Day 21 — Defaulted and Deleted Functions

1. Problem It Solves

= default requests the compiler's normal implementation, while = delete makes an unwanted operation participate in diagnostics but remain unusable. It makes an important constraint visible instead of leaving readers to guess. This lesson keeps only the C++11 core that fits one focused day.

2. Prerequisites

The ideas from Day 20, plus basic variables, functions, and output already introduced.

3. Core Idea

Mental model: = default requests the compiler's normal implementation, while = delete makes an unwanted operation participate in diagnostics but remain unusable. Identify the relevant value or state, who owns it, and whether the rule acts during compilation or execution.

4. Minimal Syntax

Token() = default; Token(const Token&) = delete;

5. How It Works

The example creates a tiny fixed state with no keyboard input.
C++11 or the standard-library contract applies today's rule.
The program prints the important result so it can be checked against the source.

6. Common Mistakes

Deleting copy construction also affects code that passes or returns the type by value; the resulting error may appear far from the declaration.

7. When to Use It

Use it when expressing that compiler behavior is correct or an operation is intentionally forbidden.
Avoid it when it hides ownership, lifetime, type, ordering, or cost.

8. Simple Example

A token can be default-constructed but its copy constructor is explicitly deleted; the forbidden copy remains as a commented compile-time example. The .cpp keeps the data fixed and avoids unrelated abstraction.

Complete sample code

Source file

cpp11/21_defaulted_deleted_functions/main.cpp

C++

#include <iostream>

class Token {
public:
    Token() = default;
    Token(const Token&) = delete;
    Token& operator=(const Token&) = delete;

    void use() const {
        std::cout << "token-used\n";
    }
};

int main() {
    Token token; // compiler-generated default constructor
    token.use();

    // Token copy(token); // error: copy constructor is deleted
}

9. Key Takeaways

The feature is part of the C++11 scope used in this course.
Understand its lifetime, ownership, type, and ordering consequences.
Compile with warnings and prefer the smallest form that makes the rule obvious.

10. Self-Check Questions

Easy — In class Token, what do Token() = default and Token(const Token&) = delete ask the compiler to do? Is Token copy(token) allowed?
Medium — Read the small example described above. What value or state should it print, and which rule produces that result?
Hard — Find and explain the subtle bug in this situation: Deleting copy construction also affects code that passes or returns the type by value; the resulting error may appear far from the declaration. What is the smallest C++11-safe correction?

Practice this lesson's cards

#include <iostream> class Token { public: Token() = default; Token(const Token&) = delete; Token& operator=(const Token&) = delete; void use() const { std::cout << "token-used\n"; } }; int main() { Token token; // compiler-generated default constructor token.use(); // Token copy(token); // error: copy constructor is deleted }

10. Self-Check Questions

Easy — In class Token, what do Token() = default and Token(const Token&) = delete ask the compiler to do? Is Token copy(token) allowed?
Medium — Read the small example described above. What value or state should it print, and which rule produces that result?
Hard — Find and explain the subtle bug in this situation: Deleting copy construction also affects code that passes or returns the type by value; the resulting error may appear far from the declaration. What is the smallest C++11-safe correction?