Extra Clang Tools 3.8 documentation

clang-tidy - readability-implicit-bool-cast

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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 cast:

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

void use(Foo& foo) {
  bool value = foo.getFoo(); // warning: implicit cast 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 occurences of bool, and the remaining code is no longer correct, yet it still compiles without any visible warnings.

In addition to issuing warnings, FixIt hints are provided to help solve the reported issues. This can be used for improving readabilty 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;
  // ^ propose replacement: functionTakingInt(static_cast<int>(boolean));

  // ^ propose replacement: functionTakingInt(1);
In general, the following cast types are checked:
  • integer expression/literal to boolean,
  • floating expression/literal to boolean,
  • pointer/pointer to member/nullptr/NULL to boolean,
  • boolean expression/literal to integer,
  • boolean expression/literal to floating.
The rules for generating FixIt hints are:
  • in case of casts from other built-in type to bool, an explicit comparison is proposed to make it clear what exaclty 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 cast to bool, the proposed replacement with comparison is simplified:
    • if (!pointer) is changed to if (pointer == nullptr),
  • in case of casts from bool to other built-in types, an explicit static_cast is proposed to make it clear that a cast is taking place:
    • int integer = boolean; is changed to int integer = static_cast<int>(boolean);,
  • if the cast 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 cast to pointer is detected,
  • instead of nullptr literal, 0 is proposed as replacement.

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

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