Implementing a Convolutional Neural Network from Scratch in C++: A Complete Practice Without Any Deep Learning Frameworks

This article introduces the CNN-MNIST-From-Scratch-CPP project published by rebwarai on GitHub. This project implements a convolutional neural network entirely from scratch in C++, covering core components like custom tensors, convolutional layers, fully connected layers, backpropagation, and Adam optimization, without relying on any deep learning frameworks. The project aims to help developers deeply understand the underlying principles of neural networks while providing C++ performance optimization experience, making it highly valuable for learning and practice.

Most deep learning developers rely on frameworks like TensorFlow/PyTorch to quickly build models, but often lack a deep understanding of the underlying principles. This project, by implementing a CNN from scratch in C++ (without external deep learning libraries), allows learners to grasp every detail from tensor operations to backpropagation, addressing the problem of insufficient principle understanding caused by framework dependency.

The project's core components include:

1. Custom Tensor System: Supports arbitrary dimension storage and operations, providing a foundation for subsequent layers;
2. Convolutional Layer: Implements kernel sliding, multi-channel processing, padding/stride control, and gradient propagation;
3. Fully Connected Layer: Weight initialization, matrix multiplication, and backpropagation updates;
4. Pooling and Regularization: Max pooling (dimensionality reduction + translation invariance), Dropout (overfitting prevention);
5. Loss and Optimization: Cross-entropy loss (classification tasks), Adam optimizer (adaptive learning rate);
6. Custom JPEG Decoder: Reads MNIST images directly without libraries like OpenCV.

The training process consists of:

1. Forward Propagation: Input images go through convolutional layers for feature extraction → activation function → pooling layers for dimensionality reduction → fully connected layers for classification;
2. Backpropagation: Uses the chain rule to reversely calculate gradients for each layer, determining the contribution of parameters to the loss;
3. MNIST Processing: Loads 60,000 training/10,000 test images, and implements normalization and batch training logic.

The project's value lies in:

1. Principle Understanding: Implementing components by hand helps master underlying logic like convolution operations, backpropagation, and optimizer updates;
2. C++ Optimization: Learn high-performance programming techniques such as manual memory management, loop optimization, and cache-friendly code;
3. Interview Advantage: Implementing CNN from scratch is a classic question in deep learning interviews, demonstrating solid fundamentals and distinguishing "framework users" from "principle-focused engineers."

Current Limitations:

• Performance: Pure C++ implementation is faster than Python but not as fast as GPU-accelerated libraries like cuDNN;
• Functionality: Lacks modern techniques like residual connections and batch normalization;
• Dataset: Only supports MNIST; complex datasets require architecture adjustments. Improvement Directions:
• GPU acceleration: Migrate computations using CUDA/OpenCL;
• Modern architectures: Implement ResNet/Inception, etc.;
• Data augmentation: Add random rotation, scaling, etc.;
• Batch normalization: Speed up training.

This project is an excellent learning resource for deep learning, proving that a fully functional CNN can be implemented without frameworks. Recommendations for learners:

1. Read through the code to understand the overall architecture;
2. Debug step-by-step to observe changes in tensor shapes;
3. Modify the network structure and observe the impact on accuracy;
4. Try to implement a simplified version yourself. Although this path is steep, it can build a solid foundation and help with learning more complex models later.