构造函数调用机制
struct my
{
my(){ std::cout<<"Default";}
my(const my& m){ std::cout<<"Copy";}
~my(){ std::cout<<"Destructor";}
};
int main()
{
my m(); //1
my n(my()); //2
}
预期输出：
1 ) Default
2 ) Copy
实际输出：
我对构造函数调用机制的理解有什么问题？
注意 为了简洁起见，我省略了头文件。
struct my
{
my(){ std::cout<<"Default";}
my(const my& m){ std::cout<<"Copy";}
~my(){ std::cout<<"Destructor";}
};
int main()
{
my m(); //1
my n(my()); //2
}
Expected output :
1 ) Default
2 ) Copy
Actual output :
What's wrong with my understanding of the constructor invoking mechanism?
Note I have omitted header files for brevity.
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评论（2）
情况 1)
m被解释为函数返回my并且不带任何参数。要查看预期的输出，请删除
()，即使用my m;情况 2)
这是更广为人知的“最令人烦恼的解析”。
n被解释为返回my的函数，该函数接受一个指针类型的参数，该函数返回不带任何参数的my函数。要查看这种情况下的预期输出，请尝试
my n((my()));[编译器不再像前一种情况那样将其视为参数规范，而是将其解释为表达式，因为额外的()]我的解释：
my n((my()))相当于my n = my()。现在，右值表达式my()创建一个临时对象[即对默认构造函数的调用]，并且n被复制初始化为该临时对象[不调用复制向量由于一些编译器优化]PS ：我对我的答案的最后部分不是100%确定。如果我错了请纠正我。
Case 1)
mis interpreted as a function returnmyand taking no arguments.To see the expected output remove
()i.e usemy m;Case 2)
This is something better known as the "Most vexing parse".
nis interpreted as a function returningmythat takes an argument of type pointer to function returningmytaking no arguments.To see the expected output in this case try
my n((my()));[Instead of treating as an argument specification as in the former case the compiler would now interpret it as an expression because of the extra()]My interpretation:
my n((my()))is equivalent tomy n = my(). Now the rvalue expressionmy()creates a temporary[i.e a call to the default constructor] andnis copy initialized to that temporary object[no call to the copy-ctor because of some compiler optimization]P.S: I am not 100% sure about the last part of my answer. Correct me if I am wrong.
与 Prasoon 一样，我怀疑 C++ 编译器正在以您意想不到的方式解析您的代码。例如，我认为它将该行解析
为函数原型声明，而不是变量声明并调用构造函数 - 这就是为什么你看不到输出。
Like Prasoon, I suspect the C++ compiler is parsing your code in a way you don't expect. For example, I think it is parsing the line
as a function prototype declaration, not as a variable declaration and call to the constructor - hence why you see no output.