Result and error handling

The Aws::WeaverRuntime::Result<T> class uses a third-party Outcome library. You can use the following pattern to check the Result and catch errors returned by API calls.

void DoBeginUpdate(Application& app)
{
    Result<Transaction> transactionResult = Api::BeginUpdate(app);
    
    if (transactionResult)
    {
        Transaction transaction = 
            std::move(transactionResult).assume_value();
        
        /**
         * Do things with transaction ...
         */
    }
    else
    {     
        ErrorCode errorCode = WEAVERRUNTIME_EXPECT_ERROR(transactionResult);
        /**
         * Macro compiles to:
         * ErrorCode errorCode = transactionResult.assume_error();
         */
    }
}

Result control statement macro

Inside a function with a return type Aws::WeaverRuntime::Result<T>, you can use the WEAVERRUNTIME_TRY macro instead of the previous code pattern. The macro will execute the function passed to it. If the passed function fails, the macro will make the enclosing function return an error. If the passed function succeeds, execution progresses to the next line. The following example shows a rewrite of the previous DoBeginUpdate() function. This version uses the WEAVERRUNTIME_TRY macro instead of the if-else control structure. Note that the return type of the function is Aws::WeaverRuntime::Result<void>.

Aws::WeaverRuntime::Result<void> DoBeginUpdate(Application& app)
{
    /**
     * Execute Api::BeginUpdate() 
     * and return from DoBeginUpdate() if BeginUpdate() fails.
     * The error is available as part of the Result.
     */
    WEAVERRUNTIME_TRY(Transaction transaction, Api::BeginUpdate(m_app));
    
    /**
     * Api::BeginUpdate executed successfully.
     *
     * Do things here.
     */
    
    return Aws::Success();
}

If BeginUpdate() fails, the macro makes DoBeginUpdate() return early with a failure. You can use the WEAVERRUNTIME_EXPECT_ERROR macro to get the Aws::WeaverRuntime::ErrorCode from BeginUpdate(). The following example shows how the Update() function calls DoBeginUpdate() and gets the error code on failure.

void Update(Application& app)
{
    Result<void> doBeginUpdateResult = DoBeginUpdate(app);
    
    if (doBeginUpdateResult)
    {
        /**
         * Successful.
         */
    }
    else
    {    
        /**
         * Get the error from Api::BeginUpdate().
         */ 
        ErrorCode errorCode = WEAVERRUNTIME_EXPECT_ERROR(doBeginUpdateResult);

    }
}

You can make the error code from BeginUpdate() available to a function that calls Update() by changing the return type of Update() to Aws::WeaverRuntime::Result<void>. You can repeat this process to keep sending the error code further down the call stack.