异常和复制构造函数:C ++ [英] Exception and Copy Constructor : C++

问题描述

引用 http://en.wikipedia.org/wiki/Copy_elision

我运行以下代码:

#include <iostream>

struct C {
  C() {}
  C(const C&) { std::cout << "Hello World!\n"; }
};

void f() {
  C c;
  throw c; // copying the named object c into the exception object.
}          // It is unclear whether this copy may be elided.

int main() {
  try {
    f();
  }
  catch(C c) {  // copying the exception object into the temporary in the exception declaration.
  }             // It is also unclear whether this copy may be elided.
}

我得到的输出:

Gaurav@Gaurav-PC /cygdrive/d/Trial
$ make clean
rm -f Trial.exe Trial.o

Gaurav@Gaurav-PC /cygdrive/d/Trial
$ make
g++ -Wall Trial.cpp -o Trial

Gaurav@Gaurav-PC /cygdrive/d/Trial
$ ./Trial
Hello World!
Hello World!

我了解编译器可能已经通过不必要的复制对代码进行了优化，而这在这里没有做.

I understand that the compiler might have optimized the code with unnecessary copying, which it is not doing here.

但是我想问的是，如何两次调用复制构造函数?

catch(C c)-由于我们按值传递，因此此处将调用复制构造函数.

catch(C c) - Since we are passing by value, hence here the copy constructor is being called.

但是在 throw c 处如何调用拷贝构造函数?有人可以解释吗?

But at throw c how is copy constructor being called? Can someone explain?

推荐答案

throw c;     

创建一个临时对象，并抛出该临时对象.临时文件的创建可以通过复制/移动构造函数进行.是的，可以删除此副本/移动.

Creates a temporary object and it is this temporary object that is thrown. The creation of the temporary might be through copy/move constructor. And yes this copy/move can be elided.

参考文献:
C ++ 11 15.1引发异常

§3:

throw-expression会初始化一个临时对象，称为异常对象，其类型是通过从throw操作数的静态类型中删除所有顶级cv限定符并进行调整来确定的类型.........

A throw-expression initializes a temporary object, called the exception object, the type of which is determined by removing any top-level cv-qualifiers from the static type of the operand of throw and adjusting the type.........

§5:

当抛出的对象是类对象时，即使省略了复制/移动操作(12.8)，复制/移动构造函数和析构函数也应可访问.

When the thrown object is a class object, the copy/move constructor and the destructor shall be accessible, even if the copy/move operation is elided (12.8).

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