In C 11, =default is used to explicitly require the compiler to generate default special member functions, and =delete is used to explicitly disable functions; 1. Use = default to restore the generation of default constructors, destructors, copy constructors and copy assignment operators, even if other constructors have been defined; 2. Use = delete to prohibit copy constructs, copy assignments, implicit type conversion constructs or specific function overloading, thereby preventing undesired calls; 3. Both = default and = delete can improve code security and readability, clearly express the design intention, and = delete will trigger a compilation error when called to ensure that the disable logic is valid.
In C 11 and later standards, default and delete are two keywords used to explicitly control special member functions of the class (such as constructors, destructors, assignment operators, etc.). They can help us express design intent more clearly and improve code readability and security.
The following is a specific example to illustrate the usage of default and delete .
? Use = default : explicitly ask the compiler to generate default functions
Sometimes we want the compiler to generate default special member functions for us, but because other constructors are defined, the compiler no longer automatically generates certain functions (such as the default constructor). At this time, you can use = default to explicitly require generation.
class MyClass {
public:
// 1. Explicitly require the default constructor MyClass() = default;
// 2. Constructor with parameters MyClass(int value) : data(value) {}
// 3. Explicitly require the default destructor ~MyClass() = default;
// 4. Explicitly require the default copy constructor MyClass(const MyClass& other) = default;
// 5. Explicitly require the default copy assignment operator MyClass& operator=(const MyClass& other) = default;
private:
int data = 0;
};? illustrate:
- Although we define
MyClass(int), the default constructor is explicitly declared by= default, soMyClass obj;is still legal. -
= defaultcan be written inside the class (inline) or defined outside the class (such asMyClass::~MyClass() = default;).
?Use = delete : explicitly disable certain functions
Used to prohibit certain functions from being called, often used to prevent copying, assignment, or disable certain types of conversions.
class NonCopyable {
public:
NonCopyable() = default;
// NonCopyable(const NonCopyable& other) = delete;
// NonCopyable& operator=(const NonCopyable& other) = delete;
// Construct via int is prohibited (prevent implicit conversion)
NonCopyable(int) = delete;
};
class Logger {
public:
void write(const std::string& msg) {
std::cout << "[LOG] " << msg << std::endl;
}
// Some types of calls to write are prohibited, such as int
void write(int) = delete; // Prevent write(123)
};? Usage example:
int main() {
NonCopyable a;
// NonCopyable b = a; // ? Compilation error: The copy construct was deleted
// NonCopyable c;
// c = a; // ? Compilation error: assignment is deleted
// NonCopyable d(42); // ? Compilation error: the int construct was deleted
Logger log;
log.write("Hello"); // ? Normal call // log.write(100); // ? Compilation error: write(int) was deleted
return 0;
}? Practical application scenarios
| Scene | usage |
|---|---|
| Implement singleton or resource management classes | = delete copy construction and assignment |
| Disable copy in mobile semantics | MyClass(const MyClass&) = delete;
|
| Prevent implicit type conversion | MyClass(int) = delete;
|
| Explicitly state "I could have generated, but I chose to declare it explicitly" | ~MyClass() = default;
|
? Tips
-
= defaultand= deletecan only be used for special member functions or overloaded functions. - If the function
= deleteis called, an error will be reported during compilation, which is very safe. -
= deletecan be used for ordinary function overloading and is used to prevent calls of certain parameter types (such asLogger::write(int)above).
Basically that's it. default and delete are simple and powerful tools in modern C. The rational use can make the interface safer and the intentions clearer.
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