可能重复: 实施no-op std :: ostream
在c ++中有没有相当于NULL的流?我想写一个函数,如果用户想要将内部输出到某个地方,但如果没有,输出会进入一些假的地方,
Is there any stream equivalent of NULL in c++? I want to write a function that takes in a stream if the user wants to have the internal outputted to somewhere, but if not, the output goes into some fake place
void data(std::stream & stream = fake_stream){ stream << "DATA" ; }我想要能够选择 ()或 data(std :: cout)
推荐答案编辑:取自@Johannes Schaub - litb的邮件这里,稍加修改:
Edit: Taken from @Johannes Schaub - litb's mail here with slight modifications:
template<typename Ch, typename Traits = std::char_traits<Ch> > struct basic_nullbuf : std::basic_streambuf<Ch, Traits> { typedef std::basic_streambuf<Ch, Traits> base_type; typedef typename base_type::int_type int_type; typedef typename base_type::traits_type traits_type; virtual int_type overflow(int_type c) { return traits_type::not_eof(c); } }; // convenient typedefs typedef basic_nullbuf<char> nullbuf; typedef basic_nullbuf<wchar_t> wnullbuf; // buffers and streams // in some .h extern std::ostream cnull; extern std::wostream wcnull; // in a concrete .cpp nullbuf null_obj; wnullbuf wnull_obj; std::ostream cnull(&null_obj); std::wostream wcnull(&wnull_obj);使用这些:
void data(std::ostream& stream = cnull){ // whatever... }
现在,这看起来很酷和所有,但下面是更短和工作,因为如果一个空指针提供给 ostream 的构造函数,它会自动设置badbit,并默默忽略任何写入:
Now, this looks cool and all, but the following is way shorter and works, because if a null pointer is provided to the constructor of ostream, it automatically sets the badbit and silently ignores any writes:
// in .h extern std::ostream cnull; extern std::wostream wcnull; // in .cpp std::ostream cnull(0); std::wostream wcnull(0);标准保证这个工作,从 27.6.2.2 [lib.ostream .cons] p1 其中描述了 ostream 的构造函数,它接受一个指向 streambuf :
The standard guarantees this works, beginning from 27.6.2.2 [lib.ostream.cons] p1 which describes the constructor of ostream that takes a pointer to a streambuf:
效果:构造类 basic_ostream 的一个对象,基本类通过调用 basic_ios< charT,traits> :: init(sb)。
Effects: Constructs an object of class basic_ostream, assigning initial values to the base class by calling basic_ios<charT,traits>::init(sb).
basic_ios , 27.4.4.1 [lib.basic.ios.cons] p3 :
void init(basic_streambuf< charT,traits> * sb); 后置条件:此函数的后置条件如表89所示:
void init(basic_streambuf<charT,traits>* sb); Postconditions: The postconditions of this function are indicated in Table 89:
表89中的重要行:
rdstate() - goodbit if sb不是空指针,否则为badbit。
rdstate() -- goodbit if sb is not a null pointer, otherwise badbit.
如果 badbit 被设置,会在 27.6.2.6 [lib.ostream.unformatted] :
每个未格式化的输出函数通过构造 sentry 。如果此对象返回true,则在转换为bool类型的值时,该函数会尝试生成请求的输出。
Each unformatted output function begins execution by constructing an object of class sentry. If this object returns true, while converting to a value of type bool, the function endeavors to generate the requested output.
,如果 sentry 为false,则不会。下面是 sentry 如何转换为 bool ,取自 27.6.2.3 [lib.ostream :: sentry] p3& p5 :
This implies that, in case the sentry is false, it does not. Here is how the sentry converts to bool, taken from 27.6.2.3 [lib.ostream::sentry] p3 & p5:
3)如果在任何准备工作完成后, os.good ()是 true , ok_ == true ok_ == false 。
3) If, after any preparation is completed, os.good() is true, ok_ == true otherwise, ok_ == false.
5) operator bool(); 效果:返回确定_。
5) operator bool(); Effects: Returns ok_.
( ok _ ostream :: sentry 类型 bool 。)
请注意,这些引号仍然存在于C ++ 11中,只是在不同的地方。按照此回答中的出现顺序:
Note that these quotes are still present in C++11, just in different places. In order of appearance in this answer:
- 27.6.2.2 [lib.ostream.cons] p1 => 27.7.3.2 [ostream.cons] p1
- 27.4.4.1 [lib .basic.ios.cons] p3 => 27.5.5.2 [basic.ios.cons]
- 表89 =>表128
- 27.6.2.6 [lib.ostream.unformatted] => .3.7 [ostream.unformatted] p1
- 27.6.2.3 [lib.ostream :: sentry] p3& p5 => 27.7.3.4 [ostream :: sentry] p4& p5
- 27.6.2.2 [lib.ostream.cons] p1 => 27.7.3.2 [ostream.cons] p1
- 27.4.4.1 [lib.basic.ios.cons] p3 => 27.5.5.2 [basic.ios.cons]
- Table 89 => Table 128
- 27.6.2.6 [lib.ostream.unformatted] => 27.7.3.7 [ostream.unformatted] p1
- 27.6.2.3 [lib.ostream::sentry] p3 & p5 => 27.7.3.4 [ostream::sentry] p4 & p5
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