item-2-understand-auto-type-deduction

Item 2. Understand auto type deductions

Test Codes

$ # TODO: 

Notes

// When a variable is declared using auto, auto plays the role of T in the template, 
//   and the type specifier for the variable acts as ParamType.

// In this example, 
//   auto plays the role of T, 
//   const auto& acts as ParamType.              
const auto& x = 100;    

With only one curious exception, auto type deduction is template type deduction.

Same part with Item 1 (template type deduction)

• Case 1: The type specifier is a pointer or reference, but not a universal reference.
• Case 2: The type specifier is a universal reference.
• Case 3: The type specifier is neither a pointer nor a reference.
• Array Arguments
• Function Arguments

Different part

The only real difference between auto and template type deduction is that auto assumes that a braced initializer(i.e. { }) represents a std::initializer_list, but template type deduction doesn't.

Code	auto's Type	Comment
auto x = 100;	int
auto x(100);	int
auto x = {100};	std::intializer_list<int>	See comment-1
auto x {100};	std::intializer_list<int>	See comment-1
auto x = {1, 2, 3.0};	ERROR!	can't deduce T for std::initializer_list<T>

comment-1: be noted that some compilers may result the type by int instead of std::initializer_list<int> under N3922(only proposal, not part of C++11 or C++14 standard).

	ParamType	Template Function	expr	Deduced T	Deduced ParamType	Comment
T	T	template<typename T> void f(T)	f({1, 2});	ERROR!	ERROR!
T	std::intializer_list<T>	template<typename T> void f(std::intializer_list<T>)	f({1, 2});	int	std::intializer_list<int>	The std::initializer_list<T> should be ParamType if want to deduce template function by braced initializer(i.e. { })

When use auto as return type or lambda parameter type

• C++14 permits auto to indicate that a function's return type should be deduced, and C++14 lambdas may use auto in parameter declarations.
• However, these uses of auto employ template type deduction, not auto type deduction.

References

• (Chinese) auto vs. decltype