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develop/doc/_sources/design/cpp_data_feeding.md.txt

+# C++ Data Feeding
+
+In training with Paddle V2 API, data feeding wholly dependents on Python code. To get rid of the Python environment and achieve the goal of "wrapping the whole training by a while loop op" in Paddle Fluid, a C++ data feeding mechanism is required. 
+
+In this document we show the fundamental design of C++ data feeding process, which includes the data reading, shuffling and batching.
+
+## Reader
+
+A new concept named 'Reader' is introduced. `Reader` is a series of inherited classes which can be hold by our `Variable` and they are used to read or process file data.
+
+
+### `ReaderBase`
+
+`ReaderBase` is the abstract base class of all readers. It defines the all readers' interfaces.
+
+```cpp
+class ReaderBase {
+ public:
+  explicit ReaderBase(const std::vector<DDim>& shapes) : shapes_(shapes) {
+    PADDLE_ENFORCE(!shapes_.empty());
+  }
+  // Read the next batch of data. (A 'batch' can be only one instance)
+  virtual void ReadNext(std::vector<LoDTensor>* out) = 0;
+  // Show whether the next bacth exists.
+  virtual bool HasNext() const = 0;
+  
+  // Reinitialize the reader and read the file from the begin.
+  virtual void ReInit() = 0;
+  
+  // Get a certain read in data's shape.
+  DDim shape(size_t idx) const;
+  // Get shapes of all read in data.
+  std::vector<DDim> shapes() const { return shapes_; }
+  // Set shapes of read in data.
+  void set_shapes(const std::vector<DDim>& shapes) { shapes_ = shapes; }
+
+  virtual ~ReaderBase() {}
+
+ protected:
+  std::vector<DDim> shapes_;
+};
+```
+
+### `FileReader` and `DecoratedReader`
+
+These two classes are derived from the `ReaderBase` and will further be derived by respective specific readers. That is to say, in our design, there are two kinds of readers: file readers and decorated readers. A file reader reads from a file of some specific format, and yield only one instance of data at a time. e.g. RecordIO reader, jpg reader, .... A decorated reader takes another reader(both file reader and decorated reader are OK) as its 'underlying reader'. It gets data from its underlying reader, does some process on them(shuffling, or batching), then yields processed data. The output data of a decorated reader can be a single instance or a batch. `ShuffleReader` and `BatchReader` are both decorated readers.
+
+All the readers share exactly the same interfaces defined in `ReaderBase`. So they can be decorated for more than one time: We can **shuffle** a reader's outputs and then **batch** the shuffle outputs. The interface consistency also allows related ops use readers without knowing what they are exactly.
+
+
+### `ReaderHolder`
+
+Different readers belong to different class types. It leads to a problem: How can we drop them into `Variable`s and fetch them out by a unified method? For example, if a Variable holds a `BatchReader`, we can not get it by the following code:
+
+```cpp
+var->Get<ReaderBase>("batch_reader");
+```
+
+we have to write:
+
+```cpp
+var->Get<BatchReader>("batch_reader");
+```
+
+This requires each time getting a reader from a variable we must know the reader's type exactly. It is nearly impossible.
+
+To solve this problem, we introduce `ReaderHolder` as a wrapper. It acts as an empty decorator of `ReaderBase`, which erases reader's type. With `ReaderHolder` we are able to fetch all types of readers by `var->Get<ReaderHolder>("...")` and regard the obtained object as a reader.
+
+## Related Operators
+
+To create and invoke readers, some now ops are introduced:
+
+### `CreateReaderOp`
+
+Each reader has its creating op. File readers' creating ops have no input and yield the created file reader as its output. Decorated readers' creating ops take the underlying readers as inputs and then yield new decorated readers.
+
+### `ReadOp`
+
+A reader is only a Variable. It cannot trigger the reading process by itself. So we add the `ReadOp` to execute it. A `ReadOp` takes a reader Variable as its input. Each time it runs, it invokes the reader‘s `ReadNext()` function and gets a new batch of data(or only one instance of data, if we use file reader directly). The output data of a reader are in the form of `std::vector<LoDTenosr>`, so the `ReadOp` also needs to split the vector and move LoDTensors to their respective output Variables.

