C++ typedef typename classname::template

Question

I am not able to parse the meaning of the following line of code:

typedef typename Allocator::template rebind<Mapped>::other mapped_type_allocator;

This is the code for allocator rebinding (line 63 of https://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-api-4.5/a00756_source.html)

How is this different from the following?

typedef typename Allocator::rebind<Mapped>::other mapped_type_allocator;

Answer 1

typedef typename Allocator::template rebind<Mapped>::other mapped_type_allocator;

This is a templated typedef - it establishes mapped_type_allocator as an alias for a template.

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typedef typename Allocator::rebind<Mapped>::other mapped_type_allocator;

This is a typedef for a type. To compile OK, the Mapped would need to be defined/known.

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The Allocator::rebind<typename X>::other (as a concept) is expected to define a template, not a type.