readability-implicit-bool-conversion

This check can be used to find implicit conversions between built-in types and booleans. Depending on use case, it may simply help with readability of the code, or in some cases, point to potential bugs which remain unnoticed due to implicit conversions.

The following is a real-world example of bug which was hiding behind implicit bool conversion:

class Foo {
  int m_foo;

public:
  void setFoo(bool foo) { m_foo = foo; } // warning: implicit conversion bool -> int
  int getFoo() { return m_foo; }
};

void use(Foo& foo) {
  bool value = foo.getFoo(); // warning: implicit conversion int -> bool
}

This code is the result of unsuccessful refactoring, where type of m_foo changed from bool to int. The programmer forgot to change all occurrences of bool, and the remaining code is no longer correct, yet it still compiles without any visible warnings.

In addition to issuing warnings, fix-it hints are provided to help solve the reported issues. This can be used for improving readability of code, for example:

void conversionsToBool() {
  float floating;
  bool boolean = floating;
  // ^ propose replacement: bool boolean = floating != 0.0f;

  int integer;
  if (integer) {}
  // ^ propose replacement: if (integer != 0) {}

  int* pointer;
  if (!pointer) {}
  // ^ propose replacement: if (pointer == nullptr) {}

  while (1) {}
  // ^ propose replacement: while (true) {}
}

void functionTakingInt(int param);

void conversionsFromBool() {
  bool boolean;
  functionTakingInt(boolean);
  // ^ propose replacement: functionTakingInt(static_cast<int>(boolean));

  functionTakingInt(true);
  // ^ propose replacement: functionTakingInt(1);
}

In general, the following conversion types are checked:

• integer expression/literal to boolean (conversion from a single bit bitfield to boolean is explicitly allowed, since there's no ambiguity / information loss in this case),
• floating expression/literal to boolean,
• pointer/pointer to member/nullptr/NULL to boolean,
• boolean expression/literal to integer (conversion from boolean to a single bit bitfield is explicitly allowed),
• boolean expression/literal to floating.

The rules for generating fix-it hints are:

• in case of conversions from other built-in type to bool, an explicit comparison is proposed to make it clear what exactly is being compared:
  • bool boolean = floating; is changed to bool boolean = floating == 0.0f;,
  • for other types, appropriate literals are used (0, 0u, 0.0f, 0.0, nullptr),
• in case of negated expressions conversion to bool, the proposed replacement with comparison is simplified:
  • if (!pointer) is changed to if (pointer == nullptr),
• in case of conversions from bool to other built-in types, an explicit static_cast is proposed to make it clear that a conversion is taking place:
  • int integer = boolean; is changed to int integer = static_cast<int>(boolean);,
• if the conversion is performed on type literals, an equivalent literal is proposed, according to what type is actually expected, for example:
  • functionTakingBool(0); is changed to functionTakingBool(false);,
  • functionTakingInt(true); is changed to functionTakingInt(1);,
  • for other types, appropriate literals are used (false, true, 0, 1, 0u, 1u, 0.0f, 1.0f, 0.0, 1.0f).

Some additional accommodations are made for pre-C++11 dialects:

• false literal conversion to pointer is detected,
• instead of nullptr literal, 0 is proposed as replacement.

Occurrences of implicit conversions inside macros and template instantiations are deliberately ignored, as it is not clear how to deal with such cases.

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