Differences between auto and decltype inference methods (learning notes)

auto and decltype There are three main differences :

First of all ：auto The type specifier uses the compiler to calculate the initial value of a variable to infer its type , and decltype Although it also allows the compiler to analyze the expression and get its type , But it doesn't actually evaluate the value of the expression .

second ： The compiler infers auto The type is sometimes not exactly the same as the type of the initial value , The compiler will change the result type appropriately to make it more consistent with the initialization rules . for example ,auto The top floor is usually ignored const, And put the bottom const Keep it . By contrast ,decltype The top level of the variable is retained const.

Third ： And auto Different ,decltype The result type of is closely related to the expression form , If you add a pair of parentheses to the variable name , The type obtained will be different from that without parentheses . If decltype It's a variable without parentheses , The result is the type of the variable ; If you add one or more parentheses to a variable , Then the compiler will infer the reference type .

Code parsing ：

#include<iostream>
#include<typeinfo>
using namespace std;

int main(){
	int a = 3;
	auto c1 = a;
	decltype(a) c2 = a;
	decltype((a)) c3 = a;
	
	const int d = 5;
	auto f1 = d;
	decltype(d) f2 = d;
	
	cou<<typeid(c1).name()<<endl;
	cou<<typeid(c2).name()<<endl;
	cou<<typeid(c3).name()<<endl;
	cou<<typeid(f1).name()<<endl;
	cou<<typeid(f2).name()<<endl;
	
	c1++;
	c2++;
	c3++;
	c3++;
	f1++;   //auto Ignore the top level const, So no mistake  
	f2++;   // Report errors ：f2 Is an integer constant , You can't add  
	cout<<a<<" "<<c1<<" "<<c2<<" "<<c3<<" "<<f1<<" "<<f2<<endl;
	return 0;
}

         For the first set of type inference ,a Is a non constant integer ,c1 The inference result is an integer ,c2 The inference result of is also an integer ,c3 The inference result is due to the variable a An extra pair of parentheses is added, so it is an integer reference .c1、c2、c3 Perform the auto increment operation in turn , because c3 It's a variable. a Another name for , therefore c3 Self increase is equivalent to a Self increasing , Final a、c1、c2、c3 The value of is changed to 4.

         For the second set of type inference ,d Is a constant integer , Containing top layer const, Use auto The inferred type automatically ignores the top level const, therefore f1 The inference result is an integer ;decltype Keep the top layer const, therefore f2 The inference result is an integer constant .f1 The auto increment operation can be performed normally , And constant f2 The value of cannot be changed , So it can't increase itself .