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+            
+  <div class="section" id="c-data-feeding">
+<span id="c-data-feeding"></span><h1>C++ Data Feeding<a class="headerlink" href="#c-data-feeding" title="Permalink to this headline">¶</a></h1>
+<p>In training with Paddle V2 API, data feeding wholly dependents on Python code. To get rid of the Python environment and achieve the goal of &#8220;wrapping the whole training by a while loop op&#8221; in Paddle Fluid, a C++ data feeding mechanism is required.</p>
+<p>In this document we show the fundamental design of C++ data feeding process, which includes the data reading, shuffling and batching.</p>
+<div class="section" id="reader">
+<span id="reader"></span><h2>Reader<a class="headerlink" href="#reader" title="Permalink to this headline">¶</a></h2>
+<p>A new concept named &#8216;Reader&#8217; is introduced. <code class="docutils literal"><span class="pre">Reader</span></code> is a series of inherited classes which can be hold by our <code class="docutils literal"><span class="pre">Variable</span></code> and they are used to read or process file data.</p>
+<div class="section" id="readerbase">
+<span id="readerbase"></span><h3><code class="docutils literal"><span class="pre">ReaderBase</span></code><a class="headerlink" href="#readerbase" title="Permalink to this headline">¶</a></h3>
+<p><code class="docutils literal"><span class="pre">ReaderBase</span></code> is the abstract base class of all readers. It defines the all readers&#8217; interfaces.</p>
+<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ReaderBase</span> <span class="p">{</span>
+ <span class="k">public</span><span class="o">:</span>
+  <span class="k">explicit</span> <span class="n">ReaderBase</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">DDim</span><span class="o">&gt;&amp;</span> <span class="n">shapes</span><span class="p">)</span> <span class="o">:</span> <span class="n">shapes_</span><span class="p">(</span><span class="n">shapes</span><span class="p">)</span> <span class="p">{</span>
+    <span class="n">PADDLE_ENFORCE</span><span class="p">(</span><span class="o">!</span><span class="n">shapes_</span><span class="p">.</span><span class="n">empty</span><span class="p">());</span>
+  <span class="p">}</span>
+  <span class="c1">// Read the next batch of data. (A &#39;batch&#39; can be only one instance)</span>
+  <span class="k">virtual</span> <span class="kt">void</span> <span class="n">ReadNext</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">LoDTensor</span><span class="o">&gt;*</span> <span class="n">out</span><span class="p">)</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
+  <span class="c1">// Show whether the next bacth exists.</span>
+  <span class="k">virtual</span> <span class="kt">bool</span> <span class="nf">HasNext</span><span class="p">()</span> <span class="k">const</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
+  
+  <span class="c1">// Reinitialize the reader and read the file from the begin.</span>
+  <span class="k">virtual</span> <span class="kt">void</span> <span class="nf">ReInit</span><span class="p">()</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
+  
+  <span class="c1">// Get a certain read in data&#39;s shape.</span>
+  <span class="n">DDim</span> <span class="nf">shape</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">idx</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
+  <span class="c1">// Get shapes of all read in data.</span>
+  <span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">DDim</span><span class="o">&gt;</span> <span class="n">shapes</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="n">shapes_</span><span class="p">;</span> <span class="p">}</span>
+  <span class="c1">// Set shapes of read in data.</span>
+  <span class="kt">void</span> <span class="n">set_shapes</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">DDim</span><span class="o">&gt;&amp;</span> <span class="n">shapes</span><span class="p">)</span> <span class="p">{</span> <span class="n">shapes_</span> <span class="o">=</span> <span class="n">shapes</span><span class="p">;</span> <span class="p">}</span>
+
+  <span class="k">virtual</span> <span class="o">~</span><span class="n">ReaderBase</span><span class="p">()</span> <span class="p">{}</span>
+
+ <span class="k">protected</span><span class="o">:</span>
+  <span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">DDim</span><span class="o">&gt;</span> <span class="n">shapes_</span><span class="p">;</span>
+<span class="p">};</span>
+</pre></div>
+</div>
+</div>
+<div class="section" id="filereader-and-decoratedreader">
+<span id="filereader-and-decoratedreader"></span><h3><code class="docutils literal"><span class="pre">FileReader</span></code> and <code class="docutils literal"><span class="pre">DecoratedReader</span></code><a class="headerlink" href="#filereader-and-decoratedreader" title="Permalink to this headline">¶</a></h3>
