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Error on Expression Using Overloaded Logical Operator

Symptoms

Microsoft C++ error C2440: '?' : cannot convert from 'type' to 'bool'
GNU C++ error: could not convert 'type' to 'bool'
Clang error: value of type 'type' is not contextually convertible to 'bool'

Cause

A declaration of operator&& or operator|| is incompatible with operands or a result of type bool. Defining these operators with operand type or return type other than bool conflicts with condition/decision coverage at a basic level, and is not compatible with BullseyeCoverage.

Resolution

It is recommended to first try the latest release .

Alternative options are listed below in order of preference.

  1. Rename the overloaded operators && and || to conventional function names.
  2. Overload operator& or operator| rather than operator&& or operator||.
  3. Modify your source code to invoke the overloaded operators using the names operator&& and operator||. For example, 
    // a = b && c;              ← change this
a = b.operator&&(c);        ← to this
  4. Modify the source code so that instead of defining operator&& or operator||, instead define operator bool() to convert all expressions to boolean values, and then use the built-in && and || operators on the boolean values. For example, 
    class C {
public:
	//bool operator&&(const C&) const;      ← remove this
	//bool operator||(const C&) const;      ← remove this
	operator bool() const;                  ← add this
};

C c1, c2;
if (c1 && c2)
if (c1 || c2)
  5. Exclude the code that invokes the overloaded operator&& or operator||.

More Information

See also Best Practice Recommendations.

Updated: 3 Sep 2024