c++ passing a rhs object as parameter to the constructor of anther obejct

Question

I have searched the internet and SO for couple of hours on this question. There are similar questions but I could not find an answer to mine. Basically I am trying to pass a rhs object as a parameter of the constructor of another class. I did not receive any errors but neither of the two class's constructors are triggered. If I break the process into two steps: creating one object first and then pass it to the other as lhs, then it works. I tried to have a copy constructor with class2 below and it dose not work either.

Below is the coding. The program runs without errors, but the console records no output.

struct class1
{
    class1()
    {
        std::cout << "class1 constructed" << std::endl;
    }
};


struct class2
{
    class2()
    {
        std::cout << "class2 default constructed" << std::endl;
    }
    template <typename T>
    class2(T)
    {
        std::cout << "class2 with template constructed" << std::endl;
    }

};

int main()
{
     class2 test(class1()); 
     return 0;
}

That isn't doing what you think it is. Google *C++ most-vexing parse*. — WhozCraig, Aug 04 '17 at 02:56
`warning C4930: 'class2 test(class1 (__cdecl *)(void))': prototyped function not called (was a variable definition intended?)` Add extra parens: `class2 test((class1()));` — Retired Ninja, Aug 04 '17 at 02:58
@RetiredNinja thanks I think that post is helpful. I would argue even though my question may be duplicate, but it was asked in a meaningfully different way:). — DavidY, Aug 04 '17 at 03:09
It's not like your question will go away if it is marked as a duplicate. — Retired Ninja, Aug 04 '17 at 03:16

score 1 · Answer 1 · answered Aug 04 '17 at 02:58

1

class2 test(class1());

In this line Parentheses were disambiguated as a function declaration

answered Aug 04 '17 at 02:58

Hariom Singh

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score 1 · Accepted Answer · answered Aug 04 '17 at 03:01

1

This is a most vexing parse issue.

class2 test(class1()); is not a variable definition (as you might expect), but a function declaration, the function is named test and returns class2, takes one unnamed parameter which is a pointer to function (which takes nothing and returns class1).

You can use braces instead (since C++11). e.g.

class2 test(class1{});

answered Aug 04 '17 at 03:01

songyuanyao

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got you seems like I should really get used to use the universal initialization list. – DavidY Aug 04 '17 at 03:08

c++ passing a rhs object as parameter to the constructor of anther obejct

2 Answers2