C：参数的数目不详 - 无效美孚（） [英] C: Unspecified number of parameters - void foo()

问题描述

我读<一个href=\"http://stackoverflow.com/questions/51032/is-there-a-difference-between-foovoid-and-foo-in-c-or-c\">here在 C 无效美孚（）办法函数foo接受的参数数目不详未指定类型。

谁能给我或点我到一个 C 函数调用参数数目不详的例子吗？可这有什么在 C 被应用到？我无法在网上找到任何东西。

Can anyone give me or point me to an example where a C function takes an unspecified number of arguments? What can this be applied to in C? I couldn't find anything on the web.

谢谢！

推荐答案

这是一个的旧式的函数声明。

That's an old-style function declaration.

这声明：

void foo();

声明富是一个函数返回无效接受一个不确定的，但固定的数量和类型（S）参数。这并不意味着调用任意参数是有效的;这意味着编译器不能诊断与错号码或参数类型不正确的电话。

declares that foo is a function returning void that takes an unspecified but fixed number and type(s) of arguments. It doesn't mean that calls with arbitrary arguments are valid; it means that the compiler can't diagnose incorrect calls with the wrong number or type of arguments.

某处，也许在另一个翻译单元（源文件），必须有一个的定义的功能，也许是：

Somewhere, perhaps in another translation unit (source file), there has to be a definition of the function, perhaps:

void foo(x, y)
long x;
double *y;
{
    /* ... */
}

这意味着，富任何调用的不的传递型长和双* 是无效的，并已未定义的行为。

This means that any call to foo that doesn't pass two arguments of type long and double* is invalid, and has undefined behavior.

在此之前的1989年的ANSI C标准，这些是唯一的一种函数声明和定义中的语言版本，并编写正确的函数调用的负担完全在程序员。 ANSI C添加的原型的，即指定一个类型函数的参数，它允许编译时的函数调用检查函数声明。 （此功能从早期的C ++借来的。）的现代相当于上面会：

Prior to the 1989 ANSI C standard, these were the only kind of function declaration and definition available in the language, and the burden of writing correct function calls was entirely on the programmer. ANSI C added prototypes, function declarations that specify the types of a function's parameters, which allow compile-time checking of function calls. (This feature was borrowed from early C++.) The modern equivalent of the above would be:

void foo(long x, double *y);

/* ... */

void foo(long x, double *y) {
    /* ... */
}

旧样式（非原型）的声明和定义仍然是合法的，但他们是正式的过时的，这意味着，在原则上，他们可以从语言的未来版本中删除 - 尽管因为他们仍然围绕在2011的标准，我不知道这永远不会真正发生

Old-style (non-prototype) declarations and definitions are still legal, but they're officially obsolescent, which means that, in principle, they could be removed from a future version of the language -- though since they're still around in the 2011 standard I don't know that that will ever actually happen.

有没有好的理由使用旧式函数声明和定义在现代的C code。 （我见过的论点在某些角落情况下使用它们，但我觉得他们缺乏说服力。）

There is no good reason to use old-style function declarations and definitions in modern C code. (I've seen arguments for using them in some corner cases, but I find them unconvincing.)

C也支持的可变参数的功能，如的printf ，它需要做的参数任意数量，但是这是一个显着特点。一个可变参数函数必须用原型，其中包括尾随 ... 声明。 （呼叫无可见原型可变参数函数并不违法，但它未定义的行为。）函数本身使用的中所定义的宏; STDARG.H＆GT; 来处理其参数。与旧式函数声明，没有编译时检查相应的参数 ... （尽管一些编译器可以检查一些调用，例如海湾合作委员会警告如果在的printf 调用的参数与格式字符串不一致）。

C also supports variadic functions like printf, which do take an arbitrary number of arguments, but that's a distinct feature. A variadic function must be declared with a prototype, which includes a trailing , .... (Calling a variadic function with no visible prototype isn't illegal, but it has undefined behavior.) The function itself uses macros defined in <stdarg.h> to process its parameters. As with old-style function declarations, there is no compile-time checking for arguments corresponding to the , ... (though some compilers may check some calls; for example gcc warns if the arguments in a printf call are inconsistent with the format string).

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