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"ANSI C++ forbids floating-point template arguments" ???

Posted on 2003-11-04
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Last Modified: 2008-01-09
Hi,

I have a template that is called:
template <class T, T Min, T Max> class Test

I call this template with the following type defs:
typedef Test<unsigned char> Test_byte;
typedef Test<int> Test_int;
typedef Test<float> Test_float;

The compiler does not allow the third typedef: typedef Test<float> Test_float;
It says: "ANSI C++ forbids floating-point template arguments"

I don't understand why this is forbidden! Is there any way around it?
What would be the best solution when you use a template, but you want to use specific values for every instanciated class?

Who can help me out here?

Thanks.

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Question by:Klaren
6 Comments
 
LVL 8

Expert Comment

by:Exceter
ID: 9679451
>> I don't understand why this is forbidden! Is there any way around it?

I don't think it is. After all, you can say,

std::vector <float> data; // Works just fine.

>>template <class T, T Min, T Max> class Test
...
>> typedef Test<float> Test_float;

Why are you only using one of the three template arguments? Have you tried,

typedef Test <float, 45.0, 45.0> Test_float;

or,

template <class T> class Test
...
typedef Test<float> Test_float;

Exceter
0
 
LVL 4

Accepted Solution

by:
PhilipNicoletti earned 50 total points
ID: 9682528
The error message is correct.

Section 14.1 paragraph 7 of the ISO C++ Standard :

7 A non-type template-parameter shall not be declared to have floating point, class, or void type. [Example:

template<double d> class X; // error
template<double* pd> class Y; // OK
template<double& rd> class Z; // OK

--end Example]

Based on this, you can probably do this (but I don't know how useful it will be) :

#include <iostream>

using namespace std;

extern const float my_min1 = 2.3f;
extern const float my_max1 = 10.0f;

extern const float my_min2 = -1.0f;
extern const float my_max2 =  1.0f;


template <typename T, const T* MIN, const T* MAX>
class A
{
public:

    A() : min_allowed(*MIN) , max_allowed(*MAX){}
    T value;
    T min_allowed;
    T max_allowed;
};

int main()
{
    A<float,&my_min1,&my_max1> a1;
    A<float,&my_min2,&my_max2> a2;

    cout << a1.min_allowed << " " << a1.max_allowed << endl;
    cout << a2.min_allowed << " " << a2.max_allowed << endl;

    return 0;
}
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LVL 8

Expert Comment

by:Exceter
ID: 9683931
Weird... Then why does this work?

std::vector <float> data;
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LVL 31

Expert Comment

by:Zoppo
ID: 9685132
>Then why does this work
Because the template paramete is the Type 'double', not a double value.

The problem (as PhilipNicoletti statet) in
template <class T, T Min, T Max> class Test
doesn't come from the 'class T' but from 'T Min' and 'T Max'

ZOPPO
0
 

Author Comment

by:Klaren
ID: 9685794
Thank you all for your input.
However, I still don't understand why it isn't allowed to declare a template-parameter to have floating-point type. I understand it is a ISO C++ Standard rule, but WHY?

Doesn't anybody else run into this problem? And although I'm grateful for the solution of PhilipNicoletti, I wonder if there isn't an easier way to introduce some floating point parameters to a template. What do you guys do if you want to use specific values for every instanciated class of  a template?

Thanks!
Klaren
0
 
LVL 8

Expert Comment

by:Exceter
ID: 9739328
>> What do you guys do if you want to use specific values for every instanciated class of  a template?

Depending on what you are trying to do you can pass the values through the constructor, use a macro, a global variable, or a local variable.
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