constructor with one, default argument

Question

i searched but could not find the answer.

So I have my c++ constructor:

MyClass(string username = "something");

note this is the only constructor I have.

in my main, I do:

MyClass one();
MyClass two = MyClass();

are these two expressions equivalent? is the compiler gonna call my constructor with the default string, or is it gonna call the default (empty) constructor?

What would change if I did have a constructor MyClass(); ? I guess that would not compile, right?

`MyClass one();` [Refferred to as the most Vexing parse](http://stackoverflow.com/questions/1424510/most-vexing-parse-why-doesnt-a-a-work) Its not actually a declaration of the variable one. But a forward declaration of a function called one. — Martin York, Mar 19 '11 at 07:27

Prasoon Saurav · Answer 1 · 2011-03-19T05:18:30.333

4

MyClass one();

This declares a function one returning a MyClass object and taking no arguments.

If you have both default constructor (of the form MyClass()) and default argument constuctor (of the form MyClass(string s = "string") ) which one would be called if you don't pass any argument?

For example this would not compile

class MyClass
{
  public:
   MyClass(std::string username = "something") {}
   MyClass(){}
};

int main()
{
   MyClass one();
   MyClass two = MyClass(); //ambiguous call here
}

edited Mar 19 '11 at 05:18

answered Mar 19 '11 at 04:30

Prasoon Saurav

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Also, `two = MyClass()` will not compile because the type is missing. – Jon Mar 19 '11 at 04:31
@Jon : I am assuming that he somehow missed the type. – Prasoon Saurav Mar 19 '11 at 04:35
Unless it is previously declared in that function, or defined in the class. By itself, you're right. – Merlyn Morgan-Graham Mar 19 '11 at 04:36
@Prasoon: I know -- it's just for the sake of completeness of the answer. – Jon Mar 19 '11 at 04:38
"You can't have both default constructor and default argument constructor defined within a class." this is incorrect. and it's sort of fractally incorrect: both constructors in the OP's example are default constructors, and yes you can have two or more of them. it makes it practically impossible to call the one that doesn't have formal arguments, but it's OK to have it there. Please correct. – Cheers and hth. - Alf Mar 19 '11 at 05:08
@Alf : My mistake. Corrected!! – Prasoon Saurav Mar 19 '11 at 05:19
@Martin : Where did I mention that it declares a variable? – Prasoon Saurav Mar 19 '11 at 07:23
There are thousands of folks around on SO. Why do you think it's @Martin who downvoted you? – Johannes Schaub - litb Mar 19 '11 at 11:54
@Johannes Schaub - litb : That was a wild guess. :P – Prasoon Saurav Mar 19 '11 at 11:56
@Prasoon Saurav: Not me. But it was late last night. Removed comments. But the explanation could be clearer. – Martin York Mar 19 '11 at 15:50

score 2 · Answer 2 · answered Mar 19 '11 at 06:47

2

MyClass one;
MyClass two = MyClass();

If this is what was meant then one and two will call the same constructor, which happens to be your constructor, which happens to be the only constructor.

answered Mar 19 '11 at 06:47

travis

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1

score 0 · Answer 3 · answered Mar 19 '11 at 04:31

It will call your constructor. If you define a constructor yourself--any constructor regardless of the number of arguments--the compiler will not generate a default constructor.

[edit] As Prasoon Saurav says, MyClass one(); declares a function. You want MyClass one;, note: no parentheses.

score 0 · Answer 4 · answered Mar 19 '11 at 04:31

0

The expressions will have the same nett effect, but the aren't strictly equivalent. The first calls your constructor on one. The second constructs a temporary and then assigns it to two, thus invoking the class's copy-constructor.

answered Mar 19 '11 at 04:31

Marcelo Cantos

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While the copy-constructor call can be optimized away, it still has to be accessible. – Ben Voigt Mar 19 '11 at 05:23

score 0 · Answer 5 · answered Apr 16 '19 at 15:50

are these two expressions equivalent?

They are not equivalent.

MyClass one();

it is function one(), return MyClass.

if you want to declare object of MyClass, It should be:

MyClass one;

or

MyClass one = MyClass();

for question:

is the compiler gonna call my constructor with the default string, or is it gonna call the default (empty) constructor?

if you declare your own constructor, compiler will never generate default constructor, then there is no default constructor now. for example:

MyClass(string username); // declare constructor take 1 parameter
MyClass three; // error, no matching constructor

constructor with one, default argument

5 Answers5