+<p>These two classes are derived from the <code class="docutils literal"><span class="pre">ReaderBase</span></code> and will further be derived by respective specific readers. That is to say, in our design, there are two kinds of readers: file readers and decorated readers. A file reader reads from a file of some specific format, and yield only one instance of data at a time. e.g. RecordIO reader, jpg reader, .... A decorated reader takes another reader(both file reader and decorated reader are OK) as its &#8216;underlying reader&#8217;. It gets data from its underlying reader, does some process on them(shuffling, or batching), then yields processed data. The output data of a decorated reader can be a single instance or a batch. <code class="docutils literal"><span class="pre">ShuffleReader</span></code> and <code class="docutils literal"><span class="pre">BatchReader</span></code> are both decorated readers.</p>
+<p>All the readers share exactly the same interfaces defined in <code class="docutils literal"><span class="pre">ReaderBase</span></code>. So they can be decorated for more than one time: We can <strong>shuffle</strong> a reader&#8217;s outputs and then <strong>batch</strong> the shuffle outputs. The interface consistency also allows related ops use readers without knowing what they are exactly.</p>
+</div>
+<div class="section" id="readerholder">
+<span id="readerholder"></span><h3><code class="docutils literal"><span class="pre">ReaderHolder</span></code><a class="headerlink" href="#readerholder" title="Permalink to this headline">¶</a></h3>
+<p>Different readers belong to different class types. It leads to a problem: How can we drop them into <code class="docutils literal"><span class="pre">Variable</span></code>s and fetch them out by a unified method? For example, if a Variable holds a <code class="docutils literal"><span class="pre">BatchReader</span></code>, we can not get it by the following code:</p>
+<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">var</span><span class="o">-&gt;</span><span class="n">Get</span><span class="o">&lt;</span><span class="n">ReaderBase</span><span class="o">&gt;</span><span class="p">(</span><span class="s">&quot;batch_reader&quot;</span><span class="p">);</span>
+</pre></div>
+</div>
+<p>we have to write:</p>
+<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">var</span><span class="o">-&gt;</span><span class="n">Get</span><span class="o">&lt;</span><span class="n">BatchReader</span><span class="o">&gt;</span><span class="p">(</span><span class="s">&quot;batch_reader&quot;</span><span class="p">);</span>
+</pre></div>
+</div>
+<p>This requires each time getting a reader from a variable we must know the reader&#8217;s type exactly. It is nearly impossible.</p>
+<p>To solve this problem, we introduce <code class="docutils literal"><span class="pre">ReaderHolder</span></code> as a wrapper. It acts as an empty decorator of <code class="docutils literal"><span class="pre">ReaderBase</span></code>, which erases reader&#8217;s type. With <code class="docutils literal"><span class="pre">ReaderHolder</span></code> we are able to fetch all types of readers by <code class="docutils literal"><span class="pre">var-&gt;Get&lt;ReaderHolder&gt;(&quot;...&quot;)</span></code> and regard the obtained object as a reader.</p>
+</div>
+</div>
+<div class="section" id="related-operators">
+<span id="related-operators"></span><h2>Related Operators<a class="headerlink" href="#related-operators" title="Permalink to this headline">¶</a></h2>
+<p>To create and invoke readers, some now ops are introduced:</p>
+<div class="section" id="createreaderop">
+<span id="createreaderop"></span><h3><code class="docutils literal"><span class="pre">CreateReaderOp</span></code><a class="headerlink" href="#createreaderop" title="Permalink to this headline">¶</a></h3>
+<p>Each reader has its creating op. File readers&#8217; creating ops have no input and yield the created file reader as its output. Decorated readers&#8217; creating ops take the underlying readers as inputs and then yield new decorated readers.</p>
+</div>
+<div class="section" id="readop">
+<span id="readop"></span><h3><code class="docutils literal"><span class="pre">ReadOp</span></code><a class="headerlink" href="#readop" title="Permalink to this headline">¶</a></h3>
+<p>A reader is only a Variable. It cannot trigger the reading process by itself. So we add the <code class="docutils literal"><span class="pre">ReadOp</span></code> to execute it. A <code class="docutils literal"><span class="pre">ReadOp</span></code> takes a reader Variable as its input. Each time it runs, it invokes the reader‘s <code class="docutils literal"><span class="pre">ReadNext()</span></code> function and gets a new batch of data(or only one instance of data, if we use file reader directly). The output data of a reader are in the form of <code class="docutils literal"><span class="pre">std::vector&lt;LoDTenosr&gt;</span></code>, so the <code class="docutils literal"><span class="pre">ReadOp</span></code> also needs to split the vector and move LoDTensors to their respective output Variables.</p>
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develop/doc_cn/_sources/design/cpp_data_feeding.md.txt

+# C++ Data Feeding
+
+In training with Paddle V2 API, data feeding wholly dependents on Python code. To get rid of the Python environment and achieve the goal of "wrapping the whole training by a while loop op" in Paddle Fluid, a C++ data feeding mechanism is required. 
