关于solaris (x86)上的std::basic_string的一些困惑
关于solaris (x86)上std::basic_string的一些困惑
#include <iostream>
#include <string>
int main()
{
const wchar_t* s = L"abcdef";
std::wstring ws(s, s+6);
for(int i = 0; i < ws.size(); ++i)
{
std::cout << ws[i] << std::endl;
}
return 0;
}
运行结果是：
97
99
101
0
0
0
为什么不是
97
98
99
100
101
102
并且代码
#include <iostream>
#include <string>
int main()
{
const wchar_t* s = L"abcdef";
std::wstring ws;
ws.resize(6);
for(int i = 0; i < ws.size(); ++i)
{
std::cout << (ws[i] = s[i]) << std::endl;
}
return 0;
}
可以得到预期的结果。 我使用gcc 3.4.6，构建命令是g++ -fshort-wchar stringtest.cpp。任何人都可以解释一下吗？
Some puzzles about std::basic_string on solaris (x86)
#include <iostream>
#include <string>
int main()
{
const wchar_t* s = L"abcdef";
std::wstring ws(s, s+6);
for(int i = 0; i < ws.size(); ++i)
{
std::cout << ws[i] << std::endl;
}
return 0;
}
The running result is:
97
99
101
0
0
0
Why is not
97
98
99
100
101
102
And the code
#include <iostream>
#include <string>
int main()
{
const wchar_t* s = L"abcdef";
std::wstring ws;
ws.resize(6);
for(int i = 0; i < ws.size(); ++i)
{
std::cout << (ws[i] = s[i]) << std::endl;
}
return 0;
}
can get the expected result.
I use gcc 3.4.6, build command is g++ -fshort-wchar stringtest.cpp. Any one can give an expain?
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-fshort-wchar 的文档读到，因此，似乎这个标志导致了观察到的差异，并且由于语言规范没有讨论这样的标志，因此该行为可以归类为实现定义的或未定义的。
作为旁注，在处理宽字符时，您应该使用
wcout而不是cout，因为wcout旨在处理宽字符：basic_ostream类型的对象。wcout是basic_ostream类型的对象。我认为，在这种情况下，问题不在于您用来打印值的内容，因为您告诉编译器将
wchar_t视为short unsigned int。The documentation of
-fshort-wcharreads,So it seems that this flag is causing the observed discrepancy, and since the language specification doesn't talk about such flag, the behavior can be categorized as either implementation-defined or undefined.
As a sidenote, you should use
wcoutinstead ofcoutwhen dealing with wide-characters, aswcoutis designed to handle wide-characters:coutis an object of typebasic_ostream<char>.wcoutis an object of typebasic_ostream<wchar_t>.Thought, in this case, the problem is not what you use to print the value, as you're telling the compiler to treat
wchar_tasshort unsigned int, anyway.标准库很可能不是用
--short-wchar编译的。该标志更改了 ABI，尽管这不会被检测到，因为名称修改不会更改。The standard library was most likely not compiled with
--short-wchar. This flag changes the ABI, although this goes undetected because name mangling does not change.看起来问题是
ws[i]给出了错误的结果；当我查看原始内存时，该字符串似乎包含预期的数据。为什么会发生这种情况真是令人费解；据我所知，operator[] 只是取消引用指向wchar_t的指针，它在其他地方可以正常工作（例如打印s[i] 在你的第二个例子中）。该问题也出现在较新版本的 GCC（我尝试过 4.6.1）和 Linux 上。您可以使用
*(ws.begin() + i)来解决这个问题。It looks like the problem is
ws[i]giving the wrong result; the string seems to contain the expected data when I look at the raw memory. It's quite baffling why this happens; as far as I can see,operator[]is simply dereferencing a pointer towchar_t, which works correctly in other places (e.g. printings[i]in your second example). The problem also occurs on more recent versions of GCC (I tried 4.6.1), and on Linux.You can work around it by using
*(ws.begin() + i)instead.