struct constructor error, why use default constructor function will call a error?

Question

#include <iostream>
using namespace std;

struct Foo{
    Foo(){}
    Foo(int){}
    void fun(){}
};

void main()
{
    Foo a(10);
    a.fun();
    Foo b();
    b.fun();//error
    Foo c = Foo(); // this is the right way to use default constructor?
    c.fun();
}

The code has an error when compiling, because b is not a type of class, who can tell me what is b? And the meaning of Foo b()?

score 1 · Accepted Answer · edited May 23 '17 at 12:05

1

This problem is commonly known as C++ "most vexing parse".

Foo b();

This declares a function named b returning Foo.

It should be just;

Foo b;

This will declare a variable b of type Foo that will be default initialised (Foo has a default constructor). Foo c = Foo(); yes, this is an alternative default initialisation (copy initialisation) but it is generally used more with POD data types.

It is worth noting that with C++11 (uniform initialisation), the following will compile and is possibly an alternative for you

Foo b{};

But it does the same thing in this case (with respect to constructing b).

edited May 23 '17 at 12:05

Community

1
1

answered Aug 29 '14 at 07:50

Niall

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This is vexing, but doesn't quite qualify as *most vexing*. – T.C. Aug 29 '14 at 08:45
@T.C., :), Scott did write that around 2001, so in the 13 or so years since, things may have got worse... – Niall Aug 29 '14 at 08:49
`std::vector v(std::istream_iterator(ifs), std::istream_iterator());` is the standard "most vexing" example :) – T.C. Aug 29 '14 at 08:51

score 0 · Answer 2 · answered Aug 29 '14 at 07:51

0

The right way should be:

Foo b;

You just declared a function named b.

answered Aug 29 '14 at 07:51

songyuanyao

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struct constructor error, why use default constructor function will call a error?

2 Answers2