C++ Copy Initialization | "Copy Initialization" Guide

What is Copy Initialization?

Copy initialization is a way to initialize variables using the = expr syntax. It is distinct from list initialization {} and is often optimized away through move semantics or RVO/NRVO. Understanding it alongside copy and move constructors helps clarify its behavior.

int x = 10;           // Copy initialization
std::string s = "Hi"; // Copy initialization

int y(10);            // Direct initialization
std::string s2("Hi"); // Direct initialization

Direct Initialization vs Copy Initialization

class Widget {
public:
    explicit Widget(int x) {}
};

// ❌ Copy initialization: explicit constructor not allowed
// Widget w1 = 10;

// ✅ Direct initialization: explicit constructor allowed
Widget w2(10);
Widget w3{10};

Practical Examples

Example 1: Primitive Types

// Copy initialization
int x = 10;
double d = 3.14;
char c = 'A';

// Direct initialization
int x(10);
double d(3.14);
char c('A');

// Uniform initialization
int x{10};
double d{3.14};
char c{'A'};

Example 2: Classes

class String {
    std::string data;
    
public:
    String(const char* str) : data(str) {}
    String(const String& other) : data(other.data) {
        std::cout << "Copy constructor" << std::endl;
    }
};

// Copy initialization
String s1 = "Hello";  // const char* -> String

// Direct initialization
String s2("Hello");

Example 3: Implicit Conversions

class Number {
    int value;
    
public:
    Number(int v) : value(v) {}  // Allows implicit conversion
};

void func(Number n) {}

int main() {
    func(42);  // int -> Number (copy initialization)
    
    Number n = 42;  // Copy initialization
}

Example 4: Return Values

std::string getName() {
    return "Alice";  // Copy initialization
}

int getValue() {
    int x = 10;
    return x;  // Copy initialization
}

explicit and Initialization

class Widget {
public:
    explicit Widget(int x) {}
};

// ❌ Copy initialization
// Widget w1 = 10;
// void func(Widget w) {}
// func(10);

// ✅ Direct initialization
Widget w2(10);
Widget w3{10};

Common Issues

Issue 1: Explicit Constructor

class Array {
public:
    explicit Array(size_t size) {}
};

// ❌ Copy initialization
// Array arr = 10;

// ✅ Direct initialization
Array arr(10);
Array arr2{10};

Issue 2: Conversion Operators

class Fraction {
public:
    Fraction(int n, int d) {}
    
    operator double() const {
        return /* ... */;
    }
};

Fraction f(1, 2);
double d = f;  // Copy initialization (conversion)

Issue 3: Initialization Lists

// Copy initialization
std::vector<int> v1 = {1, 2, 3};

// Direct initialization
std::vector<int> v2{1, 2, 3};

// Both produce the same result

Issue 4: auto Type Deduction

auto x = 10;      // int
auto y = {10};    // initializer_list<int>
auto z{10};       // C++17: int, C++11/14: initializer_list<int>

Performance Considerations

// Copy initialization
std::string s1 = "Hello";  // Temporary object may be created

// Direct initialization
std::string s2("Hello");   // Direct construction

// Optimized with RVO

FAQ

Q1: What is copy initialization?

A: Uses =. Allows implicit conversions.

Q2: What is direct initialization?

A: Uses () or {}. Allows explicit constructors.

Q3: What’s the difference?

A: Whether explicit constructors can be invoked.

Q4: What about performance?

A: Optimized with RVO. No significant difference.

Q5: When should I use each?

A:

• Copy: For simple initializations.
• Direct: When explicit constructors are needed.

Q6: Where can I learn more about copy initialization?

A:

• “Effective C++”
• “C++ Primer”
• cppreference.com

Related Posts: List Initialization, Move Semantics, Copy and Move Constructors, RVO/NRVO.

같이 보면 좋은 글 (내부 링크)

이 주제와 연결되는 다른 글입니다.

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• C++ 복사/이동 생성자 | “Rule of Five” 가이드
• C++ RVO/NRVO | “Return Value Optimization” 가이드

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