+
+In this document we show the fundamental design of C++ data feeding process, which includes the data reading, shuffling and batching.
+
+## Reader
+
+A new concept named 'Reader' is introduced. `Reader` is a series of inherited classes which can be hold by our `Variable` and they are used to read or process file data.
+
+
+### `ReaderBase`
+
+`ReaderBase` is the abstract base class of all readers. It defines the all readers' interfaces.
+
+```cpp
+class ReaderBase {
+ public:
+  explicit ReaderBase(const std::vector<DDim>& shapes) : shapes_(shapes) {
+    PADDLE_ENFORCE(!shapes_.empty());
+  }
+  // Read the next batch of data. (A 'batch' can be only one instance)
+  virtual void ReadNext(std::vector<LoDTensor>* out) = 0;
+  // Show whether the next bacth exists.
+  virtual bool HasNext() const = 0;
+  
+  // Reinitialize the reader and read the file from the begin.
+  virtual void ReInit() = 0;
+  
+  // Get a certain read in data's shape.
+  DDim shape(size_t idx) const;
+  // Get shapes of all read in data.
+  std::vector<DDim> shapes() const { return shapes_; }
+  // Set shapes of read in data.
+  void set_shapes(const std::vector<DDim>& shapes) { shapes_ = shapes; }
+
+  virtual ~ReaderBase() {}
+
+ protected:
+  std::vector<DDim> shapes_;
+};
+```
+
+### `FileReader` and `DecoratedReader`
+
+These two classes are derived from the `ReaderBase` and will further be derived by respective specific readers. That is to say, in our design, there are two kinds of readers: file readers and decorated readers. A file reader reads from a file of some specific format, and yield only one instance of data at a time. e.g. RecordIO reader, jpg reader, .... A decorated reader takes another reader(both file reader and decorated reader are OK) as its 'underlying reader'. It gets data from its underlying reader, does some process on them(shuffling, or batching), then yields processed data. The output data of a decorated reader can be a single instance or a batch. `ShuffleReader` and `BatchReader` are both decorated readers.
+
+All the readers share exactly the same interfaces defined in `ReaderBase`. So they can be decorated for more than one time: We can **shuffle** a reader's outputs and then **batch** the shuffle outputs. The interface consistency also allows related ops use readers without knowing what they are exactly.
+
+
+### `ReaderHolder`
+
+Different readers belong to different class types. It leads to a problem: How can we drop them into `Variable`s and fetch them out by a unified method? For example, if a Variable holds a `BatchReader`, we can not get it by the following code:
+
+```cpp
+var->Get<ReaderBase>("batch_reader");
+```
+
+we have to write:
+
+```cpp
+var->Get<BatchReader>("batch_reader");
+```
+
+This requires each time getting a reader from a variable we must know the reader's type exactly. It is nearly impossible.
+
+To solve this problem, we introduce `ReaderHolder` as a wrapper. It acts as an empty decorator of `ReaderBase`, which erases reader's type. With `ReaderHolder` we are able to fetch all types of readers by `var->Get<ReaderHolder>("...")` and regard the obtained object as a reader.
+
+## Related Operators
+
+To create and invoke readers, some now ops are introduced:
+
+### `CreateReaderOp`
+
+Each reader has its creating op. File readers' creating ops have no input and yield the created file reader as its output. Decorated readers' creating ops take the underlying readers as inputs and then yield new decorated readers.
+
+### `ReadOp`
+
+A reader is only a Variable. It cannot trigger the reading process by itself. So we add the `ReadOp` to execute it. A `ReadOp` takes a reader Variable as its input. Each time it runs, it invokes the reader‘s `ReadNext()` function and gets a new batch of data(or only one instance of data, if we use file reader directly). The output data of a reader are in the form of `std::vector<LoDTenosr>`, so the `ReadOp` also needs to split the vector and move LoDTensors to their respective output Variables.

