c++ - 用 g++ 编译的奇怪代码

Question

以下代码使用g++ 4.8.1编译成功:

int main()
{
    int(*)();
}

它看起来像一个简单的函数指针声明:

int(*f)();

它不能用 clang 3.4 和 vc++ 2013 编译。

它是编译器错误还是标准的黑暗地方之一？

---

用 g++ 4.8.1 编译好的类似奇怪代码片段列表(更新):

1. int(*)();

2. int(*);

3. int(*){};

4. int(*());

Live example with these strange code pieces .

更新 1: @Ali在评论中添加了一些有趣的信息:

All 4 cases give a compile error with clang 3.5 trunk (202594) and compile fine with gcc 4.9 trunk (20140302). The behavior is the same with -std=c++98 -pedantic, except for int(*){}; which is understandable; extended initializer lists only available with -std=c++11.

更新 2: 作为 @CantChooseUsernames在 his answer 中注明即使进行了初始化，它们仍然可以正常编译，并且即使没有启用任何优化，g++ 也不会为它们生成程序集(无论是否初始化):

1. int(*)() = 0;

2. int(*) = 0;

3. int(*){} = 0;

4. int(*()) = 0;

Live example with initializations .

更新 3: 我很惊讶地发现 int(*)() = "Hello, world!"; 也可以正常编译(而 int(*p)() = "Hello, world!"; 当然不会编译)。

更新 4: 非常棒，但 int(*){} = Hello, world!; 编译良好。还有下面这段极其奇怪的代码:int(*){}() = -+*/%&|^~.,:!?$()[]{}; ( live example)。

更新 5: 作为 @zwol在 his comment 中注明

This and a number of related syntactic problems are being tracked as gcc bug 68265.

Answer 1

根据 C++ 标准(第 7 节声明的第 6 页)

6 Each init-declarator in the init-declarator-list contains exactly one declarator-id, which is the name declared by that init-declarator and hence one of the names declared by the declaration

所以这只是一个编译器错误。

虽然我无法用我的 MS VC++ 2010 编译它，但有效代码可能看起来像示例(除了您显示的函数指针声明)。

int(*p){};

您用于测试的编译器似乎允许不带 declarator-id 的声明。

还要考虑第 8.1 节类型名称的以下段落

1 To specify type conversions explicitly, and as an argument of sizeof, alignof, new, or typeid, the name of a type shall be specified. This can be done with a type-id, which is syntactically a declaration for a variable or function of that type that omits the name of the entity.