Solved

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

Posted on 2003-11-04
6
617 Views
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.

0
Comment
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;
}
0
 
LVL 8

Expert Comment

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

std::vector <float> data;
0
Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

 
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.
0

Featured Post

Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Go is an acronym of golang, is a programming language developed Google in 2007. Go is a new language that is mostly in the C family, with significant input from Pascal/Modula/Oberon family. Hence Go arisen as low-level language with fast compilation…
Container Orchestration platforms empower organizations to scale their apps at an exceptional rate. This is the reason numerous innovation-driven companies are moving apps to an appropriated datacenter wide platform that empowers them to scale at a …
The goal of the tutorial is to teach the user how to use functions in C++. The video will cover how to define functions, how to call functions and how to create functions prototypes. Microsoft Visual C++ 2010 Express will be used as a text editor an…
The viewer will learn additional member functions of the vector class. Specifically, the capacity and swap member functions will be introduced.

943 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

17 Experts available now in Live!

Get 1:1 Help Now