Why is the constructor not called when () is used to declare an object?

Question

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Why is it an error to use an empty set of brackets to call a constructor with no arguments?

$ cat cons.cpp
#include <iostream>

class Matrix {
private:
    int m_count;

public:
    Matrix() {
        m_count = 1;
        std::cout << "yahoo!" << std::endl;
    }
};

int main() {
    std::cout << "before" << std::endl;
    Matrix m1();                         // <----
    std::cout << "after" << std::endl;
}
$ g++ cons.cpp
$ ./a.out
before
after
$

What does the syntax Matrix m1(); do?

I believed that it is the same as Matrix m1;. Obviously I am wrong.

score 12 · Accepted Answer · edited Feb 26 '12 at 11:11

12

Matrix m1(); // m1 is a function whose return type is Matrix.

Also this C++ FAQ lite entry should be helpful.

Is there any difference between List x; and List x();

edited Feb 26 '12 at 11:11

Peter Mortensen

30,738
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answered Feb 26 '12 at 07:47

Mahesh

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1

@Lazer: to convince yourself, try to use m1 (for example accessing m1.m_count )... – Francesco Feb 26 '12 at 07:50
In C++11 you can use braces for all constructor calls this avoids c++'s most vexing parse i.e Matrix m1{}; – mark Feb 26 '12 at 08:58
This isn't the "most vexing parse", the most vexing parse is when you try to declare and object and pass a value initialized temporary to one or more of it's constructor parameters. E.g. `A b(A());`. `Matrix m1();` is just a regular function declaration. – CB Bailey Feb 26 '12 at 09:14
@CharlesBailey: the Most Vexing Parse is about treating a declaration as a function declaration if at all possible. This question is indeed just yet another Most Vexing Parse question. – Cheers and hth. - Alf Feb 26 '12 at 10:58

score 3 · Answer 2 · answered Feb 26 '12 at 07:51

Matrix m1() declares a function that takes no parameters and returns a Matrix. You can see this is so by adding a method to Matrix and trying to invoke it on m1:

#include <iostream>

class Matrix {
private:
    int m_count;

public:
    Matrix() {
        m_count = 1;
        std::cout << "yahoo!" << std::endl;
    }
    void foo() {}
};

int main() {
    std::cout << "before" << std::endl;
    Matrix m1();
    m1.foo();
    std::cout << "after" << std::endl;
}

gives error: request for member 'foo' in 'm1', which is of non-class type 'Matrix()'

score 1 · Answer 3 · answered Feb 26 '12 at 09:00

Think from C language perspective:

int data_member();

is actually a prototype for function taking void and returning int. When you change it like:

T data();

it still is a function declaration, retuning T. When you need to declare it as variable, you do:

T data; // int data;

score 0 · Answer 4 · answered Feb 26 '12 at 08:34

This will do what you're trying to do:

int main() {
    std::cout << "before" << std::endl;
    Matrix m1;                         // <----
    std::cout << "after" << std::endl;
}

In C++, if you initialize a variable with parens, it actually declares a function that takes no parameters and returns that type.

Why is the constructor not called when () is used to declare an object?

4 Answers4