Use of the C + + Tutorial Auto keyword

One autoKeywords in previous lives

Starting with the C language, the auto keyword is treated as a storage type modifier for a variable that represents an automatic variable (local variable). It cannot be used alone, or the compiler will give a warning.

 #include <stdio.h>int main () {int a = 123 Span class= "K" >auto int b = 234 Span class= "K" >auto c = 345printf (\n" Span class= "P", abc ); return 0;}        

Compile run Result:

$ gcc main.cmain.c:7:7: warning: type specifier missing, defaults to ‘int‘ [-Wimplicit-int] auto c = 345; ~~~~ ^1 warning generated.$ ./a.out a = 123, b = 234, c = 345

Two autoKey words of this life

In the C + + 11 standard, a new type derivation attribute has been added, which, given the auto very few uses of the keyword, gives it the ability to re-assert the variable that the type was deduced by the compiler. In C + + 11, a auto variable with a definition cannot be decorated with another type modifier, and the type of the variable is determined automatically by the compiler based on the initialization data. autoA variable of type must be initialized.

 #include <iostream>int main () {int a = 21 Span class= "K" >auto b = a//typeid can get the name of the variable or data type std::cout << typeid (b).  Name () << std::endl return 0;}        

1. When compiling using the C + + 98 standard, a warning appears:

$ clang++ main.cpp -std=c++98main.cpp:6:2: warning: ‘auto‘ type specifier is a C++11 extension      [-Wc++11-extensions] auto b = a; ^1 warning generated.$ ./a.out i #输出结果为整数类型

Use C + + 11 for compilation:

$ clang++ main.cpp -std=c++11$ ./a.out i

2. However, if it is still auto used as a storage type modifier, a warning will appear under the C + + 11 standard:

 #include <iostream>int main () {int a = 21 Span class= "K" >auto int b = a Span class= "C1" >//typeid can get the name of the variable or data type std::cout << typeid (b).  Name () << std::endl return 0;}        

Compiling with C + + 98:

$ clang++ main.cpp -std=c++98$ ./a.out i

Instead of using C + + 11 compilation, a warning appears and is no longer allowed as a storage type modifier:

$ clang++ main.cpp -std=c++11main.cpp:6:2: warning: ‘auto‘ storage class specifier is not permitted in C++11,      and will not be supported in future releases [-Wauto-storage-class] auto int b; ^~~~~1 warning generated.

3. A variable of auto type must be initialized, and the definition variable cannot be used alone in C + + 98 auto .

$ clang++ MAIN.CPP-STD=c++98main.cpp:6:2:warning: ' auto ' type specifier is a c++11 extension [-wc++11-extensions ' B ' with type  ' auto ' requires an initializer auto B ^1 warning and 1 error generated. $ clang++ main.cpp main.cpp:6:2:warning:  ' auto ' type specifier is a c++11 extension [-wc++11-extensions "auto B ^main.cpp:6:7:error: Declaration of variable  ' B ' with type  ' auto ' requires an initializer Auto B ^1 warning and 1 error generated.      

Third, expansion

It can also be used in C + + decltype to get the type of a variable or expression and to define a new variable.

 #include <iostream>int main () {int a = 21decltype (a) bstd::cout << typeid  (b Name () << std::endl return 0;}        

Compile run Result:

$ clang++ main.cpp -std=c++98$ ./a.out i$ clang++ main.cpp -std=c++11$ ./a.out i

It is important to note that the standard only changes the use of Auto in C + +, but it does not affect auto in the C language!

This document is organized by the Changsha Camp David Education.

Use of the C + + Tutorial Auto keyword