I've just found out about a strange syntax for default template parameters
template<class T = class Z>
struct X
{};
What does the second "class" keyword mean in this context?
It's nothing special really. C++ allows you to refer to a class via an elaborated type specifier . Eg
void foo(class bar*);
This declares a function foo
that accepts an argument of the type bar*
. If bar
was not declared previously, this elaborate type specifier constitutes a declaration of bar
in the namespace containing foo
. Ie as if you had written:
class bar;
void foo(bar*);
Back to your example, X
is a class template that expects a single type parameter, denoted by class T
, but could have been denoted just the same as typename T
. Said type parameter has a default argument, named by the elaborated class specifier class Z
. That declaration can be rewritten just like the function above:
class Z;
template<class T = Z>
struct X
{};
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