Deducing Types

• 变化

C++98只支持函数模板(function template)的类型推导
C++11增加了auto 和decltypes
C++14在C++11的基础上扩展了auto和decltype的适用范围

• 好处

在一处改变原类型，可自动推导出其它地方的类型

• 特点

大部分类型自动推导发生在函数模板调用上

Item1: Understand template type deduction

• Bad news

When the template type deduction rules are applied in the context of auto, they sometimes seem less intuitive than when they’re applied to templates.

• 函数模板类型参数推导三种可能

  • 参数是指针或者引用
  • 参数是universal reference
  • 参数不是指针也不是引用

• Universal reference type

  • T&&
  • type deduction distinguishes between lvalue and rvalue

• ParamType is Neither a Pointer nor a Reference

  • That means that param will be a copy–a new object
  • That’s why expr’s constness (and volatileness, if any) is gnored

• Array Arguments

  • array-to-pointer rule

 void myFunc(int param[])
 void myFunc(int* param) // the same as above
 f(name);

• const char (&)[13]

• T (&)[N]4

• Function Arguments

  • fucntion-to-pointer decay

Item2: Understand auto type deduction

• deducing types for auto is, with only one exception, the same as deducing types for templates

auto x1 = 27;
auto x2(27);
auto x3 = {27};
auto x4{ 27 4};

• The treatment of braced initializers is the only way in which auto type deduction and template type deduction differ

Item3: Understand decltype4

• decltype almost always yields the type of a variable or expression without any modifications

Item4：Know how to view deduced types

Things to remember:

• Deduced types can often be seen using IDE editors, compiler error messages, and the Boost TypeIndex library
• The results of some tools may be neither helpful nor accurate, so an undersatnding of C++'s type deduction rules remains essential.

标签：27,Effective,auto,Modern,C++,deduction,template,type
来源： https://blog.csdn.net/tanshiqian/article/details/112915461