Member Variables and Member Methods

Intermediate12 min read•Updated: 2026-10-22

⚙️ Member Variables and Member Methods

Introduction

A class binds state (data) and behavior (functions) into a unified type. Understanding how to organize declarations in header files (.h/.hpp) and method implementations in source files (.cpp) using the scope resolution operator (::) is fundamental to C++ project architecture.

🏛️ Separating Declaration from Definition

1#include <iostream>
2#include <string>
3 
4class BankAccount {
5private:
6    std::string m_accountHolder;
7    std::string m_accountNumber;
8    double m_balance;
9 
10public:
11    // Method Declarations (Prototypes)
12    void initialize(const std::string& holder, const std::string& accNum, double initialDeposit);
13    void deposit(double amount);
14    bool withdraw(double amount);
15    double getBalance() const;
16    void printStatement() const;
17};
18 
19// Method Definitions using Class::Method syntax
20void BankAccount::initialize(const std::string& holder, const std::string& accNum, double initialDeposit) {
21    m_accountHolder = holder;
22    m_accountNumber = accNum;
23    m_balance = (initialDeposit >= 0.0) ? initialDeposit : 0.0;
24}
25 
26void BankAccount::deposit(double amount) {
27    if (amount > 0.0) {
28        m_balance += amount;
29        std::cout << "Deposited ₹" << amount << " into " << m_accountNumber << "\n";
30    }
31}
32 
33bool BankAccount::withdraw(double amount) {
34    if (amount > 0.0 && amount <= m_balance) {
35        m_balance -= amount;
36        std::cout << "Withdrew ₹" << amount << " from " << m_accountNumber << "\n";
37        return true;
38    }
39    std::cerr << "[ERROR]: Withdrawal denied. Insufficient funds or invalid amount.\n";
40    return false;
41}
42 
43double BankAccount::getBalance() const {
44    return m_balance;
45}
46 
47void BankAccount::printStatement() const {
48    std::cout << "Holder: " << m_accountHolder << " | Acc: " << m_accountNumber 
49              << " | Balance: ₹" << m_balance << "\n";
50}
51 
52int main() {
53    BankAccount account;
54    account.initialize("Priya Verma", "MSK-SB-8001", 15000.0);
55    account.deposit(5000.0);
56    account.withdraw(3000.0);
57    account.printStatement();
58    return 0;
59}

💡 Naming Conventions for Member Variables

To immediately distinguish class member fields from local variables and function parameters, popular engineering conventions include:

Prefix with m_: m_balance, m_accountHolder (Google / LLVM style)
Suffix with trailing underscore: balance_, account_holder_ (C++ Core Guidelines)

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Constructors: Default, Parameterized, and Member Initializer Lists

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Classes vs Structs in C++ and Access Specifiers (public, private, protected)	Constructors: Default, Parameterized, and Member Initializer Lists

Practice Quiz

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🏛️ Separating Declaration from Definition

1#include <iostream>
2#include <string>
3 
4class BankAccount {
5private:
6    std::string m_accountHolder;
7    std::string m_accountNumber;
8    double m_balance;
9 
10public:
11    // Method Declarations (Prototypes)
12    void initialize(const std::string& holder, const std::string& accNum, double initialDeposit);
13    void deposit(double amount);
14    bool withdraw(double amount);
15    double getBalance() const;
16    void printStatement() const;
17};
18 
19// Method Definitions using Class::Method syntax
20void BankAccount::initialize(const std::string& holder, const std::string& accNum, double initialDeposit) {
21    m_accountHolder = holder;
22    m_accountNumber = accNum;
23    m_balance = (initialDeposit >= 0.0) ? initialDeposit : 0.0;
24}
25 
26void BankAccount::deposit(double amount) {
27    if (amount > 0.0) {
28        m_balance += amount;
29        std::cout << "Deposited ₹" << amount << " into " << m_accountNumber << "\n";
30    }
31}
32 
33bool BankAccount::withdraw(double amount) {
34    if (amount > 0.0 && amount <= m_balance) {
35        m_balance -= amount;
36        std::cout << "Withdrew ₹" << amount << " from " << m_accountNumber << "\n";
37        return true;
38    }
39    std::cerr << "[ERROR]: Withdrawal denied. Insufficient funds or invalid amount.\n";
40    return false;
41}
42 
43double BankAccount::getBalance() const {
44    return m_balance;
45}
46 
47void BankAccount::printStatement() const {
48    std::cout << "Holder: " << m_accountHolder << " | Acc: " << m_accountNumber 
49              << " | Balance: ₹" << m_balance << "\n";
50}
51 
52int main() {
53    BankAccount account;
54    account.initialize("Priya Verma", "MSK-SB-8001", 15000.0);
55    account.deposit(5000.0);
56    account.withdraw(3000.0);
57    account.printStatement();
58    return 0;
59}

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Classes vs Structs in C++ and Access Specifiers (public, private, protected)

Constructors: Default, Parameterized, and Member Initializer Lists