What is the type of an 'auto' return type when returning *this in an anonymous class?

What is type of auto in here ?

The type is decltype(m_some_class) - I.e., the return value is of the same type as the variable m_some_class.

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Note that the function will return a copy of *this.

If a reference to *this is wanted instead, you can use auto& or, since C++14, the more generic decltype(auto).

For anonymous structure types, internally the compiler creates a name and the auto in your case return the structure.

You can see below, that your anonymous structure is given name __anon_1_1 and the operator[] function returns object of __anon_1_1 structure. m_some_class is instance of type __anon_1_1

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your code

struct
{
    auto operator[](const char*)
    {
        return *this;
    }

}m_some_class;

compiler version

struct __anon_1_1
{
  inline __anon_1_1 operator[](const char *)
  {
    return __anon_1_1(*this);
  }
  
  // inline constexpr __anon_1_1() noexcept = default;
  // inline constexpr __anon_1_1(const __anon_1_1 &) noexcept = default;
};

__anon_1_1 m_some_class = __anon_1_1();

The line in the given code:

return *this;

returns the struct m_some_class itself, i.e. the type of the operator[] is:

decltype(m_some_class); // i.e. the type returned is the same as the struct

Also, notice that this will only return a copy instance of the struct since the passed argument isn't given any reference-to operator. Any changes made to the copy of the struct won't affect the original struct.

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What's the auto keyword?

The auto keyword is typically used in those situations when the type of something is not known to the programmer or it's too lengthy to type either.

Also, the type defined by auto may vary dependent upon the various situations. For instance:

auto len = vector.size(); // len is now defined as size_t in compile time

In some systems, the type of len maybe unsigned long and in my case, it's unsigned long long, here you can't explicitly define which qualifier to use correctly in this indeterminate place. Here we use auto keyword.