英文:
Using `volatile` has unexpected results
问题
我对尝试复制不同的错误时发现的以下差异感到困惑。此程序按预期工作：
#include <memory>
#include <string>
#include <iostream>
int main() {
std::string s = "Hi\n";
std::cout << s;
auto p = std::make_unique<const char *>(s.c_str());
std::cout << *(p.get());
return 0;
}
它打印：
Hi
Hi
但是添加单个volatile关键字：
#include <memory>
#include <string>
#include <iostream>
int main() {
std::string s = "Hi\n";
std::cout << s;
auto p = std::make_unique<volatile const char *>(s.c_str());
std::cout << *(p.get());
return 0;
}
会使其打印：
Hi
1
这也没有帮助：
#include <memory>
#include <string>
#include <iostream>
int main() {
std::string s = "Hi\n";
std::cout << s;
auto p = std::make_unique<volatile const char *>(reinterpret_cast<volatile const char *>(s.c_str()));
std::cout << *(p.get());
return 0;
}
为什么？
英文:
I am puzzled by following difference discovered by trying to replicate different bug. This program works as expected:
#include <memory>
#include <string>
#include <iostream>
int main() {
std::string s = "Hi\n";
std::cout << s;
auto p = std::make_unique<const char *>(s.c_str());
std::cout << *(p.get());
return 0;
}
It prints
Hi
Hi
But adding single volatile keyword:
#include <memory>
#include <string>
#include <iostream>
int main() {
std::string s = "Hi\n";
std::cout << s;
auto p = std::make_unique<volatile const char *>(s.c_str());
std::cout << *(p.get());
return 0;
}
makes it print:
Hi
1
This did not help either:
#include <memory>
#include <string>
#include <iostream>
int main() {
std::string s = "Hi\n";
std::cout << s;
auto p = std::make_unique<volatile const char *>(reinterpret_cast<volatile const char *>(s.c_str()));
std::cout << *(p.get());
return 0;
}
Why?
答案1
得分: 4
std::ostream对const char*和bool（指针可以转换为bool）重载了运算符<<，但没有重载volatile const char*。
然后，可以通过以下方式简单复制差异：
#include <iostream>
int main() {
/*volatile*/ const char * s = "abc\n";
std::cout << s;
return 0;
}
取消注释volatile后，输出为abc，注释它后输出为1（没有换行符）。
英文:
The answer is in the comment by Jarod42:
> std::ostream has overload for operator << for const char* and bool (pointer has conversion to bool), not for volatile const char *
The difference then can be replicated simply by:
#include <iostream>
int main() {
/*volatile*/ const char * s = "abc\n";
std::cout << s;
return 0;
}
Gives abc with volatile commented out and 1 (without newline) with uncommenting it.
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