这篇 post 是主要是写给 Association of Robots and Artificial Intelligence 的小伙伴的～主要说说为什么 void main() 是非法的，以及 main() 函数的返回值到底有什么用～
先从 void main() 讲起吧～这里就先引用 C++ 之父，Bjarne Stroustrup，在他的博客中写过的一篇问答：Can I write "void main()"?
Can I write "void main()"?
The definition
void main() { /* ... */ }
is not and never has been C++, nor has it even been C. See the ISO C++ standard 3.6.1[2] or the ISO C standard 5.1.2.2.1. A conforming implementation accepts
int main() { /* ... */ }
and
int main(int argc, char* argv[]) { /* ... */ }
A conforming implementation may provide more versions of main(), but they must all have return type int. The int returned by main() is a way for a program to return a value to "the system" that invokes it. On systems that doesn't provide such a facility the return value is ignored, but that doesn't make "void main()" legal C++ or legal C. Even if your compiler accepts "void main()" avoid it, or risk being considered ignorant by C and C++ programmers.
翻译：这种写法 void main() { /* ... */ }，从来都没有在 C/C++ 中存在过，根据 ISO C++ 标准 3.6.1 或者 ISO C 标准 5.1.2.2.1，只有 int main() {/* ... */} 或者是 int main(int argc, char* argv[]) {/* ... */} 才是可接受的。
A conforming implementation may provide more versions of main(), but they must all have return type int. The int returned by main() is a way for a program to return a value to “the system” that invokes it. On systems that doesn’t provide such a facility the return value is ignored, but that doesn’t make “void main()” legal C++ or legal C. Even if your compiler accepts “void main()” avoid it, or risk being considered ignorant by C and C++ programmers. —— Bjarne Stroustrup
翻译：符合规范的写法可能会有一些别的版本，但是他们都必须返回 int。这个main()返回的 int是用来会返回给调用这个程序的“系统”的。在没有提供这样的功能的系统中，这个返回值会被忽略，但是那也绝不使得void main在 C/C++ 中是合法的。即便你的编译器让你编译过了，或者就是仔细考虑过这么写的后果/风险
比如，我们来编译一下如下的 C 代码
void main() {
}
那么编译时就会报一个警告
编译器告诉我们 main() 函数应该有的返回类型是 int 而不是 void。
此外，void main() 也不在 C89 或者 ANSI C 规范中受支持，要么会报错，要么会产生警告。事实上，没有任何一个 C/C++ 标准支持这种形式的 main() 函数。以下是依次以 C89 标准和 ANSI C 标准编译时会有的输出。