“const Allocator& alloc = Allocator()”是什么意思？

Question

I refer to c++ std::string constructor on both cplusplus.com and cppreference.com .

And I find the syntax on cppreference.com is more complex to me. On the contrary, cplusplus.com is clearer and easier to understand.

For example:

// cplusplus.com
string (const string& str, size_t pos, size_t len = npos);

// cppreference.com
basic_string( const basic_string& other,
              size_type pos,
              size_type count = std::basic_string::npos,
              const Allocator& alloc = Allocator() );

I wonder why " const Allocator& alloc = Allocator() " appears here and why it is needed (I am not sure if it is omitted by default while constructing).

It would be more helpful if anyone can show more differences between the two references in details. Thank you!

Answer 1

What is the meaning of “const Allocator& alloc = Allocator()”?

In this context, this is syntax for a function parameter. It allows arguments to be passed into functions.

I wonder why "const Allocator& alloc = Allocator()" appears here

It appears so that an allocator could be passed as an argument.

and why it is needed

Because The container supports custom allocators.