CODEWITHSUNDEEP
c++
Zhongkun Ma
Zhongkun Ma

Reputation: 429

Declare a function with class constructor as function parameter

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.

}

The question is in the comment, I hope some people can help me to understand it. Thank you very much.

Upvotes: 1

Views: 81

Answers (2)

Brian Bi
Brian Bi

Reputation: 119467

It declares a function named b that returns B, which has a single parameter of type A (*)(), that is, pointer to function taking no arguments and returning A. The declarator A() means "function taking no arguments and returning A", but whenever you declare a parameter to have function type, it's rewritten to become a pointer to function. The parameter in this declaration is unnamed (you don't have to specify a name for a parameter if you don't want to).

To implement such a function you would need a definition, e.g.,

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

See, e.g., Is there any use for local function declarations?

Upvotes: 3

Code-Apprentice
Code-Apprentice

Reputation: 83567

B b(A()) declares a function named b which returns a B and takes a function pointer as an argument. The function pointer points to a function which returns a A and takes no arguments.

Upvotes: 2

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