Implementing a Convolutional Neural Network from Scratch in C++: Deep Dive into Core Deep Learning Mechanisms

This article introduces an open-source project called convolution-neural-network-cpp, published by vanshdangi on GitHub. It implements a convolutional neural network (CNN) from scratch using pure C++, including a custom tensor class, manual forward/backward propagation, and training on the CIFAR-10 dataset. The project aims to help developers step out of their comfort zone with high-level APIs and gain a deep understanding of the underlying mechanisms of deep learning.

Original Author and Source

• Author/Maintainer: vanshdangi
• Platform: GitHub
• Project Name: convolution-neural-network-cpp
• Link: https://github.com/vanshdangi/convolution-neural-network-cpp
• Release Date: June 14, 2026

Project Value

1. Educational Significance: Manual implementation of components to deeply understand CNN working principles
2. Performance Optimization Foundation: Mastering low-level implementation helps hardware-specific optimization
3. Framework-Agnostic Understanding: Grasp core concepts to flexibly use any framework

CIFAR-10 (10 classes, 60000 32x32 color images) is chosen as the validation benchmark, suitable for testing basic CNN functions.

Custom Tensor Class

Design considerations: Memory layout (row/column major), dimension management, operation support (addition/multiplication/convolution), gradient tracking (automatic differentiation)

Convolution Layer Implementation

• Mathematical Essence: Filter sliding dot product calculation
• Key Challenges: Boundary handling (zero padding/ignoring/mirroring), stride control, multi-channel processing, batch processing

Backward Propagation

• Convolution Layer Gradients: Gradient calculation for kernel/input/bias
• Numerical Stability: Gradient clipping, activation function selection (ReLU/Leaky ReLU), weight initialization

Other Components

• Pooling Layers: Max pooling (record max value positions), average pooling
• Activation Functions: ReLU (simple but prone to neuron death), Sigmoid (gradient vanishing), Tanh (zero-centered)

CIFAR-10 Dataset

Contains 10 classes (airplane/car/bird etc.), 50000 training images, 10000 test images, 32x32 size suitable for validation

Key Training Decisions

1. Learning rate scheduling (fixed/decay)
2. Batch size (affects memory and gradient stability)
3. Regularization (L2/Dropout to prevent overfitting)
4. Early stopping strategy (avoid overfitting on validation set)

1. Debugging Ability: Locate root cause when model is abnormal
2. Architecture Design: Understand component roles to design task-appropriate architecture
3. Performance Optimization: Identify computation bottlenecks and optimize key paths
4. Innovation Foundation: Deepen understanding of existing technologies to propose effective improvements

1. First implement the same task with PyTorch/TensorFlow to build intuitive understanding
2. Read the project source code and understand details against formulas
3. Modify network structure and observe performance impact
4. Try to implement a simplified version independently

Implementing CNN from scratch is a challenging but rewarding experience. It deepens the understanding of core deep learning mechanisms and cultivates the ability to read source code and debug complex systems. In today's fast-iterating AI field, low-level understanding is the key to distinguishing ordinary users from experts. The best way to learn is not to call APIs, but to build every component with your own hands and feel the beauty of mathematics behind the code.