C ++构造函数初始化列表抛出异常 [英] C++ constructor initializer list throw exceptions

问题描述

我有以下代码的问题。我们可以看到我已经处理了A的构造函数在C的构造函数中抛出的异常，为什么我应该在主函数中再次捕获和处理异常？

I have a problem with the following code. As we can see I have already handled the exception thrown by A's constructor in C's constructor, why should I bother to catch and handle the exception again in the main function?

#include <iostream>

class WException : public std::exception
{
public:
    WException( const char* info ) : std::exception(info){}
};

class A
{
public:
    A( int a ) : a(a) 
    {
        std::cout << "A's constructor run." << std::endl;
        throw WException("A constructor throw exception.");
    }

private:
    int a;
};

class B
{
public:
    B( int b ) : b(b) 
    {
        std::cout << "B's constructor body run." << std::endl;
        throw WException("B constructor throw exception");
    }

private:
    int b;
};

class C : public A, public B
{
public:
    C( int a, int b ) try : A(a), B(b)
    {
        std::cout << "C's constructor run." << std::endl;
    }
    catch( const WException& e )
    {
        std::cerr << "In C's constructor" << e.what() << std::endl;
    }
};

int main( int argc, char* argv[] )
{
    try
    {
        C c( 10, 100 );
    }
    catch( const WException& e )
    {
        std::cerr << "In the main: " << e.what() << std::endl;
    }   

    return 0;
}

推荐答案

捕获构造函数中的异常。你可以处理它，但你必须重新抛出它或另一个异常。原因是关于对象完整性和对象生命周期：

You cannot actually catch an exception in a constructor. You can handle it, but you have to rethrow it or another exception. The reason is about object integrity and object lifetimes:

如果构造 a throws， c $ c> c 尚未初始化并完全丢失 - a 的生命周期永远不会开始。 a 不是 C 的可选部分，否则必须是指针或 std :: optional （因为C ++ 14 - boost :: optional 之前）。

If the construction of a throws, a part of c has not been initialized and is completely missing - lifetime of a never starts. a is not an optional part of C, otherwise it had to be a pointer or a std::optional (since C++14 - boost::optional before that).

那么，如果不能构造一个 C 的重要部分，那么如何组装它呢？你不能。 c 永远不会开始作为一个完整的对象存在，所以没有办法你可以正常退出构造函数。这就是为什么如果构造一个成员对象失败，整个对象的构造必须失败，即必须抛出异常。

So how do you assemble a C if one of its vital parts cannot be constructed? You can't. c can never start to exist as a complete object, so there is no way you can exit the constructor normally. That's the reason why if construction of a member object fails, construction of the whole object has to fail, i.e. has to throw an exception.

如果你不在 C :: C 的catch块中抛出异常，编译器会为你这样做。

If you don't throw an exception in C::C's catch block, the compiler will do so for you.

C ++标准，§15.3.15：

如果控制达到

The currently handled exception is rethrown if control reaches the end of a handler of the function-try-block of a constructor or destructor.

有关该主题的更广泛的处理，请参阅 http://www.gotw.ca/gotw/066.htm

For a broader treatment on that topic, see http://www.gotw.ca/gotw/066.htm

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