[Effective modern C + +] Item 3. Understand Decltype-Learn about Decltype

about DecltypeBasic Knowledge

A variable or expression is provided, and Decltype returns its type, but the content returned is surprising.

Here are some simple inference types:

Const inti =0;//Decltype (i), const intBOOLFConstwidget& W);//Decltype (w), const widget&, Decltype (f), BOOL (const widget&)structPoint {intx, y;} //Decltype (point::x), intWidget W;//Decltype (w) Widgetif(f (w));//Decltype (f (w) ), BOOLTemplate<typename t>classVector { Public: T&operator[] (std::size_t index);}; Vector<int> v;//Decltype (v), vector<int>if(v[0] ==0);//Decltype (v[0]), int&

In c++11, the primary function of Decltype is to infer the return type inferred from the formal parameter type.

For std::vector<bool>,operator[] The return is not bool&, but a new object.

An inferred use of a return type:

Template<typename Container, TypeName index>Auto Authandaccess (Container& C, Index i)  Decltype (C[i]) {    authenticateuser ();     return c[i];}

C++11 allows inference for single-statement lambda return types, c+14 extends to all lambda and functions.

// c++14 version, but there will be a problem template<typename Container, typename index>Auto Authandaccess (Container & C, Index i) {    authenticateuser ();     return c[i];}

Although the above function uses auto, the rules that apply auto are problematic. As above C[i] returns INT&, but the reference type is removed according to the inference rule, causing no modification.

The following code can return the type correctly:

// c++14 version, the type Template<typename Container can be returned correctly , TypeName index> decltype(auto) authandaccess ( Container& C, Index i) {    authenticateuser ();     return c[i];}

The difference between auto and Decltype (auto):

Widget W; Const widget& CW =//  myWidget1 Widget//  MyWidget2, const widget& 

In order for the function to pass both Lvalue and Rvalue, the function introduces a generic reference, the correct form is as follows:

// final c++14 versionTemplate<typename Container, TypeName index>decltype (auto) authandaccess ( Container&& C, Index i) {    authenticateuser ();     return std::forward<container>(c) [i];} // final c++1 versionTemplate<typename Container, typename index>Auto Authandaccess (Container && C, Index i)decltype (std::forward<container>(c) [i]) {    authenticateuser () ;     return std::forward<container>(c) [i];}

When using Decltype, the parentheses outside the variable change the inferred type:

int 0  ///  int/ /- int&

Summary

• Decltype always produces types of variables or expressions that have not been modified
• For an lvalue expression of type T and not a variable name, Decltype always returns the t& type
• C++14 supports Decltype (auto), which infers types from initialization like auto, but uses Decltype to infer types

[Effective modern C + +] Item 3. Understand Decltype-Learn about Decltype