如何将标准:: ostringstream转换为bool？ [英] How would std::ostringstream convert to bool?

问题描述

我碰到这个code绊倒了。

I stumbled across this code.

	std::ostringstream str;
	/// (some usage)
	assert( ! str );

这是什么 ostringstream 表示在 使用时BOOL 背景？

What does ostringstream signify when used in a bool context?

这是可能发生这种情况编译和运行一个不正确的使用情况如何？

Is this possibly an incorrect usage that happens to compile and run?

推荐答案

它会告诉你，如果流目前是否有效。这一点，所有的数据流可以做到。的文件流，例如，如果文件没有被正确打开可以是无效的。

It tells you if the stream is currently valid. This is something that all streams can do. A file stream, for example, can be invalid if the file was not opened properly.

作为一个方面说明，此功能（测试流为bool）是由超载达到明确的运营商BOOL 的在C ++ 11及更高版本，并在版本超载无效* 转换运算符的之前，C ++ 11 的

As a side note, this functionality (testing a stream as a bool) is achieved by overloading explicit operator bool in C++11 and later and by overloading the void* cast operator in versions before C++11.

下面是包含的为什么流可能会失败的一些例子。这不是特定字符串流，但是它确实适用于他们。

Here is a link containing some examples of why a stream might fail. This isn't specific to string streams, but it does apply to them.

编辑：修改布尔到无效* 之后，马丁纽约指出我的错误

changed bool to void* after Martin York pointed out my mistake.

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