C++98 에서 타입추론은 template 에만 있다. C++ 11 에서는 2개를 추가한다: auto 와 decltype 이다. C++14 에서는 auto와 decltype 을 쓸수있는 상황을 확대한다.

타입추론은 여러번의 반복되는 type 선언을 줄여준다. 코드 한곳에서 타입을 바꿀시 여러곳에 영향을 줄수있고 타입추론은 이걸 편하게 해준다.

Item 1: Understand template type deduction

The type deduced for T is dependent not just on the type of expr, but also on th e form of paramType. There are cases:

• ParamType is a pointer or reference type, but not a universal reference
• ParamType is a universal reference
• ParamType is neither a pointer nor a reference

Case 1: Param Type is a Reference or Pointer, but not a Universal Reference

template<typename T>
void f(T& param);

int x = 27
const int cx = x;
const int &rx = x;

f(x); // T is int, param's type is int &
f(cx); // T is const int, param's type is const int &
f(rx); // param's type is const int &

constness of the constness of the object becomes part of the type deduced for T. rx’s referenceness is ignored during type deduction.

If param were a pointer (or a pointer to const) instead of a reference, things would essentially the same way

Case 2: ParamType is a Universal Reference

Universal reference (T&&) behaves liked (T&), but with additional capability of binding to rvalues (temporary objects).