为什么stringstream>>更改故障目标值？ [英] Why does stringstream >> change value of target on failure?
问题描述
从Stroustrup的TC ++ PL，第3版，第21.3.3节：
From Stroustrup's TC++PL, 3rd Edition, Section 21.3.3:
如果我们尝试读入变量v和操作失败，v的值应该保持不变（如果v是istream或ostream成员函数处理的类型之一，则不会改变）。
If we try to read into a variable v and the operation fails, the value of v should be unchanged (it is unchanged if v is one of the types handled by istream or ostream member functions).
以下示例似乎与上述报价相矛盾。基于上面的引用，我期望v的值保持不变 - 但它被归零。这个明显矛盾的行为的解释是什么？
The following example appears to contradict the above quote. Based on the above quote, I was expecting the value of v to remain unchanged -- but it gets zeroed. What's the explanation for this apparent contradictory behaviour?
#include <iostream>
#include <sstream>
int main( )
{
std::stringstream ss;
ss << "The quick brown fox.";
int v = 123;
std::cout << "Before: " << v << "\n";
if( ss >> v )
{
std::cout << "Strange -- was successful at reading a word into an int!\n";
}
std::cout << "After: " << v << "\n";
if( ss.rdstate() & std::stringstream::eofbit ) std::cout << "state: eofbit\n";
if( ss.rdstate() & std::stringstream::failbit ) std::cout << "state: failbit\n";
if( ss.rdstate() & std::stringstream::badbit ) std::cout << "state: badbit\n";
return 1;
}
输出我使用x86_64-w64-mingw32-g ++。exe（rubenvb -4.7.2-release）4.7.2 is：
The output I get using x86_64-w64-mingw32-g++.exe (rubenvb-4.7.2-release) 4.7.2 is:
Before: 123
After: 0
state: failbit
感谢。
推荐答案
从此参考资料：
如果提取失败（例如，如果一个字母输入到一个数字的预期位置），值保持未修改，并设置failbit（，直到C ++ 11 ）
如果提取失败，则将零写入值，并设置failbit。如果提取导致值太大或太小而无法容纳值，那么将写入std :: numeric_limits :: max（）或std :: numeric_limits :: min（），并设置failbit标志。 （自C ++ 11 ）
If extraction fails, zero is written to value and failbit is set. If extraction results in the value too large or too small to fit in value, std::numeric_limits::max() or std::numeric_limits::min() is written and failbit flag is set. (since C++11)
似乎你的编译器是在C ++ 11模式，这改变了行为。
It seems that your compiler is compiling in C++11 mode, which changes the behavior.
输入操作符使用locale facet std :: num_get 其 get 函数调用 do_get 。对于C ++ 11，指定使用 std :: strtoll 等。 et al。类型的函数。在C ++ 11之前，它显然使用了 std :: scanf 样式解析（通过引用，我没有访问C ++ 03规范）提取数字。行为的变化是由于解析输入的这种变化。
The input operator uses the locale facet std::num_get whose get function invokes do_get. For C++11 it's specified to use std::strtoll et. al. type of functions. Before C++11 it apparently used std::scanf style parsing (going by the reference, I don't have access to the C++03 specification) to extract the numbers. The change in behavior is due to this change in parsing the input.
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