QGenericMatrix with custom Template Type

[Qtonimo]

Hi,
I want to know if it is possible to create a QGenericMatrix of my own Object Type.

Like this:

QGenericMatrix<5,5,MyClass> matrix;

QGenericMatrix<5,5,MyClass> matrix;

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thanks in advance

[amleto]

isn't that the whole meaning of 'generic'?

Isn't it faster to try than to ask here?

[Qtonimo]

I tried it but the compiler doesn't like it....

class MyClass
{
private:
    QString value;
 
public:
    MyClass();
    MyClass(QString value);
 
    inline QString getValue() const { return value; }
 
    bool operator== ( const MyClass & other ) const;
};

class MyClass
{
private:
    QString value;

public:
    MyClass();
    MyClass(QString value);

    inline QString getValue() const { return value; }

    bool operator== ( const MyClass & other ) const;
};

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#include "myclass.h"
 
MyClass::MyClass()
{
 
}
 
MyClass::MyClass(QString value) :
    value(value)
{
}
 
bool MyClass::operator ==(const MyClass & other) const
{
    return (value == other.getValue());
}

#include "myclass.h"

MyClass::MyClass()
{

}

MyClass::MyClass(QString value) :
    value(value)
{
}

bool MyClass::operator ==(const MyClass & other) const
{
    return (value == other.getValue());
}

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int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
 
    QGenericMatrix<4,4,MyClass>m;
    return a.exec();
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QGenericMatrix<4,4,MyClass>m;
    return a.exec();
}

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[spirit]

You should implement assignment operator for your class.

[amleto]

I would imagine assignment is implicit.

More likely there are problems with +, *, / etc.

If op would show the compiler error then we would know for sure...

[spirit]

Nope, the problem is in QGenericMatrix::setToIdentity uses assignment operator which looks like ... operator = (qreal other).
But I agree with you that other common operators have to be reloaded too.

[amleto]

he just needs another ctor then for that issue. The assignment will be implicit

class MyClass
{
private:
    QString value;
 
public:
    MyClass();
    MyClass(float f); // <<<<<<<
    MyClass(QString value);
 
    inline QString getValue() const { return value; }
 
    bool operator== ( const MyClass & other ) const;
};

class MyClass
{
private:
    QString value;
     
public:
    MyClass();
    MyClass(float f); // <<<<<<<
    MyClass(QString value);
     
    inline QString getValue() const { return value; }
     
    bool operator== ( const MyClass & other ) const;
};

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[spirit]

Well, yes it's another way to solve the issue, but in this case a new temporary object will be created. In the case of using assignment operation that will not happen.
Implicit anything it's evil.

[Qtonimo]

I tried it with MyClass(float f) and it works...thanks

@spirit maybe you can show me the assignment operator in this example....

is this correct?

MyClass & MyClass::operator= ( const MyClass & other )
{
    this->value = other.getValue();
    return *this;
}

MyClass & MyClass::operator= ( const MyClass & other )
{
    this->value = other.getValue();
    return *this;
}

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[yeye_olive]

I tried it with MyClass(float f) and it works...thanks

@spirit maybe you can show me the assignment operator in this example....

is this correct?

MyClass & MyClass::operator= ( const MyClass & other )
{
    this->value = other.getValue();
    return *this;
}

MyClass & MyClass::operator= ( const MyClass & other )
{
    this->value = other.getValue();
    return *this;
}

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This code is correct but not very useful: it is probably equivalent to what the compiler generates automatically.

What spirit and amleto pointed out is this:
MyClass needs a way to be assigned a qreal. One way to do that is to add a constructor MyClass(qreal), which you did. However this may be suboptimal, as assigning a qreal to a MyClass first instanciates a temporary MyClass with MyClass(qreal), then assigns it with operator(const MyClass &), then destroys the temporary. You could add an assignment operator operator=(qreal) to avoid this unnecessary construction and destruction. However since MyClass is essentially a wrapper around QString which itself uses implicit sharing, I am not convinced it will make a huge difference.

Also, it is possible that some other operators (such as binary arithmetic one) are missing from MyClass which are needed for QGenericMatrix to function properly. Errors will probably show up as soon as you try arithmetic operations on your matrices.

[amleto]

Well, yes it's another way to solve the issue, but in this case a new temporary object will be created. In the case of using assignment operation that will not happen.
Implicit anything it's evil.

only in debug build and/or poor compiler implementations, and definitely not in c++11 with move semantics

[spirit]

Here we go:

...
MyClass &operator=(qreal other)
{
        value = QString("%1").arg(other);
        return *this;
}
...

...
MyClass &operator=(qreal other)
{
        value = QString("%1").arg(other);
        return *this;
}
...

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[yeye_olive]

only in debug build and/or poor compiler implementations, and definitely not in c++11 with move semantics

Good point.

Here we go:

...
MyClass &operator=(qreal other)
{
        value = QString("%1").arg(other);
        return *this;
}
...

...
MyClass &operator=(qreal other)
{
        value = QString("%1").arg(other);
        return *this;
}
...

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An alternative solution would be value.setNum(other).

[spirit]

only in debug build and/or poor compiler implementations, and definitely not in c++11 with move semantics

686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00) -- still shows this.

[amleto]

hmmm, ok, I may have 'jumped the gun' with that comment, suggesting rvo when it does not apply.

[Qtonimo]

Thank you guys

You brought light into the darkness.

[Qtonimo]

What I have to change, if I want to store pointers in my QGenericMatrix?

QGenericMatrix<4,4,MyClass*> m;

QGenericMatrix<4,4,MyClass*> m;

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The compiler cannot convert float to a pointer in assignment at setToIdentity()... The message is clear, but I'm not able to work arround this.

[spirit]

Well, it's gonna be challenging...
So, I think, you have chosen wrong type of container -- What are you using it for?

[amleto]

That wont work. You will need to create a wrapper.

If you want QGenericMatrix<4,4,T>, then it must support

T t;
t = 1.0f

since T is MyClass*, there is no opportunity to overload operators.

Therefore you are left with using a thin wrapper or keeping with value semantics.

template<class T>
class Wrapper
{
public:
  Wrapper(T* ptr) : ptr_(ptr) {}
 
  Wrapper& operator=(float f)
  {
    (*ptr) = f; // operator= (or suitable ctor) must be supported by T!
  }
 
private:
T* ptr_;
};

template<class T>
class Wrapper
{
public:
  Wrapper(T* ptr) : ptr_(ptr) {}

  Wrapper& operator=(float f)
  {
    (*ptr) = f; // operator= (or suitable ctor) must be supported by T!
  }

private:
T* ptr_;
};

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[Qtonimo]

Pointers where my intention, because I receive lots of pointers (MyClass* pointer1, MyClass* pointer2, etc.) and want to insert them in the matrix.
If I could store pointers I would save insertion time (matrix(0,0) = pointer1...).

So I have to create every incoming myclass object on the stack and delete the pointer object on the heap : (