Call constructor inside a call to another constructor-CodePudding

Suppose I have classes A, B:

class A
{
    int a;
public:

    A(int a) : a(a) { }
};

class B
{
    A a;
public:

    B(A a) : a(a) { }
};

And I want to create an instance of B as:

int i = 1;
B b(A(i));

But when I actually try to use b, I get the problem described here. That is, b is not an instance of B, it's a function that returns an object of type B. However, it's a different case from the linked question, and I can't figure out why it's happening here.

I could create B as:

int i = 1;
A a(i);
B b(a);

Or even as:

B b(A(1));

And it'll work. However, in the real case the first option requires too many lines, and the second option requires very long line (because instead of int I have an object which needs to be constructed, and I have a chain of some objects). How can I create the int in his own line, and then create A and B in the same line?

CodePudding user response：

The problem is that B b(A(i)); is a function declaration and not a declaration for a variable named b of type B.

This is due to what is called most vexing parse. In particular, the statement:

B b(A(i)); //this is a declaration for a function named `b` that takes parameter of type A and has no return type

the above is a declaration for a function named b that takes one parameter of type A and with return type of B.

To solve this, you can either use curly braces {} or double () to solve this:

Method 1

Use {} around A(i).

//----v----v------->declares a variable of type B and is not a function declaration
   B b{A(i)};

Method 2

Use double parenthesis (()).

//-v--------v------>declares a variable of type B and is not a function declaration
B b( (A(i)) );

CodePudding user response：

One obvious way to rule out any possible function declaration is:

B b { A {i} };