CODEWITHSUNDEEP
c++c++11templatestemplate-specialization
ampawd
ampawd

Reputation: 1010

Empty angle brackets on user defined types

There are only 2 use cases known to me for empty angle brackets <> in c++ templates, which are encountered:

1) During template full specialization e.g.

template<typename T>
class X
{};

template<>
class X<char>
{};

2) During the templated function call which argument type's can be deduced e.g.

template<typename T>
T add(T a, T b);

...  

add<>(1, 2);

But this case is completely new to me, and I can't understand the meaning of such syntax (please have a look at rapidxml::xml_node<>*):

class TestWidget : public GUI::Widget
{
public:
    TestWidget(const std::string& name, rapidxml::xml_node<>* elem);
    // ...
};

What semantics are given to the xml_node<> type here in the constructor parameter ???

Upvotes: 6

Views: 1024

Answers (2)

max66
max66

Reputation: 66220

Not this case (default template argument, as in the Jarod42's answer) but that syntax can also declare an object of a variadic template type with empty list of template arguments.

Or both.

#include <iostream>

// default value
template <typename = void>
struct foo
 { };

// variadic list
template <int ...>
struct bar
 { };

// both
template <int = 0, typename ...>
struct foobar
 { };


int main ()
 {    
   foo<>     f;
   bar<>     b;
   foobar<>  fb;
 }

The funny part it that you can't distinguish the cases from variable declarations.

Upvotes: 8

Jarod42
Jarod42

Reputation: 217663

There is also with default template argument:

template<typename T = char>
class X
{};

so

X<> x; // X<char>

Upvotes: 8

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