std :: ifstream :: read或std :: ofstream :: write与零参数？ [英] std::ifstream::read or std::ofstream::write with a zero parameter?

问题描述

它是完全确定（=根据标准良好定义的行为）调用：

Is it perfectly ok (= well defined behaviour according to the standard) to call :

mystream.read(buffer, 0);

或

mystream.write(buffer, 0);

（当然不会读取或写入任何内容）。
我想知道如果在调用这两个函数之一之前，我必须测试所提供的大小是否为null。

(and of course nothing will be read or written). I would like to know if I have to test if the provided size is null before calling one of these two functions.

推荐答案

是的，行为是明确定义的：两个函数将通过未格式化的输入/输出函数的运动（构造哨兵，设置failbit如果eofbit设置，刷新绑定流，如果必要），然后他们将获得到此子句：

Yes, the behavior is well-defined: both functions will go through the motions for unformatted input/output functions (constructing the sentry, setting failbit if eofbit is set, flushing the tied stream if necessary), and then they will get to this clause:

§27.7.2.3[istream.unformatted] / 30

§27.7.2.3[istream.unformatted]/30

提取字符并存储，直到发生以下任一情况：

Characters are extracted and stored until either of the following occurs:

- 存储n个字符;

— n characters are stored;

§27.7.3.7[ostream.unformatted] / 5

§27.7.3.7[ostream.unformatted]/5

字符插入，直到发生以下任一情况

Characters are inserted until either of the following occurs

- 插入n个字符;

— n characters are inserted;

插入在存储或提取任何内容之前为真。

"zero characters are stored/inserted" is true before anything is stored or extracted.

看一下实际的实现，我看到 for（; gcount< n; ++ gcount） 在libc ++ 或 sgetn（buffer，n）; in stdlibc ++ ，其中包含等效循环

Looking at actual implementations, I see for (; gcount < n; ++gcount) in libc++ or sgetn(buffer, n); in stdlibc++ which has the equivalent loop

这篇关于std :: ifstream :: read或std :: ofstream :: write与零参数？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！