Default template parameters: Why does the compiler complain about not specifying template argument?
c++, default-parameters, template-argument-deduction, templates
Solution
The correct syntax is this (demo):
B<> b;
The default argument `A` is assumed for the class template `B`. The `<>` part tells the compiler that `B` is a class template and asks it to take the default parameter as the template argument to it.
Problem
I have this 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 I make `foo()` as a template function with the same template 'signature', the compiler doesn't complain about not specifying the template arguments: ``` struct A {}; struct B { template<class T = A> void foo() {} }; B b; //OK b.foo() ``` So why do I need to specify an argument for a template class with a default parameter, but not for a template function? Is there some subtlety I am missing? The reason is because of template argument deduction failure for sure. But I want to know why.