decltype，result_of或typeof？ [英] decltype, result_of, or typeof?

问题描述

我有：

  class A {
 public：
 B toCPD 
  

和：

  template< typename T> 
 class Ev {
 public：
 typedef result_of（T :: toCPD（））D; 
  

实例化 Ev< A> 后，编译器说：

meta.h：12：错误：'T :: toCPD'不是类型

解决方案

由于你获得的结果取决于模板参数，

$ typedef typename 是必需的。

decltype 是标准的C ++ 11功能。它是一个接受表达式并返回类型的操作符。

  typedef typename decltype（T ）D; //不能使用T ::作为非静态
  

如果 T ）不是有效的（ T 不是默认可构造的），您将需要 declval 这是一个接受一个类型并返回该类型的无意义无效值的函数。 declval 只能用于未评估的上下文，例如 decltype 。

  typedef typename decltype（std :: declval< T>（）。toCPD（））D; 
  

在C ++ 11之前， decltype Microsoft的MSVC编译器的非标准扩展。

---

typeof 是GCC等效的前C ++ 11扩展，如 decltype ，它也被克隆在其他编译器中。 此处是其来自GCC的文档。该页面不提供功能之间的比较，但它指出，当使用标准模式时， typeof 必须调用 __ typeof __ -std = c ++ YY ，你应该总是），它可以在C和C ++。

为了C兼容性， __ typeof __ 不会从glvalue表达式解析引用类型。所以，它真的只适用于C.这可能解释了为什么C ++功能没有继承更自我解释的名称：GNU不愿意牺牲向后兼容性，而Microsoft不太在乎C，也许需要更少的更改。

---

result_of 是一个C ++ 11元函数TR1库从2006）。它是一个模板，它接受一个可调用的类型（如函数 int（void），函数指针 int（*）（void），函子类实现 operator（）或指向成员函数& T :: toCPD



要使用 result_of

code>带有指向成员函数的指针，你必须在参数列表中包含父对象类型作为 this 的替代。

  typedef typename std :: result_of< decltype（& T :: toCPD）（T *）> :: type D; 
  

这很脆弱，因为& T :: toCPD 无法解决，如果有任何重载，如非const版本。这是真的，尽管事实上必须明确写出 T * 或 T const * ！在大多数情况下，你最好使用 decltype 和 declval 。

I have:

class A {
public:
    B           toCPD() const;

And:

template<typename T>
class Ev {
public:
    typedef result_of(T::toCPD()) D;

After instantiating Ev<A>, the compiler says:

meta.h:12: error: 'T::toCPD' is not a type

neither decltype nor typeof work either.

解决方案

Since whatever result you obtain depends on the template parameter, typedef typename is necessary.

decltype is a standard C++11 feature. It is an "operator" which takes an expression and returns a type.

typedef typename decltype( T().toCPD() ) D; // can't use T:: as it's nonstatic

If T() isn't a valid (T not default-constructible) you will want declval which is a function that takes a type and returns a meaningless, invalid value of that type. declval can only be used in unevaluated contexts such as decltype.

typedef typename decltype( std::declval<T>().toCPD() ) D;

Before C++11, decltype was a non-standard extension by Microsoft's MSVC compiler. Its behavior might have been changed slightly by standardization.

---

typeof is GCC's equivalent pre-C++11 extension like decltype, which was also cloned in other compilers. Here is its documentation from GCC. That page provides no comparison between the features, but it notes that typeof must be called __typeof__ when using a standard mode (-std=c++YY, which you should always do), and it is available in C as well as C++.

For the sake of C compatibility, __typeof__ will not resolve a reference type from a glvalue expression. So, it's really only suitable for C. This probably explains why the C++ feature didn't inherit the more self-explanatory name: GNU was unwilling to sacrifice backward compatibility, whereas Microsoft cares less about C and perhaps needed fewer changes.

---

result_of is a C++11 metafunction (previously standardized in the ISO TR1 library from 2006). It is a template which takes a callable type (such as a function int(void), function pointer int(*)(void), functor class implementing operator(), or pointer-to-member-function &T::toCPD) and an argument type-list for that type, and provides the return type if the call would work.

To use result_of with a pointer to member function, you must include the parent object type in the argument list as a surrogate for this.

typedef typename std::result_of< decltype( & T::toCPD ) ( T * ) >::type D;

This is very brittle, though, because &T::toCPD cannot be resolved if there's any overloading, such as a non-const version. This is true despite the fact that T * or T const * must be explicitly written out! In most cases, you're better off with decltype and declval.

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