Dynamic Invocation and Dispatch in C++

This page describes the C++ mapping for the ice_invoke proxy function and the Blobject class.

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ice_invoke in C++

The mapping for ice_invoke is shown below:

C++
bool ice_invoke(
    const std::string& operation,
    Ice::OperationMode mode,
    const std::vector< Ice::Byte >& inParams,
    std::vector< Ice::Byte >& outParams
);

Another overloading of ice_invoke (not shown) adds a trailing argument of type Ice::Context.

As an example, the code below demonstrates how to invoke the operation op, which takes no in parameters:

C++
Ice::ObjectPrx proxy = ...
try {
    std::vector<Ice::Byte> inParams, outParams;
    if (proxy->ice_invoke("op", Ice::Normal, inParams, outParams)) {
        // Handle success
    } else {
        // Handle user exception
    }
} catch (const Ice::LocalException& ex) {
    // Handle exception
}

As a convenience, the Ice run time accepts an empty byte sequence when there are no input parameters and internally translates it into an empty encapsulation. In all other cases, the value for inParams must be an encapsulation of the encoded parameters.

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Using Streams with ice_invoke in C++

The streaming interfaces provide the tools an application needs to dynamically invoke operations with arguments of any Slice type. Consider the following Slice definition:

Slice
module Calc {
    exception Overflow {
        int x;
        int y;
    };
    interface Compute {
        idempotent int add(int x, int y)
            throws Overflow;
    };
};

Now let's write a client that dynamically invokes the add operation:

C++
Ice::ObjectPrx proxy = ...
try {
    std::vector< Ice::Byte > inParams, outParams;

    Ice::OutputStreamPtr out = Ice::createOutputStream(communicator);
    out->startEncapsulation();
    out->writeInt(100); // x
    out->writeInt(-1);  // y
    out->endEncapsulation();
    out->finished(inParams);

    if (proxy->ice_invoke("add", Ice::Idempotent, inParams, outParams)) {
        // Handle success
        Ice::InputStreamPtr in = Ice::createInputStream(communicator, outParams);
        in->startEncapsulation();
        int result = in->readInt();
        in->endEncapsulation();
        assert(result == 99);
    } else {
        // Handle user exception
    }
} catch (const Ice::LocalException& ex) {
    // Handle exception
}

You can see here that the input and output parameters are enclosed in encapsulations.

We neglected to handle the case of a user exception in this example, so let's implement that now. We assume that we have compiled our program with the Slice-generated code, therefore we can call throwException on the input stream and catch Overflow directly:

C++
    if (proxy->ice_invoke("add", Ice::Idempotent, inParams, outParams)) {
        // Handle success
        // ...
    } else {
        // Handle user exception
        Ice::InputStreamPtr in = Ice::createInputStream(communicator, outParams);
        try {
            in->startEncapsulation();
            in->throwException();
        } catch (const Calc::Overflow& ex) {
            cout << "overflow while adding " << ex.x << " and " << ex.y << endl;
        } catch (const Ice::UserException& ex) {
            // Handle unexpected user exception
        }
    }

This is obviously a contrived example: if the Slice-generated code is available, why bother using dynamic dispatch? In the absence of Slice-generated code, the caller would need to manually unmarshal the user exception, which is outside the scope of this manual.

As a defensive measure, the code traps Ice::UserException. This could be raised if the Slice definition of add is modified to include another user exception but this segment of code did not get updated accordingly.

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Subclassing Blobject in C++

Implementing the dynamic dispatch model requires writing a subclass of Ice::Blobject. We continue using the Compute interface to demonstrate a Blobject implementation:

C++
class ComputeI : public Ice::Blobject {
public:
    virtual bool ice_invoke(
        const std::vector<Ice::Byte>& inParams,
        std::vector<Ice::Byte>& outParams,
        const Ice::Current& current);
};

An instance of ComputeI is an Ice object because Blobject derives from Object, therefore an instance can be added to an object adapter like any other servant.

For the purposes of this discussion, the implementation of ice_invoke handles only the add operation and raises OperationNotExistException for all other operations. In a real implementation, the servant must also be prepared to receive invocations of the following Object operations:

  • string ice_id()
    Returns the Slice type ID of the servant's most-derived type.
  • StringSeq ice_ids()
    Returns a sequence of strings representing all of the Slice interfaces supported by the servant, including "::Ice::Object".
  • bool ice_isA(string id)
    Returns true if the servant supports the interface denoted by the given Slice type ID, or false otherwise. This operation is invoked by the proxy function checkedCast.

With that in mind, here is our simplified version of ice_invoke:

C++
bool ComputeI::ice_invoke(
    const std::vector<Ice::Byte>& inParams,
    std::vector<Ice::Byte>& outParams,
    const Ice::Current& current)
{
    if (current.operation == "add") {
        Ice::CommunicatorPtr communicator = current.adapter->getCommunicator();
        Ice::InputStreamPtr in = Ice::createInputStream(communicator, inParams);
        in->startEncapsulation();
        int x = in->readInt();
        int y = in->readInt();
        in->endEncapsulation();
 
        Ice::OutputStreamPtr out = Ice::createOutputStream(communicator);
        if (checkOverflow(x, y)) {
            Calc::Overflow ex;
            ex.x = x;
            ex.y = y;
            out->startEncapsulation();
            out->writeException(ex);
            out->endEncapsulation();
            out->finished(outParams);
            return false;
        } else {
            out->startEncapsulation();
            out->writeInt(x + y);
            out->endEncapsulation();
            out->finished(outParams);
            return true;
        }
    } else {
        Ice::OperationNotExistException ex(__FILE__, __LINE__);
        ex.id = current.id;
        ex.facet = current.facet;
        ex.operation = current.operation;
        throw ex;
    }
}

If an overflow is detected, the code "raises" the Calc::Overflow user exception by calling writeException on the output stream and returning false, otherwise the return value is encoded and the function returns true.

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Using the Array Mapping for ice_invoke and Blobject in C++

Ice for C++ supports an alternative mapping for sequence input parameters that avoids the overhead of extra copying. Since the ice_invoke functions treat the encoded input parameters as a value of type sequence<byte>, the dynamic invocation and dispatch facility includes interfaces that use the array mapping for the input parameter blob.

Ice provides two overloaded versions of the proxy function ice_invoke that use the array mapping. The version that omits the trailing Ice::Context argument is shown below:

C++
bool ice_invoke(
    const std::string& operation,
    Ice::OperationMode mode,
    const std::pair< const Ice::Byte*, const Ice::Byte* >& in,
    std::vector< Ice::Byte >& out
);

A Blobject servant uses the array mapping by deriving its implementation class from Ice::BlobjectArray and overriding its ice_invoke function:

C++
class BlobjectArray {
public:
    virtual bool ice_invoke(
        const std::pair<const Ice::Byte*, const Ice::Byte*>& in,
        std::vector<Ice::Byte>& out,
        const Ice::Current& current) = 0;
};

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See Also