c++ - Default type parameter error in template code -

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1)template <class t = int, class u = double> //compiles  2)template <class t, class u =double> //compiles  3)template <class t = int, class u> //fails

why 1 , 2 compile whereas 3 not?

for same reason why:

void f(int = 0, int);

fails.

there no way use 3rd version default parameter:

template<class t = int, class u> class b { ... };  b<, short> var; // ??? no such syntax

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