C++ .NET - how to dispose a bitmap?

问题

I want to load a bitmap from file, perform some operations on it, and save it back under the same file name. The pattern is this:

Bitmap in = gcnew Bitmap(fileName);
Bitmap out = gcnew Bitmap(in.Width, in.Height, in.PixelFormat);

fill [out] with data from [in]

out.Save(fileName);

but this doesn't work. That's obvious. I cannot save to a file which is still opened (because of bitmap in). The question is: how the heck do I close bitmap in?! I've tried many ways but nothing works. Calling Dispose worked in C# but this method is protected in C++. Calling delete also doesn't work. What's the solution?

EDIT: Operating on one bitmap doesn't work either. But I found a problem. Calling delete worked. I forgot to declare my bitmaps as pointers

Bitmap^ in = gcnew Bitmap(fileName);
Bitmap^ out = gcnew Bitmap(in.Width, in.Height, in.PixelFormat);

fill [out] with data from [in]

delete in;
out.Save(fileName);

回答1:

This is a common trap in C++/CLI coding, you are using stack semantics. In other words, you didn't declare the reference type variable with the ^ hat. Which makes the compiler automatically emit the Dispose() call at the end of the scope block. Very convenient and a simulation of the RAII pattern in C++ but it gets in the way here. You want to dispose the in bitmap before saving the new bitmap.

Two ways to do this. You could play a game with the scope blocks by adding braces:

Bitmap^ out;
try {
    {
        Bitmap in(fileName);
        out = gcnew Bitmap(in.Width, in.Height, in.PixelFormat);
        // etc..
    }   // <== "in" gets disposed here
    out->Save(fileName);
}
finally {
    delete out;
}

But that's kinda ugly, especially since it needs to be mixed up for out in this very specific case. The alternative is to just do everything explicitly:

Bitmap^ out;
Bitmap^ in;
try {
    in = gcnew Bitmap(fileName);
    out = gcnew Bitmap(in->Width, in->Height, in->PixelFormat);
    // etc..
    delete in;
    in = nullptr;
    out->Save(fileName);
}
finally {
    delete in;
    delete out;
}

回答2:

You don't need an out Bitmap. Just edit in and save it. Also I'd advise using the CImage class instead

CImage image;
image.Load(filename);
fill [image] with whatever data you want
image.Save(filename);

来源：https://stackoverflow.com/questions/10673468/c-net-how-to-dispose-a-bitmap

标签

c++

.net

bitmap

c++-cli