Linker error when using qMin(), qMax(), qBound()

[zavulon]

Trying to use static const members in functions qMin(), qMax(), qBound() gives a linker error:
undefined reference to `<the static constant>'.

Here is an example code:

class Test
{
public:
static const float KValue=3.0;
Test();
};
 
Test::Test()
{
// Can be any function, not only a constructor
float foo = 1.0;
foo = qMax( foo, KValue );
}
 
int main()
{
// somewhere in a program
Test obj;
}

class Test
{
public:
static const float KValue=3.0;
Test();
};

Test::Test()
{
// Can be any function, not only a constructor
float foo = 1.0;
foo = qMax( foo, KValue );
}

int main()
{
// somewhere in a program
Test obj;
}

To copy to clipboard, switch view to plain text mode 

On the other hand, when using usual local constants, qMax (and friends) works ok:

float foo = 1.0;
const float value = KValue; // Make a copy of the static constant
 
foo = qMax( foo, value );

float foo = 1.0;
const float value = KValue; // Make a copy of the static constant

foo = qMax( foo, value );

To copy to clipboard, switch view to plain text mode 

Please write, if someone knowswhy this happens or any good workarounds.

Sytem info:
Kubuntu 8.04
Qt 4.4.0 and 4.4.1
gcc 4.2.4

[jpn]

You forgot something:

class Test
{
public:
    static const float KValue;
    Test();
};
 
const float Test::KValue=3.0; //<---

class Test
{
public:
    static const float KValue;
    Test();
};

const float Test::KValue=3.0; //<---

To copy to clipboard, switch view to plain text mode 

[zavulon]

Year, right, thanks.
Static const float members are not allowed to be initialized within the class declaration.
But why did then the second variant work?

[caduel]

The problem was not with qMax() etc but with your constant.
If you declare a class member (variable) as static, then you have to define it (outside of the class body). Otherwise the linker will complain.

In your second case, you did not define a static class member, so the linker was happy.

[zavulon]

Nope, in my second example I keep using the same static const float. It's initialized inside the class and used very well inside the later code, but not in function qMax. I am already beginning to suspect that the error occurs because qMax() takes its arguments by reference (not value). Something wrong, when passing address of a static constant to a function.

Here is the same example 2, but at its full glory. It works, no error here:

class Test
{
public:
static const float KValue=3.0;
Test();
};
 
Test::Test()
{
// Can be any function, not only a constructor
float foo = 1.0;
const float value = KValue; // Make a copy of the static constant
foo = qMax( foo, value );
}
 
int main()
{
// somewhere in a program
Test obj;
}

class Test
{
public:
static const float KValue=3.0;
Test();
};

Test::Test()
{
// Can be any function, not only a constructor
float foo = 1.0;
const float value = KValue; // Make a copy of the static constant
foo = qMax( foo, value );
}

int main()
{
// somewhere in a program
Test obj;
}

To copy to clipboard, switch view to plain text mode 

I've also noticed the following facts:
* Changing from float to int doesn't change the situation a bit.
* Using "normal" functions that take arguments by value (not by reference) doesn't lead to the error. Such functions can easily take static constants as arguments.

[caduel]

Probably the compiler optimizes your use of that "static const" away. (As it is static const, it is guaranteed to never change and can be replaced by its compile time value).
(Try without optimizations...)