develop/doc_cn/design/cpp_data_feeding.html

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+    <li>C++ Data Feeding</li>
+  </ul>
+</div>
+      
+      <div class="wy-nav-content" id="doc-content">
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+            
+  <div class="section" id="c-data-feeding">
+<span id="c-data-feeding"></span><h1>C++ Data Feeding<a class="headerlink" href="#c-data-feeding" title="永久链接至标题">¶</a></h1>
+<p>In training with Paddle V2 API, data feeding wholly dependents on Python code. To get rid of the Python environment and achieve the goal of &#8220;wrapping the whole training by a while loop op&#8221; in Paddle Fluid, a C++ data feeding mechanism is required.</p>
+<p>In this document we show the fundamental design of C++ data feeding process, which includes the data reading, shuffling and batching.</p>
+<div class="section" id="reader">
+<span id="reader"></span><h2>Reader<a class="headerlink" href="#reader" title="永久链接至标题">¶</a></h2>
+<p>A new concept named &#8216;Reader&#8217; is introduced. <code class="docutils literal"><span class="pre">Reader</span></code> is a series of inherited classes which can be hold by our <code class="docutils literal"><span class="pre">Variable</span></code> and they are used to read or process file data.</p>
+<div class="section" id="readerbase">
+<span id="readerbase"></span><h3><code class="docutils literal"><span class="pre">ReaderBase</span></code><a class="headerlink" href="#readerbase" title="永久链接至标题">¶</a></h3>
+<p><code class="docutils literal"><span class="pre">ReaderBase</span></code> is the abstract base class of all readers. It defines the all readers&#8217; interfaces.</p>
+<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ReaderBase</span> <span class="p">{</span>
+ <span class="k">public</span><span class="o">:</span>
+  <span class="k">explicit</span> <span class="n">ReaderBase</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">DDim</span><span class="o">&gt;&amp;</span> <span class="n">shapes</span><span class="p">)</span> <span class="o">:</span> <span class="n">shapes_</span><span class="p">(</span><span class="n">shapes</span><span class="p">)</span> <span class="p">{</span>
+    <span class="n">PADDLE_ENFORCE</span><span class="p">(</span><span class="o">!</span><span class="n">shapes_</span><span class="p">.</span><span class="n">empty</span><span class="p">());</span>
+  <span class="p">}</span>
+  <span class="c1">// Read the next batch of data. (A &#39;batch&#39; can be only one instance)</span>
+  <span class="k">virtual</span> <span class="kt">void</span> <span class="n">ReadNext</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">LoDTensor</span><span class="o">&gt;*</span> <span class="n">out</span><span class="p">)</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
+  <span class="c1">// Show whether the next bacth exists.</span>
+  <span class="k">virtual</span> <span class="kt">bool</span> <span class="nf">HasNext</span><span class="p">()</span> <span class="k">const</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
+  
+  <span class="c1">// Reinitialize the reader and read the file from the begin.</span>
+  <span class="k">virtual</span> <span class="kt">void</span> <span class="nf">ReInit</span><span class="p">()</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
+  
+  <span class="c1">// Get a certain read in data&#39;s shape.</span>
+  <span class="n">DDim</span> <span class="nf">shape</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">idx</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
+  <span class="c1">// Get shapes of all read in data.</span>
+  <span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">DDim</span><span class="o">&gt;</span> <span class="n">shapes</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="n">shapes_</span><span class="p">;</span> <span class="p">}</span>
+  <span class="c1">// Set shapes of read in data.</span>
+  <span class="kt">void</span> <span class="n">set_shapes</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">DDim</span><span class="o">&gt;&amp;</span> <span class="n">shapes</span><span class="p">)</span> <span class="p">{</span> <span class="n">shapes_</span> <span class="o">=</span> <span class="n">shapes</span><span class="p">;</span> <span class="p">}</span>
+
+  <span class="k">virtual</span> <span class="o">~</span><span class="n">ReaderBase</span><span class="p">()</span> <span class="p">{}</span>
+
+ <span class="k">protected</span><span class="o">:</span>
+  <span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">DDim</span><span class="o">&gt;</span> <span class="n">shapes_</span><span class="p">;</span>
+<span class="p">};</span>
+</pre></div>
+</div>
+</div>
+<div class="section" id="filereader-and-decoratedreader">
+<span id="filereader-and-decoratedreader"></span><h3><code class="docutils literal"><span class="pre">FileReader</span></code> and <code class="docutils literal"><span class="pre">DecoratedReader</span></code><a class="headerlink" href="#filereader-and-decoratedreader" title="永久链接至标题">¶</a></h3>
