title: Java .length equivalent in C++
body: <p>How can I change this code snippet so it runs the length of the array? The type of data in the array is a <code>Mat</code> from OpenCV.</p>
  
  <pre><code>for (int i = 0; i &lt; subImages.length(); i++) {
      UIImage *subImage = [self UIImageFromMat:subImages[i]];
      [_subImgs addObject:subImage];
  }
  </code></pre>
  
  <p>I get an error on <code>subImages.length();</code></p>