simple function

float getDistance(std::valarray<float> const& a1,
std::valarray<float> const& a2)
{
// whatever
}

should suffice.

V

[Dietmar Kuehl]

Jan Callewaert wrote:

since I want to specify an extra function for a std::valarray<float>,
Then specify an extra function! This is what non-member functions are
for.
I want to subclass it:

This is a bad idea! First of all, it is not necessary and second,
'std::valarray' is clearly not intended to be derived from as is
indicated by several aspects:
- It has no virtual functions.
- 'std::valarray' is a value type, not a reference type.

Subclassing is only appropriate in relatively few situations. Using
non-member functions or non-member function templates is the right
way to add functionality to a class. In fact, I would claim that most
standard C++ library classes are currently already unreasonably bloated
with function which had better been non-member functions.
--
<mailto:di***********@yahoo.com> <http://www.dietmar-kuehl.de/>
<http://www.eai-systems.com> - Efficient Artificial Intelligence