What does the explicit keyword mean in C++?

 

In C++, the explicit keyword is used to prevent implicit type conversions by constructors. When a constructor is marked as explicit, it ensures that the constructor is only called when an explicit conversion is requested, and it prevents automatic conversions that might lead to unexpected behavior.

Here's an example to illustrate the use of the explicit keyword:

cpp

#include <iostream>

class MyClass {
public:
    // Regular constructor
    MyClass(int x) : value(x) {}

    int getValue() const {
        return value;
    }

private:
    int value;
};

void printValue(const MyClass& obj) {
    std::cout << "Value: " << obj.getValue() << std::endl;
}

int main() {
    MyClass obj = 42;  // Implicit conversion allowed (without explicit)
    printValue(obj);   // Output: Value: 42

    MyClass newObj = MyClass(42);  // Explicit conversion required (with explicit)
    printValue(newObj);            // Output: Value: 42

    return 0;
}

In this example, the class MyClass has a constructor that takes an integer parameter. Without the explicit keyword, this constructor can be implicitly invoked to perform a conversion from an int to a MyClass object. This might lead to unexpected conversions and behavior.

However, when the constructor is marked as explicit, as shown in the example, the compiler enforces that the constructor can only be called with explicit type conversions. This prevents accidental conversions and makes the code more clear about when conversions are intended.

By using the explicit keyword, you can make your code more precise and prevent unexpected type conversions that might introduce bugs or confusion.