c++ - Default template parameters: Why does the compiler complain about not specifying template argument? -

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i have code:

struct a{};  template<class t = a> struct b {     void foo() {} };  b b; //error: missing template arguments before 'b'      //error: expected ';' before 'b'      //more errors b.foo()

if make foo() template function same template 'signature', compiler doesn't complain not specifying template arguments:

struct {};  struct b {     template<class t = a>     void foo() {} };  b b; //ok b.foo()

so why need specify argument template class default parameter, not template function? there subtlety missing?

the reason because of template argument deduction failure sure. want know why.

the correct syntax (demo):

b<> b;

the default argument a assumed class template b. <> part tells compiler b class template , asks take default parameter template argument it.


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