+<p>These two classes are derived from the <code class="docutils literal"><span class="pre">ReaderBase</span></code> and will further be derived by respective specific readers. That is to say, in our design, there are two kinds of readers: file readers and decorated readers. A file reader reads from a file of some specific format, and yield only one instance of data at a time. e.g. RecordIO reader, jpg reader, .... A decorated reader takes another reader(both file reader and decorated reader are OK) as its &#8216;underlying reader&#8217;. It gets data from its underlying reader, does some process on them(shuffling, or batching), then yields processed data. The output data of a decorated reader can be a single instance or a batch. <code class="docutils literal"><span class="pre">ShuffleReader</span></code> and <code class="docutils literal"><span class="pre">BatchReader</span></code> are both decorated readers.</p>
+<p>All the readers share exactly the same interfaces defined in <code class="docutils literal"><span class="pre">ReaderBase</span></code>. So they can be decorated for more than one time: We can <strong>shuffle</strong> a reader&#8217;s outputs and then <strong>batch</strong> the shuffle outputs. The interface consistency also allows related ops use readers without knowing what they are exactly.</p>
+</div>
+<div class="section" id="readerholder">
+<span id="readerholder"></span><h3><code class="docutils literal"><span class="pre">ReaderHolder</span></code><a class="headerlink" href="#readerholder" title="永久链接至标题">¶</a></h3>
+<p>Different readers belong to different class types. It leads to a problem: How can we drop them into <code class="docutils literal"><span class="pre">Variable</span></code>s and fetch them out by a unified method? For example, if a Variable holds a <code class="docutils literal"><span class="pre">BatchReader</span></code>, we can not get it by the following code:</p>
+<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">var</span><span class="o">-&gt;</span><span class="n">Get</span><span class="o">&lt;</span><span class="n">ReaderBase</span><span class="o">&gt;</span><span class="p">(</span><span class="s">&quot;batch_reader&quot;</span><span class="p">);</span>
+</pre></div>
+</div>
+<p>we have to write:</p>
+<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">var</span><span class="o">-&gt;</span><span class="n">Get</span><span class="o">&lt;</span><span class="n">BatchReader</span><span class="o">&gt;</span><span class="p">(</span><span class="s">&quot;batch_reader&quot;</span><span class="p">);</span>
+</pre></div>
+</div>
+<p>This requires each time getting a reader from a variable we must know the reader&#8217;s type exactly. It is nearly impossible.</p>
+<p>To solve this problem, we introduce <code class="docutils literal"><span class="pre">ReaderHolder</span></code> as a wrapper. It acts as an empty decorator of <code class="docutils literal"><span class="pre">ReaderBase</span></code>, which erases reader&#8217;s type. With <code class="docutils literal"><span class="pre">ReaderHolder</span></code> we are able to fetch all types of readers by <code class="docutils literal"><span class="pre">var-&gt;Get&lt;ReaderHolder&gt;(&quot;...&quot;)</span></code> and regard the obtained object as a reader.</p>
+</div>
+</div>
+<div class="section" id="related-operators">
+<span id="related-operators"></span><h2>Related Operators<a class="headerlink" href="#related-operators" title="永久链接至标题">¶</a></h2>
+<p>To create and invoke readers, some now ops are introduced:</p>
+<div class="section" id="createreaderop">
+<span id="createreaderop"></span><h3><code class="docutils literal"><span class="pre">CreateReaderOp</span></code><a class="headerlink" href="#createreaderop" title="永久链接至标题">¶</a></h3>
+<p>Each reader has its creating op. File readers&#8217; creating ops have no input and yield the created file reader as its output. Decorated readers&#8217; creating ops take the underlying readers as inputs and then yield new decorated readers.</p>
+</div>
+<div class="section" id="readop">
+<span id="readop"></span><h3><code class="docutils literal"><span class="pre">ReadOp</span></code><a class="headerlink" href="#readop" title="永久链接至标题">¶</a></h3>
+<p>A reader is only a Variable. It cannot trigger the reading process by itself. So we add the <code class="docutils literal"><span class="pre">ReadOp</span></code> to execute it. A <code class="docutils literal"><span class="pre">ReadOp</span></code> takes a reader Variable as its input. Each time it runs, it invokes the reader‘s <code class="docutils literal"><span class="pre">ReadNext()</span></code> function and gets a new batch of data(or only one instance of data, if we use file reader directly). The output data of a reader are in the form of <code class="docutils literal"><span class="pre">std::vector&lt;LoDTenosr&gt;</span></code>, so the <code class="docutils literal"><span class="pre">ReadOp</span></code> also needs to split the vector and move LoDTensors to their respective output Variables.</p>
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