I am working on a game with a friend and he recently checked this in:
Vector3 velEmtr = new Vector3(0.0, 0.0, 0.0);
Which to my amazement compiles and runs. I tried searching the internet for explanations but have found none. Does anyone have any insight into why this works?
I did try doing this with classes exported by libraries we are using and it did show the error I expected:
'initializing' : cannot convert from '<type> *' to '<type>'
My other question, is if that works does that memory need to be deleted?
C++: Vector3 velEmtr = new Vector3(0.0, 0.0, 0.0) compiles and runs, how?
Yep that looks like a memory leak combined with a strangely designed Vector3 class.
What does Vector3's definition look like? I'm guessing it has a constructor along the lines of: Vector3( const Vector3* o ) : m_x(o->m_x) ... or a similar assignment operator?
[Edit] BTW tell your friend to get away from Java/C# ;) (j/k)
What does Vector3's definition look like? I'm guessing it has a constructor along the lines of: Vector3( const Vector3* o ) : m_x(o->m_x) ... or a similar assignment operator?
[Edit] BTW tell your friend to get away from Java/C# ;) (j/k)
Author
Ok, yes he did just add this to the Vector3 class:
So thats the problem?
Thank you very much, for some reason I didn't even consider looking in there.
Vector3(Vector3* initFrom) { x = initFrom->x; y = initFrom->y; z = initFrom->z; };So thats the problem?
Thank you very much, for some reason I didn't even consider looking in there.
Quote:Original post by KevArnold
Which to my amazement compiles and runs. I tried searching the internet for explanations but have found none. Does anyone have any insight into why this works?
- Custom operator and function overloading
- Implicit conversion
- Assignment operator, copy constructor and implicit/explicit constructor rules
- SFINAE and Koenig lookup
Big can of worms in general with lots of undefined or problematic spots.
Quote:Original post by KevArnoldYes. Assuming that's supposed to be a copy constructor, the correct version would be:
So thats the problem?
Vector3(const Vector3& initFrom){ x = initFrom.x; y = initFrom.y; z = initFrom.z;};
Quote:Original post by Evil SteveQuote:Original post by KevArnoldYes. Assuming that's supposed to be a copy constructor, the correct version would be:
So thats the problem?
*** Source Snippet Removed ***
Even if it wasn't supposed to be a copy constructor thats better as it's easy to dereference a pointer on a function call, and would avoid shenanigans like this.
Quote:Original post by KevArnold
Ok, yes he did just add this to the Vector3 class:
*** Source Snippet Removed ***
So thats the problem?
Thank you very much, for some reason I didn't even consider looking in there.
Tag that constructor with the explicit keyword. As a general rule, you should tag any constructor that takes 1 argument with 'explicit'. Your problem is similar to this:
int main(){ Vector3* myVector = new Vector3(2.0, 4.0, 3.0); printf("myVector: %p", &myVector); printVector(myVector);}void printVector(const Vector& aVector){ printf("aVector: %p", &aVector);}Example Output:
myVector: 0x11111111aVector: 0x22222222Why? Because the aVector can be implicitly constructed using a Vector3 pointer with the Vector3(Vector3*) constructor. You supplied the wrong type for the function, so it looks to see if it can implicitly construct the right type with the argument you gave it. Since there's a constructor not tagged 'explicit' that can do the job, it uses that.
Thus, your error is hidden and you can get weird and subtle bugs.
More about 'explicit': http://www.glenmccl.com/tip_023.htm
Yeah, the explicit keyword would certainly be beneficial here =)
Seemingly little-known feature of C++ as well, haven't met many folks that know about it.
Seemingly little-known feature of C++ as well, haven't met many folks that know about it.
It's used all the time although you mightn't notice it:
std::string s = "Hello World";
Wouldn't be possible with an explicit constructor. You'd need to use
std::string s("Hello World");
std::string s = "Hello World";
Wouldn't be possible with an explicit constructor. You'd need to use
std::string s("Hello World");
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