问题描述
我正在为我正在开发的C ++应用程序编写一小段异常类层次结构,并且在从 std :: runtime_error
间接派生时遇到了麻烦.这是类似于我到目前为止编写的代码:
class RuntimeException:公共虚拟boost :: exception,公共虚拟std :: runtime_error {上市:虚拟〜RuntimeException(){}RuntimeException():runtime_error(发生RuntimeException."){}RuntimeException(const std :: string& what):runtime_error(what){}};class IllegalArgumentException:公共虚拟RuntimeException {上市:IllegalArgumentException():RuntimeException(发生IllegalArgumentException."){}IllegalArgumentException(const std :: string& what):RuntimeException(what){}};
RuntimeException
类可以毫无问题地进行编译,但是 IllegalArgumentException
拒绝在VS2015上进行编译,从而产生错误:类"std ::"不存在默认构造函数:
.挑战了我对C ++继承层次结构的理解,因为我希望这段代码可以很好地编译. IllegalArgumentException
的两个构造函数的"runtime_error"
我的理解是 IllegalArgumentException
应该进行编译,因为尽管 std :: runtime_error
确实没有默认构造函数,但是构造函数正在为 RuntimeException
调用构造函数.但是显然这一定是错误的,因为编译器正在拒绝它.似乎要我直接从 IllegalArgumentException
构造函数调用 std :: runtime_error
构造函数(当我这样做时,编译器错误会消失),但这似乎是错误的,因为这样我会两次调用 std :: runtime_error
的构造函数:一次在 RuntimeException
的构造函数中,再一次在 IllegalArgumentException
的构造函数中./p>
这样做安全和/或有效吗?如果没有,为什么编译器似乎鼓励它呢?我可以只是从 std :: exception
派生并自己实现 std :: string
作为成员变量,但我认为这样会更容易从已经实现此功能的标准类派生.这是错误的方法吗?另外,事实上我实际上是从 boost:exception
和 std :: runtime_error
派生出来的吗?
使用 virtual
继承时, virtual
基的构造函数调用是的责任大多数派生类,而不是任何中间类的责任.原因很明显:使用 virtual
继承表示期望使用基类实际上存在多个派生类.这些派生类中的哪一个负责构建 virtual
基?
因此,任何派生类的构造函数都需要为 virtual
基提供一个参数,例如:
IllegalArgumentException :: IllegalArgumentException(std :: string const& what):std :: runtime_error(什么),RuntimeException(what){}
为避免有中间基类,调用用于 virtual
继承的 virtual
基类的构造函数通常会提供默认的构造函数.当然,这开辟了以下可能性:大多数派生类不正确地依赖于由其基数之一调用的适当构造函数.
I'm writing a small hierarchy of exception classes for a C++ application I'm developing, and I'm having trouble deriving indirectly from std::runtime_error
. Here is code analogous to what I've written so far:
class RuntimeException : public virtual boost::exception, public virtual std::runtime_error {
public:
virtual ~RuntimeException() {}
RuntimeException() : runtime_error("A RuntimeException occurred.") {}
RuntimeException(const std::string& what) : runtime_error(what) {}
};
class IllegalArgumentException : public virtual RuntimeException {
public:
IllegalArgumentException() : RuntimeException("An IllegalArgumentException occurred.") {}
IllegalArgumentException(const std::string& what) : RuntimeException(what) {}
};
The RuntimeException
class compiles without issue, but IllegalArgumentException
refuses to compile on VS2015, generating the error: no default constructor exists for class "std::runtime_error"
for both constructors of IllegalArgumentException
. This challenges my understanding of C++ inheritance hierarchies, as I expected this code to compile fine.
My understanding is that IllegalArgumentException
should compile because, although it is true that std::runtime_error
does not have a default constructor, its constructor is being called by the constructor for RuntimeException
. But obviously this must be false, as the compiler is rejecting it. It seems to want me to call the std::runtime_error
constructor directly from the IllegalArgumentException
constructor (the compiler error goes away when I do so), but this seems wrong because then I would be calling the constructor for std::runtime_error
twice: once in the constructor for RuntimeException
, and again in the constructor for IllegalArgumentException
.
Is this safe and/or efficient to do? If not, why does the compiler seem to encourage it? I could just derive from std::exception
and implement the std::string
myself as a member variable, but I thought it would be easier to derive from a standard class that has already implemented this. Is this the wrong approach to take? Additionally, is the fact that I'm deriving virtually from both boost:exception
and std::runtime_error
contributing to this issue?
When using virtual
inheritance the constructor call of the virtual
base is the responsibility of the most derived class rather than the responsibility of any intermediate class. The reason is obvious: the use of virtual
inheritance indicates that there is an expectation that there are actually multiple derived classes using the base class. Which one of these derived classes would be responsible for constructing the virtual
base?
So, the constructor of any of the derived classes needs to provide an argument to the virtual
base, e.g.:
IllegalArgumentException::IllegalArgumentException(std::string const& what)
: std::runtime_error(what)
, RuntimeException(what) {
}
To avoid having intermediate bases call the constructor of the virtual
base classes intended for virtual
inheritance often provide a default constructor. Of course, that opens up the possibility that the most derived class incorrectly relies on the proper constructor being called by one of its bases.
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