c++ - 看到 std::vector<Typo> 和 std::vector<struct Typo> 时发出的编译器诊断之间存在差异的原因是什么

Question

考虑这两个程序及其尝试编译。

#include <vector>

int main() {
    std::vector<struct Typo> a; // Errors centered around `Typo` being an incomplete type
}

例如，

gcc 6.3 给出了以 Typo 为不完整类型为中心的错误消息，包括

/usr/include/c++/6/bits/stl_vector.h:161:9: error: invalid use of incomplete type ‘struct main()::Typo’

#include <vector>

int main() {
    std::vector<Typo> b; // Errors centred around `Typo` not being defined
}

例如，

gcc 6.3 给出了以 Typo 未定义为中心的错误，包括

prog.cpp:4:14: error: ‘Typo’ was not declared in this scope

当然，两个 vector 实例都是错误的，但程序注释中引用的不同诊断消息的基本原理是什么？

Answer 1

struct Typo是一个详细的类型说明符。这会影响名称查找的工作方式。

[basic.lookup.elab]

2: If the elaborated-type-specifier has no nested-name-specifier, and unless the elaborated-type-specifier appears in a declaration with the following form:

class-key attribute-specifier-seq identifier ;

the identifier is looked up according to [basic.lookup.unqual] but ignoring any non-type names that have been declared. If the elaborated-type-specifier is introduced by the enum keyword and this lookup does not find a previously declared type-name, the elaborated-type-specifier is ill-formed. If the elaborated-type-specifier is introduced by the class-key and this lookup does not find a previously declared type-name, or if the elaborated-type-specifier appears in a declaration with the form:

class-key attribute-specifier-seq identifier ;

the elaborated-type-specifier is a declaration that introduces the class-name as described in [basic.scope.pdecl].

所以在 std::vector<struct Typo> a; , 因为 struct Typo未找到先前声明的 Typo ，它作为类型的前向声明。因此，可能¹提示它得到的类型不完整的 vector 实例化，因为它会尝试用它做一些事情。

在 std::vector<Typo> b; 期间, 查找 Typo一无所获。它没有先前的声明，因此此时应该发出一个诊断，表明此标识符未声明。

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<子> ¹ - 取决于您正在编译的标准，以及您使用的 vector 的哪些成员。添加了对不完整类型的支持。