How does this conversion to bool work?

Question

I'm learning the Cinder framework. There is a class Texture in this framework, and it can be used like this:

Texture myImage;
myImage.loadImage(/*...*/);
if(myImage)
{
    // draw the image.
}

I got confused about this, because myImage is an object. Using it as a condition doesn't make sense to me. I expected something like myImage.exist();. So I stepped through the code, and it turns out that Texture class has a conversion operator defined:

public:
    //@{
    //! Emulates shared_ptr-like behavior
    typedef std::shared_ptr<Obj> Texture::*unspecified_bool_type;
    // What is this???
    operator unspecified_bool_type() const { return ( mObj.get() == 0 ) ? 0 : &Texture::mObj; }
    void reset() { mObj.reset(); }
    //@}  

Obj is defined as:

protected:      
    struct Obj {
        Obj() : mWidth( -1 ), mHeight( -1 ), mCleanWidth( -1 ), mCleanHeight( -1 ), mInternalFormat( -1 ), mTextureID( 0 ), mFlipped( false ), mDeallocatorFunc( 0 ) {}
        Obj( int aWidth, int aHeight ) : mInternalFormat( -1 ), mWidth( aWidth ), mHeight( aHeight ), mCleanWidth( aWidth ), mCleanHeight( aHeight ), mFlipped( false ), mTextureID( 0 ), mDeallocatorFunc( 0 )  {}
        ~Obj();

        mutable GLint   mWidth, mHeight, mCleanWidth, mCleanHeight;
        float           mMaxU, mMaxV;
        mutable GLint   mInternalFormat;
        GLenum          mTarget;
        GLuint          mTextureID;
        bool            mDoNotDispose;
        bool            mFlipped;   
        void            (*mDeallocatorFunc)(void *refcon);
        void            *mDeallocatorRefcon;            
    };
    std::shared_ptr<Obj>        mObj;

I know that operator int() const can implictly change the Object to int, but how is unspecified_bool_type working? The debugger stops at operator unspecified_bool_type() const { return ( mObj.get() == 0 ) ? 0 : &Texture::mObj; } when if(myImage) is executing.

And I may be a little confused about the grammar here, what does

typedef std::shared_ptr<Obj> Texture::*unspecified_bool_type;

mean?

And does

void (*mDeallocatorFunc)(void *refcon); 

in Obj mean that mDeallocatorFunc is a member of Class Obj, a function pointer to a function with prototype: void xxx(void *)?

Answer 1

This is the safe bool idiom. It doesn't use simply operator bool() because implicit conversions can cause all kinds of trouble with that operator. So instead it uses a type that is implicitly convertible to bool (like a pointer to member) and that is the least dangerous possible.

Luckily this sort of hack is not required in C++11 because we can write explicit operator bool instead and not fall prey to implicit conversions.