The explicit keyword meaning in C++

In C++, the explicit keyword is used to prevent the compiler from using a constructor for implicit type conversions. By marking a constructor as explicit, you ensure that it can only be used when the object is explicitly created with that constructor, preventing unexpected and potentially unsafe conversions that could lead to bugs or undesired behavior. This is particularly useful for single-argument constructors, which the compiler might otherwise use for implicit type conversions.

Purpose and Use

Preventing Implicit Conversions: The primary purpose of the explicit keyword is to prevent the compiler from automatically converting types using a constructor. This is important because implicit conversions can lead to subtle bugs that are hard to detect.

class Example {
public:
    explicit Example(int value) {
        // Constructor code
    }
};

Example obj = 10;  // Error: conversion from 'int' to 'Example' is explicit
Example obj2(10);  // Correct usage

Ensuring Explicit Calls: By marking a constructor as explicit, you enforce that it can only be called explicitly, which makes the code more readable and less prone to errors.

class Example {
public:
    explicit Example(int value) {
        // Constructor code
    }
};

Example obj(10);  // Explicit call, works fine

Implicit Conversions and Constructor Overloading

Implicit Conversion: Without explicit, constructors can be used for implicit type conversions, which might lead to unexpected results.

class Example {
public:
    Example(int value) {
        // Constructor code
    }
};

void func(Example ex) {
    // Function code
}

func(10);  // Implicit conversion from int to Example, might be unintended

Overloaded Constructors: When you have overloaded constructors, marking some of them as explicit can help control which conversions are allowed and which are not.

class Example {
public:
    Example(int value) {
        // Constructor code
    }
    explicit Example(double value) {
        // Constructor code
    }
};

Example obj1 = 10;   // Allowed
Example obj2 = 10.5; // Error: conversion from 'double' to 'Example' is explicit
Example obj3(10.5);  // Correct usage

Use with Conversion Operators

Explicit Conversion Operators: The explicit keyword can also be used with conversion operators to prevent implicit type conversions.

class Example {
public:
    explicit operator int() const {
        return 42;
    }
};

Example ex;
int value = static_cast(ex);  // Explicit cast required

Safety and Clarity: By using explicit with conversion operators, you ensure that type conversions are performed intentionally and are easy to spot in the code.

Preventing Ambiguities

Avoiding Ambiguities: The explicit keyword helps prevent ambiguities that might arise from implicit conversions, making the code more predictable.

class Example {
public:
    explicit Example(int value) {
        // Constructor code
    }
    Example(double value) {
        // Constructor code
    }
};

void func(Example ex) {
    // Function code
}

func(10);    // Calls Example(int), clear and unambiguous
func(10.5);  // Calls Example(double), clear and unambiguous

Improving Readability: Using explicit makes it clear when and how constructors are intended to be used, which improves the readability and maintainability of the code.

Best Practices

Default Constructors: Use explicit for single-argument constructors to prevent unintended implicit conversions.

class Example {
public:
    explicit Example(int value) {
        // Constructor code
    }
};

Conversion Operators: Mark conversion operators as explicit to ensure that conversions are intentional and explicit.

class Example {
public:
    explicit operator int() const {
        return 42;
    }
};

Consistent Use: Apply the explicit keyword consistently across your codebase to avoid implicit conversions where they are not desired.

Summary

Purpose of explicit: The explicit keyword in C++ is used to prevent implicit type conversions by ensuring that constructors and conversion operators are only called explicitly. This reduces the risk of unexpected behavior and subtle bugs that can arise from implicit conversions.

Use Cases: It is particularly useful for single-argument constructors and conversion operators, helping to improve code safety and readability.

Best Practices: Adopt the use of explicit as a standard practice for constructors and conversion operators that should not be used for implicit conversions, making your code more predictable and maintainable.