Exception Unhandled while trying to initialize dynamic array

Question

I'm allocating dynamic memory in my class as a private: variable

Then in the constructor I'm trying to initialize the array.

public
Display(int Width, int Height) {
    nScreenHeight = Height;
    nScreenWidth = Width;
    DWORD dwBytesWritten = 0;
    for (int i = 0; i < (nScreenWidth*nScreenHeight); i++) screen[i] = L'';
    SetConsoleActiveScreenBuffer(hConsole);
}

private:
    int nScreenWidth;
    int nScreenHeight;
    wchar_t *screen = new wchar_t[nScreenWidth*nScreenHeight];

If I try run the program an Exception Unhanded in thrown.

Unhandled exception thrown: write access violation. this->screen was 0x2096112.

While trying to initialize the buffer screen with L' '

wchar_t *screen = new wchar_t[nScreenWidth*nScreenHeight]; nScreenHeight and nScreenWidth are not initailized when this executes. Initialize screen in your constructor. — drescherjm
– drescherjm, Commented May 28, 2019 at 17:35
wchar_t *screen = new wchar_t[nScreenWidth*nScreenHeight]; doesn't do what you think it does. I wonder if this even compiles. Put screen = new wchar_t[nScreenWidth*nScreenHeight] to your constructor initializer list at least. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented May 28, 2019 at 17:36
Unrelated, out of curiosity, what does L'' actually evaluate as? I'm surprised that even compiles. — WhozCraig
– WhozCraig, Commented May 28, 2019 at 17:40
It is type LPCWSTR simply trying to clear the buffer with empty spaces. But indeed I was messing up what is where. — Metio_1993
– Metio_1993, Commented May 28, 2019 at 17:42
@WhozCraig I know, right? I was expecting L"", (a pointer to a null-terminated wchar_t string) not L'' . L'' should result in a compiler error: error: empty character constant. — Wyck
– Wyck, Commented May 28, 2019 at 18:05

463035818_is_not_an_ai · Accepted Answer · 2019-05-28 18:36:54Z

3

Memebers are initialized in the order they appear in the class declaration before the body of the constructor is executed. So what happens is:

nScreenWidth is default initialized (see here)
nScreenHeight is default initialized
screen is initialized with new wchar_t[nScreenWidth*nScreenHeight]
only now the constructor runs and you assign values to nScreenWidth and nScreenHeight

Use the initializer list for all members to avoid confusion:

Display(int Width, int Height) : 
   nScreenWidth(Width), nScreenHeight(Height), screen(new wchar_t[nScreenWidth*nScreenHeight])
{
    DWORD dwBytesWritten = 0;
    for (int i = 0; i < (nScreenWidth*nScreenHeight); i++) screen[i] = L'';
    SetConsoleActiveScreenBuffer(hConsole);
}

Even better would be to use a std::vector<wchar_t>, the constructor would be

Display(int Width, int Height) : 
       nScreenWidth(Width), nScreenHeight(Height), screen(Width*Height)
{
    ...

and instead of bothering about the rule of three/five you can rely on the rule of zero.

edited May 28, 2019 at 18:36

answered May 28, 2019 at 17:43

463035818_is_not_an_ai

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3 Comments

463035818_is_not_an_ai Over a year ago

@WhozCraig cannot believe I missed a chance to promote std::vector ;)

PapaDiHatti Over a year ago

@ formerlyknownas_463035818 as you mentioned that members are initialized in order they appear in class and after that constructor runs so could not understand why exception occurred ?

463035818_is_not_an_ai Over a year ago

@Kapil oh i didnt mention it explicitly. Look at my bulletpoints again, when you initialize the vector you initialize it with 0 size, because only later you assign the passed size to the members. Then when you try to access elements of the array you go out of bounds

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