C++: Default template argument only deduced when using "<>"

Question

I created this struct for some easy templated Math definitions:

template<class T = float> struct Math{
    static constexpr T PI = T(3.14159265359);
    static constexpr T E = T(2.718281828459);
    static constexpr T INF = std::numeric_limits<T>::infinity();
};

I would like to use it like this:

float pi = Math::PI;

Even if the default argument T is float Ill get the error:

'template<class T> struct Math' used without template parameters

If I use Math<>::PI it works. Is this a compiler bug or are the <> brackets mandatory?

why don't you just use `double` for all of them? – M.M Apr 25 '15 at 12:52 — M.M, Apr 25 '15 at 12:52

score 1 · Accepted Answer · edited May 23 '17 at 11:51

1

Yes the <> brackets are mandatory (see here).

But here are some other options:

use a typedef

typedef Math<> MyDefaultMath
// or
typedef Math<float> MyFloatMath

or just drop the template

struct Math
{
    static constexpr float PI = 3.14159265359f;
    // ...

edited May 23 '17 at 11:51

Community

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answered Apr 25 '15 at 12:46

Axalo

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score 1 · Answer 2 · answered Apr 25 '15 at 12:49

1

Is this a compiler bug or are the <> brackets mandatory?

No, it's not a bug they are mandatory.

However, you may want to give this a little more thought. What if someone uses Math<int>? For example the documentation of std::numeric_limits::infinity states the following:

Only meaningful if std::numeric_limits::has_infinity == true

answered Apr 25 '15 at 12:49

James Adkison

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Thanks, this is ofcourse true. I would have used the template mainly for `float` and `double`. – tly Apr 25 '15 at 13:28

score 1 · Answer 3 · answered Apr 25 '15 at 13:16

The empty brackets are mandatory and required by the standard. From [temp.arg], emphasis mine:

When template argument packs or default template-arguments are used, a template-argument list can be empty. In that case the empty <> brackets shall still be used as the template-argument-list. [ Example:
template<class T = char> class String;
String<>* p; // OK: String<char>
String* q; // syntax error

template<class ... Elements> class Tuple;
Tuple<>* t; // OK: Elements is empty
Tuple* u; // syntax error
—end example ]

C++: Default template argument only deduced when using "<>"

3 Answers3