C++ parentheses

Question

#include <iostream>
using namespace std;
class A {public: A(){cout<<3;}
void test(){cout<<4;}};
int main() {
A a();
return 0;
}

In this snippet, if I use A a; the constructor is called. If I use A a(); it says no output and does not compile error or run time error. Can someone explain me what the parentheses do?

Answer 1

A() is the default constructor, it has no parameters/arguments

We can specify a custom constructor by supplying parameters/arguments
. for example

class A {
A(); //default constructor
A(int value); // custom constructor

when creating an object with a custom constructor we specify the parameters/arguments inside the parentheses.

example:

A a(2);

when using the default constructor we do not use parentheses.

example:

A a;

See here for more information on constructors and their correct usage.

https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.2.0/com.ibm.zos.v2r2.cbclx01/cplr376.htm

Answer 2

n3337 8.2/1

"... the choice is between a function declaration with a redundant set of parentheses around a parameter name and an object declaration with a function-style cast as the initializer. Just as for the ambiguities mentioned in 6.8, the resolution is to consider any construct that could possibly be a declaration a declaration."

Similar issue for your case.

A a; // declares an object a of class A.
A a(); // declares a function of return type A, which takes no arguments.

Answer 3

removing return *b in this example does not affect the program as the return value is never used in main.

A's constructor is called when it is instantiated with the line A* b = new A;

Using return *b allows you to use the object A in the main function as it is returned to the calling function (in this case main)

A a() is a function with a return type of A

it is similar in structure to

int main()

[Return Type] functionName()

We can not use A a() to create a new object.

Answer 4

well, sure, but it won't be what you expect.

https://code.sololearn.com/cdDo30h36ueY/?ref=app

Answer 5

Thank you. I use to modify random snippets trying to get the same output with different syntax as a learning method. In this case, I removed return *b and it gave the same output. Why is that? Using return makes it possible to use A a() as a parameter?

Answer 6

Could you give me an example of when I would need A a(); but no custom constructor?