C++ one argument boolean constructor and "new" keyword: Logic Error

Question

I have the following class in c++ ``` class MyClass { public: bool flag; MyClass (bool flag = false) { this->flag = flag; } ... }; ``` In my main (used for testing) I have something like so ``` int main() { MyClass number1 = MyClass(true); MyClass number2 = MyClass(false); MyClass number3 = new MyClass(false); MyClass number4 = new MyClass(true); MyClass number5 = new MyClass(); std::cout << number1.flag << number2.flag << std::endl; std::cout << number3.flag << number4. flag << number5.flag << std::endl; } ``` And my output (imagine boolean outputs true or false and the output is formatted better) ``` true, false true, true, true ``` The problem is when I use the "new" keyword to make a "MyClass" object it will always set the flag variable to true regardless of what I do or don't put into the argument constructor. It works for the constructors not using "new" but not the ones using it. The only real difference I know between using and not using the keyword new is that if you don't us…

Accepted Answer

The pointer returned by `new` gets implicitly converted to `bool`, which is then passed to `MyClass(bool)`.

In other words, the following:

```
MyClass number3 = new MyClass(false);
```

is equivalent to:

```
MyClass number3(bool(new MyClass(false)));
```

Since the pointer returned by `new` is never null, the conversion always yields `true`, which is what gets passed to `number3`'s constructor. The newly allocated pointer gets leaked.

You probably meant to say:

```
 MyClass* number3 = new MyClass(false);
 MyClass* number4 = new MyClass(true);
 MyClass* number5 = new MyClass();
```

(note the asterisks.)

You would not have had this problem if you declared the constructor `explicit`:

```
  explicit MyClass (bool flag = false) {
    this->flag = flag;
  }
```

The code would fail to compile, which would immediate tell you that something was amiss.

C++ one argument boolean constructor and "new" keyword: Logic Error

Solution

Problem