聲明一個具有類構造函數作爲函數參數的函數

Question

struct A{ 
    A(){} 
}; 

struct B{ 
    B(const A& a){} 
}; 

int main() 
{ 
//Originally I would like to write down below code 

A a; 
B b(a); 

//Unfortunately I end up with below code by accident and there is no compile error 
//I have figured out the below does not create temporary A and call B constructor to 
//create B as the above codes, 
//it declares a function with return value B, name b, 
//and some input parameter, my question is 1) what the input parameter is ? 
//2) How to implement such a function. 

B b(A()); // There is no global function A() in my test case. 

} 

問題是在評論中，我希望有人能幫助我理解它。非常感謝你。

Answer 1

它聲明瞭一個名爲b的函數，它返回B，它具有A (*)()類型的單個參數，即指向不帶參數的函數的指針並返回A。聲明者A()的意思是「不帶參數的函數並返回A」，但是無論何時聲明具有函數類型的參數，它都會被重寫爲函數指針。此聲明中的參數未命名（如果不想爲參數指定名稱，則不需要）。

要實施這樣的功能，你需要一個定義，例如，，

B b(A a()) { 
    // do something with "a" 
    // note: the type of "a" is still pointer to function 
} 

見，例如，Is there any use for local function declarations?

Answer 2

B b(A())聲明瞭一個名爲b的函數，它返回B並將函數指針作爲參數。函數指針指向一個返回A並且不帶參數的函數。