为什么 auto a=1;用C编译? [英] Why does auto a=1; compile in C?

问题描述

代码:

int main(void)
{
    auto a=1;
    return 0;
}

当文件具有 .c 扩展名时，由 MS Visual Studio 2012 编译器编译而不会出错.我一直认为，当你使用.c扩展名时，编译应该按照C语法，而不是C++.此外，据我所知，自 C++11 以来，C++ 中仅允许没有类型的 auto，这意味着该类型是从初始化程序推导出来的.

gets compiled without errors by the MS Visual Studio 2012 compiler, when the file has the .c extension. I have always thought that when you use the .c extension, compilation should be according to the C syntax, and not C++. Moreover, as far as I know auto without a type is allowed only in C++ since C++11, where it means that the type is deduced from the initializer.

这是否意味着我的编译器没有坚持使用 C，或者代码实际上在 C 语言中是正确的?

Does that mean that my compiler isn't sticking to C, or is the code actually correct in C-language?

推荐答案

auto 是一个古老的 C 关键字，意思是本地范围".auto a 与 auto int a 相同，并且由于局部作用域是函数内部声明的变量的默认范围，因此它也与 int a<相同/code> 在这个例子中.

auto is an old C keyword that means "local scope". auto a is the same as auto int a, and because local scope is the default for a variable declared inside a function, it's also the same as int a in this example.

这个关键字实际上是 C 的前身 B 的遗留物，那里没有基类型:一切都是 int、指向 int 的指针、int<的数组/code>.(*) 声明可以是 auto 或 extrn [原文如此].C 继承了一切都是 int"作为默认规则，因此您可以使用

This keyword is actually a leftover from C's predecessor B, where there were no base types: everything was int, pointer to int, array of int.(*) Declarations would be either auto or extrn [sic]. C inherited the "everything is int" as a default rule, so you could declare integers with

auto a;
extern b;
static c;

ISO C 摆脱了这一点，但为了向后兼容，许多编译器仍然接受它.如果它看起来不熟悉，那么您应该意识到相关规则正在起作用

ISO C got rid of this, but many compilers still accept it for backward compatibility. If it seems unfamiliar, then you should realise that a related rule is at work in

unsigned d;  // actually unsigned int

这在现代代码中仍然很常见.

which is still common in modern code.

C++11 重用了关键字，如果有的话，很少有 C++ 程序员使用它的原始含义来进行类型推断.这在很大程度上是安全的，因为 C++98 中已经删除了来自 C 的一切都是 int"规则；唯一出问题的是auto T a，反正没人用过.(在他的关于语言历史的论文的某处，Stroustrup对此发表了评论，但我现在找不到确切的参考.)

C++11 reused the keyword, which few if any C++ programmers were using with the original meaning, for its type inference. This is mostly safe because the "everything is int" rule from C had already been dropped in C++98; the only thing that breaks is auto T a, which no-one was using anyway. (Somewhere in his papers on the history of the language, Stroustrup comments on this, but I can't find the exact reference right now.)

(*) 字符串处理很有趣:您将使用 int 数组并在每个成员中打包多个字符.B 实际上是具有不同语法的 BCPL.

(*) String handling in B was interesting: you'd use arrays of int and pack multiple characters in each member. B was actually BCPL with different syntax.

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