problems casting long to int

[hanvyj]

I have a problem casting a long to an int in c++

There is a long "biWidth" in the &pVideoInfoHeader->bmiHeader struct, I simply want to read this into an integer value "Width"

//check the video info header
Width=(int)(&pVideoInfoHeader->bmiHeader.biWidth);
Height=(int)(&pVideoInfoHeader->bmiHeader.biHeight);

//break point here

When I step through the code to the break point after the assignment and use the debugger to check the values it says "&pVideoInfoHeader->bmiHeader.biWidth | 176", which is fine, but "Width | 3253404"

Can anyone help me find out what is going on?

[mike_2000_17]

the (int) cast is from C, and is not recommended in C++ for good reasons. Use the "functional" casting:

//check the video info header
Width  = int(&pVideoInfoHeader->bmiHeader.biWidth);
Height = int(&pVideoInfoHeader->bmiHeader.biHeight);

//break point here

But in this case, long is implicitly convertable to int, so the cast is useless anyways.

[daviddoria]

hanvyj

In cases like this, I'd suggest making a short, compilable program outside of the context of your big program.

#include <iostream>

int main()
{

 long myLong = 20.3;
 std::cout << myLong << " " << (int)myLong << " " << int(myLong) << " " << static_cast<int>(myLong) << std::endl;

 return 0;
}

This should really help you understand what the expected results are. Once you know this, you should be able to more easily find the problem in your real code.

David

[hanvyj]

Thank you for both your replies, Once I tested it as you suggested, it should work. I realised that it was a pointer to a long, what a silly mistake!

[thelamb]

Or look up the *_cast statements in C++.
e.g.
static_cast

[Luther von Wulf]

the (int) cast is from C, and is not recommended in C++ for good reasons. Use the "functional" casting:

//check the video info header
Width  = int(&pVideoInfoHeader->bmiHeader.biWidth);
Height = int(&pVideoInfoHeader->bmiHeader.biHeight);

//break point here

The functional cast is still the same thing as the C cast, and it has all of the same problems. The only difference is more of a constructor look in the syntax. The real C++ casts that are recommended for better code are named and specialized. In the case of explicitly casting from long to int, the static_cast operator is the recommended C++ tool.

Width = static_cast<int>(pVideoInfoHeader->bmiHeader.biWidth);
Height = static_cast<int>(pVideoInfoHeader->bmiHeader.biHeight);

[Stefano Mtangoo]

And there is nice and concise tutorial on type casting right here