[英]Should throw() function always unwind stack on exception and allow exception to be catched or std::terminate must be called?
I am interested whether this is forced by standard and whether it is violated by some compiler. 我很感兴趣这是否是由标准强制执行的,以及是否被某些编译器所违反。 My observation is that: 我的观察是:
gcc (6.3.0), stack in throw() function is unwound but then std::terminate is called (exception cannot be cought by try/catch). gcc(6.3.0),未取消throw()函数中的堆栈,但随后调用了std :: terminate(try / catch无法解决异常)。 But with 7.0 (current HEAD), no stack is being unwound, std::termiante is called immediately. 但是使用7.0(当前HEAD)时,没有取消堆栈堆栈,因此立即调用std :: termiante。 Actually gcc 7.0 even warns of this : warning: throw will always call terminate() [-Wterminate]
for NoExceptFunctionWithObj2()
. 实际上,gcc 7.0甚至对此发出警告: warning: throw will always call terminate() [-Wterminate]
为NoExceptFunctionWithObj2()
warning: throw will always call terminate() [-Wterminate]
。 It makes throw() behave as noexcept(true). 它使throw()的行为与noexcept(true)相同。
clang, in all versions I have checked unwinds function stack (objects d-tors are called), and then std::terminate is called. 在所有版本中,我都检查过展开函数堆栈(调用了对象d-tors),然后调用了std :: terminate。
#include <iostream>
#include <string>
#include <vector>
struct TestDataWithoutNoexcept {
TestDataWithoutNoexcept() {
std::cout << __FUNCTION__ << "\n";
}
~TestDataWithoutNoexcept() {
std::cout << __FUNCTION__ << "\n";
}
TestDataWithoutNoexcept(TestDataWithoutNoexcept const & rhs) {
std::cout << __FUNCTION__ << "\n";
}
TestDataWithoutNoexcept(TestDataWithoutNoexcept && rhs) {
std::cout << __FUNCTION__ << "\n";
}
TestDataWithoutNoexcept& operator=(TestDataWithoutNoexcept const& rhs) {
std::cout << __FUNCTION__ << "\n";
}
};
void NoExceptFunctionWithObj1() noexcept {
TestDataWithoutNoexcept test;
throw std::runtime_error("NoExceptFunctionWithObj1 ex.");
}
void NoExceptFunctionWithObj2() throw() {
TestDataWithoutNoexcept test;
throw std::runtime_error("NoExceptFunctionWithObj2 ex.");
}
int main()
{
// Now lets see whether stack is being unwound when exception is thrown in noexcept versus throw() function.
std::cout << "\n See how dtors are called in noexcept or throw() functions\n";
try {
//NoExceptFunctionWithObj1();
}
catch (std::runtime_error& ex) {
std::cout << ex.what();
}
try {
NoExceptFunctionWithObj2();
}
catch (std::runtime_error& ex) {
std::cout << "\nShouldn't this be shown? : " << ex.what();
}
}
Yes, std::terminate
should be called. 是的,应该调用std::terminate
。 The latest published draft of the standard says : 最新发布的标准草案说:
15.4 [except.spec], paragraph 12 15.4 [规格除外],第12段
An exception-specification is non-throwing if it is of the form throw() , noexcept , or noexcept( constant-expression ) where the constant- expression yields true . 如果异常规范的形式为throw(),noexcept或noexcept(constant-expression),其中常量表达式产生true,则该异常规范为非抛出。
Which means that throw()
is strictly equivalent to noexcept(true)
. 这意味着throw()
严格等同于noexcept(true)
。 throw()
is deprecated in C++17
. 在C++17
不推荐使用throw()
。
15.5.1 [except.terminate] 15.5.1 [终止除外]
when the search for a handler (15.3) encounters the outermost block of a function with a noexcept- specification that does not allow the exception (15.4) [...] std::terminate() is called (18.8.3). 在处理程序(15.3)的搜索遇到带有noexcept-spec的函数的最外面的块时,该函数不允许出现异常(15.4)[...] std :: terminate()(18.8.3)。 In the situation where no matching handler is found, it is implementation-defined whether or not the stack is unwound before std::terminate() is called 在找不到匹配处理程序的情况下,由实现定义是否在调用std :: terminate()之前取消堆栈的堆栈
Not calling std::terminate
means MSVC is not compliant. 不调用std::terminate
表示MSVC不兼容。
Regarding the handling of the stack, the compiler does what it wants about unwinding it or not in your example — this is specified to be implementation-defined . 关于堆栈的处理,在您的示例中,编译器会执行它希望对其进行展开或不展开的操作-这被指定为实现定义的 。
Historically (before C++11) stack unwinding in this situation was mandatory. 历史上(在C ++ 11之前)在这种情况下必须进行堆栈展开。 However it turned out that the runtime cost of this enforced behaviour was too high and it was inhibiting the compiler to do some optimizations (even in the non-exception case). 然而,事实证明,这种强制行为的运行时成本太高,并且它抑制了编译器进行某些优化(即使在非异常情况下)。 As a result, the compiler is now free to leave this out. 结果,编译器现在可以自由地将其省略。
Edited after clarifications from @mike. 经过@mike澄清后进行编辑。
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