gecko-dev/xpcom/sample/nsSample.cpp

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 *
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 * Contributor(s):
 *   Pierre Phaneuf <pp@ludusdesign.com>
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/**
 *
 * A sample of XPConnect. This file contains an implementation nsSample
 * of the interface nsISample.
 *
 */
#include <stdio.h>

#include "nsSample.h"
#include "nsMemory.h"

////////////////////////////////////////////////////////////////////////

nsSampleImpl::nsSampleImpl() : mValue(nsnull)
{
    NS_INIT_ISUPPORTS();
    mValue = (char*)nsMemory::Clone("initial value", 14);
}

nsSampleImpl::~nsSampleImpl()
{
    if (mValue)
        nsMemory::Free(mValue);
}

/**
 * NS_IMPL_ISUPPORTS1 expands to a simple implementation of the nsISupports
 * interface.  This includes a proper implementation of AddRef, Release,
 * and QueryInterface.  If this class supported more interfaces than just
 * nsISupports, 
 * you could use NS_IMPL_ADDREF() and NS_IMPL_RELEASE() to take care of the
 * simple stuff, but you would have to create QueryInterface on your own.
 * nsSampleFactory.cpp is an example of this approach.
 * Notice that the second parameter to the macro is name of the interface, and
 * NOT the #defined IID.
 *
 * The _CI variant adds support for nsIClassInfo, which permits introspection
 * and interface flattening.
 */
NS_IMPL_ISUPPORTS1_CI(nsSampleImpl, nsISample);

/**
 * Notice that in the protoype for this function, the NS_IMETHOD macro was
 * used to declare the return type.  For the implementation, the return
 * type is declared by NS_IMETHODIMP
 */
NS_IMETHODIMP
nsSampleImpl::GetValue(char** aValue)
{
    NS_PRECONDITION(aValue != nsnull, "null ptr");
    if (! aValue)
        return NS_ERROR_NULL_POINTER;

    if (mValue) {
        /**
         * GetValue's job is to return data known by an instance of
         * nsSampleImpl to the outside world.  If we  were to simply return 
         * a pointer to data owned by this instance, and the client were to
         * free it, bad things would surely follow.
         * On the other hand, if we create a new copy of the data for our
         * client, and it turns out that client is implemented in JavaScript,
         * there would be no way to free the buffer.  The solution to the 
         * buffer ownership problem is the nsMemory singleton.  Any buffer
         * returned by an XPCOM method should be allocated by the nsMemory.
         * This convention lets things like JavaScript reflection do their
         * job, and simplifies the way C++ clients deal with returned buffers.
         */
        *aValue = (char*) nsMemory::Clone(mValue, strlen(mValue) + 1);
        if (! *aValue)
            return NS_ERROR_NULL_POINTER;
    }
    else {
        *aValue = nsnull;
    }
    return NS_OK;
}

NS_IMETHODIMP
nsSampleImpl::SetValue(const char* aValue)
{
    NS_PRECONDITION(aValue != nsnull, "null ptr");
    if (! aValue)
        return NS_ERROR_NULL_POINTER;

    if (mValue) {
        nsMemory::Free(mValue);
    }

    /**
     * Another buffer passing convention is that buffers passed INTO your
     * object ARE NOT YOURS.  Keep your hands off them, unless they are
     * declared "inout".  If you want to keep the value for posterity,
     * you will have to make a copy of it.
     */
    mValue = (char*) nsMemory::Clone(aValue, strlen(aValue) + 1);
    return NS_OK;
}

NS_IMETHODIMP
nsSampleImpl::Poke(const char* aValue)
{
    return SetValue((char*) aValue);
}

NS_IMETHODIMP
nsSampleImpl::WriteValue(const char* aPrefix)
{
    NS_PRECONDITION(aPrefix != nsnull, "null ptr");
    if (! aPrefix)
        return NS_ERROR_NULL_POINTER;

    printf("%s %s\n", aPrefix, mValue);
    return NS_OK;
}