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C++0x Automates Type Deduction with auto-3 : Page 3


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Pointers and References
For simple type declarations, the use of auto is rather intuitive. However, what happens when the initializer is a reference?

int& myfunc(); int yourfunc(); auto j=myfunc(); //int& or int?

The authors decided to apply the same rules of template type deduction. The type of j is therefore int, not int&. To create an auto reference variable, you need to use this form:

auto& j=myfunc(); // int&

Similarly, to add cv-qualification to the variable, use this syntax:


const auto& ci=yourfunc(); //const int&

If the initializer is a pointer, the compiler will deduce the pointer's type and assign it to the auto variable. You can omit the * in an auto declaration if the initializer is a pointer:

int n; auto pi=&n;//pi is int* auto* pi2=&n; //also int* auto* p = new Myclass; //Myclass* auto p2 = new Myclass //Myclass *

Notice however that the compiler will deduce the type based on the entire initializer expression. In the following example, the type of q is Myclass, not Myclass*:

auto q= *new Myclass;

Similarly, auto can be used in a new expression:

new auto('s');

The type of auto('s') is char. Therefore, the type of the expression new auto('s') is char *. Here's another example:

new auto(1000LL); //new long long*. const auto p= new auto(1000LL); //p is const long long*

Multi-Variable Declarations
Originally, the auto proposal allowed only one variable to be declared in every auto statement. This restriction was later removed, so now you can declare multiple variables in a single auto statement:

auto a=6.8, *pa = &a; //a is double, pa is double*

According to the latest version of the auto proposal, the status of array declarations isn't clear. At the time of writing, the following isn't allowed:

auto arr[]={1,2,3,4};//error, arrays aren't allowed

Similarly, the status of this borderline case is also being debated:

int x[4]; auto y = x; // int*?

Ready to Use
The new syntax for automatic type deduction simplifies your code by eliminating unwieldy type names from declarations and expressions. Several issues with auto still need to be resolved, though: array declarations, typedefs, and pointers to functions. At present, some implementations already support auto, Comeau for instance, so you don't have to wait for the ratification of the C++0x standard to start using this feature.



Danny Kalev is a certified system analyst and software engineer specializing in C++. He was a member of the C++ standards committee between 1997 and 2000 and has since been involved informally in the C++0x standardization process. He is the author of "The ANSI/ISO Professional C++ Programmer's Handbook" and "The Informit C++ Reference Guide: Techniques, Insight, and Practical Advice on C++."
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