Constructor without arguments doesn't work

Question

I wonder why the constructor doesn't work/get called in the first case.

#include <iostream>
#include <typeinfo>

class Test{
public:
    Test(){ std::cout << "1\n"; };
    Test(int){ std::cout << "2\n"; };
};


int main()
{
    Test a(); // actually doesn't call the constructor
    Test b(1); // "2"
    std::cout << (typeid(b).name()) << std::endl; // "4Test"
    std::cout << (typeid(a).name()); // "F4TestvE"
    return 0;
}

I've also found that typenames of created variables are strange. Can anybody explain such a behavior?

I use mingw gcc 4.7.2 for Windows to compile my projects

Thanks a lot.

http://en.wikipedia.org/wiki/Most_vexing_parse – Anton Savin Mar 30 '15 at 12:49 — Anton Savin, Mar 30 '15 at 12:49
`Test a();` is treated as a function, not a variable. – Arun A S Mar 30 '15 at 12:50 — Arun A S, Mar 30 '15 at 12:50

score 4 · Accepted Answer · answered Mar 30 '15 at 12:51

4

Test a();

Oh, that's an instantiation of a, an object of type Test.

Test a();

Oh, that's a declaration for a function of no arguments that returns something of type Test.

Oh, wait...

If you instead construct a new a(), or use an (empty) initialisation list, this ambiguity is avoided.

See Herb Sutter's excellent article for more.

answered Mar 30 '15 at 12:51

OJFord

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I like the way you answered.Thanks a lot! – James Mar 30 '15 at 16:24

Emil Laine · Answer 2 · 2019-12-19T16:57:17.927

3

Test a(); is interpreted as a declaration of a function named a that takes no parameters and returns an object of type Test.

To create an object, remove the parentheses:

Test a;

edited Dec 19 '19 at 16:57

answered Mar 30 '15 at 12:51

Emil Laine

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I know, the question is about why Test a(); does not work. Anyway, thank you! – James Mar 30 '15 at 16:25
@James A) No you don't, otherwise you wouldn't have asked. B) I just told you why. – Emil Laine Mar 30 '15 at 16:32

score 1 · Answer 3 · answered Mar 30 '15 at 12:54

1

remove the parenthesis after Test a(); it should be like test a; and it will automatically trigger the constructor with no arguments

answered Mar 30 '15 at 12:54

Sandeep Choudhary

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Constructor without arguments doesn't work

3 Answers3