为什么decltype看不到成员声明？

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尝试编译该简单类：

#include <vector> struct M { // interface auto begin() -> decltype(identities.begin()) { return identities.begin(); } // implementation private: std::vector<int> identities; };

导致错误：

$ g++-510 where.cpp -std=c++11 where.cpp:57:35: error: ‘struct M’ has no member named ‘identities’ auto begin() ->decltype(this->identities.begin()) ^ where.cpp:57:35: error: ‘struct M’ has no member named ‘identities’ $ clang++ where.cpp -std=c++11 -Wall -pedantic -Wextra where.cpp:57:35: error: no member named 'identities' in 'M' auto begin() ->decltype(this->identities.begin()) ~~~~ ^

为什么不 decltype 看到类成员？

推荐答案

来自N3337 [basic.lookup.unqual] / 7：

From N3337 [basic.lookup.unqual]/7:

在X类外部定义中使用的名称成员函数体或嵌套类定义的声明必须以下列方式之一声明：

A name used in the definition of a class X outside of a member function body or nested class definition shall be declared in one of the following ways:

• ss X或是X的基类的成员，或者...

因为尾随返回类型为是 declaration 函数的一部分，而不是 definition 的一部分，它无法向前查看该类中还声明了什么，因此您需要在该函数上方声明该成员声明。

Because the trailing return type is part of the function declaration rather than the definition, it can't look ahead to see what else is declared in the class, so you need to declare that member above the function declaration.