User Defined Conversions

Bool Typecasts

What special considerations are there for bool typecasts in C++?

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In C++, typecasts to bool have some special considerations, particularly regarding their implicit usage in control flow and logical expressions.

Even if a bool typecast operator is marked as explicit, it can still be used implicitly in certain contexts. Here’s an example:

#include <iostream>

struct MyType {
  constexpr explicit operator bool() const {
    return true;

int main() {
  MyType obj;

  // Implicit usage in control flow
  if (obj) { 
    std::cout << "Object is true in if statement\n";

  while (obj) { 
    std::cout << "Object is true in while loop\n";

  // Implicit usage in logical expressions
  if (!obj) { 
    std::cout << "Object is false\n";

  if (true || obj) { 
    std::cout << "Object is true in logical OR\n";

  // Explicit usage elsewhere
  bool value = static_cast<bool>(obj); 
  std::cout << "Explicit conversion: " << value;
Object is true in if statement
Object is true in while loop
Object is true in logical OR
Explicit conversion: 1

Special considerations for bool typecasts include:

  • Control Flow: Objects with an explicit bool operator can be used implicitly in control flow statements like if, while, and for loops. This allows for natural usage in conditions.
  • Logical Expressions: Similarly, logical operators (!, &&, ||) can implicitly use the bool operator, even if marked explicit.
  • Compile-Time Logic: When marked as constexpr, bool operators can be used in compile-time logic like static_assert and if constexpr.

The reason behind these exceptions is to maintain intuitive control flow and logical checks. If you have a class that represents a state or a condition, it makes sense to allow implicit checks without requiring explicit casts every time.

For example, consider a class that represents a network connection:

struct Connection {
  bool connected;
  constexpr explicit operator bool() const {
    return connected;

You would want to write if (connection) naturally without needing an explicit cast, even though the bool operator is explicit.

In summary, bool typecasts in C++ are given special treatment to ensure they can be used naturally in control flow and logical expressions, enhancing code readability and usability while maintaining type safety.

Answers to questions are automatically generated and may not have been reviewed.

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