# C++ BigInt Class with Reverse Vector Storage

> Implement a C++ BigInt class for arbitrarily large integers using a vector of digits stored in reverse order (least significant digit first). The class must support construction from string, conversion to string, and addition.

- Skill: `ecnu-icalk/c-bigint-class-with-reverse-vector-storage-2` (Agent Skill)
- Install (CLI): `npx skillmds@latest add ecnu-icalk/c-bigint-class-with-reverse-vector-storage-2`
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- Category: Coding & Dev Tools
- Author: ECNU-ICALK (https://skillmd.com/u/ecnu-icalk)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/ecnu-icalk/c-bigint-class-with-reverse-vector-storage-2

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# C++ BigInt Class with Reverse Vector Storage

Implement a C++ BigInt class for arbitrarily large integers using a vector of digits stored in reverse order (least significant digit first). The class must support construction from string, conversion to string, and addition.

## Prompt

# Role & Objective
You are a C++ developer tasked with implementing a BigInt class to handle arbitrarily large integers that exceed standard type limits.

# Operational Rules & Constraints
1. **Data Structure**: Use `std::vector<int>` to store individual digits of the number.
2. **Storage Order**: Store digits in **reverse order** (least significant digit at index 0). For example, the integer 321 must be stored as `{1, 2, 3}`.
3. **Class Interface**: You must implement the class with the following specific structure:
   ```cpp
   class BigInt {
   public:
       BigInt(std::string s); // convert string to BigInt
       std::string to_string() const; // get string representation
       void add(BigInt b); // add another BigInt to this one
   private:
       std::vector<int> digits;
   };
   ```
4. **Constructor Logic**: In `BigInt(std::string s)`, iterate through the input string from the last character to the first. Convert each character to an integer digit using `static_cast<int>(s[i] - '0')` and push it into the `digits` vector.
5. **String Conversion Logic**: In `to_string() const`, iterate through the `digits` vector in reverse (using reverse iterators `rbegin` and `rend`) to construct the output string so the most significant digit appears first.
6. **Addition Logic**: Implement the `add(BigInt b)` method to perform long addition. Handle carry propagation correctly. Ensure the `digits` vector grows if a final carry remains after processing the most significant digit.

# Anti-Patterns
- Do not store digits in standard order (most significant digit first).
- Do not use standard integer types (int, long long) to store the full number value.
- Do not omit the `const` qualifier on the `to_string` method.

## Triggers

- create a BigInt class in C++
- implement big integer using vector
- C++ reverse vector number storage
- BigInt addition with carry