android-CHECK_xxx分析

android-CHECK_xxx

在android源码中有不少类似这样的用法，上图中就是检查获得的hal版本是否大于等于版本1_3，满足继续往下走，不满足则assert，并报错。
接下来就展开看看CHECK_xx家族：

用法

#define CHECK_EQ(val1, val2) CHECK_OP(EQ, ==, val1, val2)
#define CHECK_NE(val1, val2) CHECK_OP(NE, !=, val1, val2)
#define CHECK_LE(val1, val2) CHECK_OP(LE, <=, val1, val2)
#define CHECK_LT(val1, val2) CHECK_OP(LT, < , val1, val2)
#define CHECK_GE(val1, val2) CHECK_OP(GE, >=, val1, val2)
#define CHECK_GT(val1, val2) CHECK_OP(GT, > , val1, val2)

#define CHECK_OP(name, op, val1, val2) CHECK((val1) op (val2))
// Helper macro for binary operators.
// Don't use this macro directly in your code, use CHECK_EQ et al below.
// The 'switch' is used to prevent the 'else' from being ambiguous when the
// macro is used in an 'if' clause such as:
// if (a == 1)
//   CHECK_EQ(2, a);
#define CHECK_OP(name, op, val1, val2)                                         \
  switch (0) case 0: default:                                                  \
  if (::logging::CheckOpResult true_if_passed =                                \
      ::logging::Check##name##Impl((val1), (val2),                             \
                                   #val1 " " #op " " #val2))                   \
   ;                                                                           \
  else                                                                         \
    ::logging::LogMessage(__FILE__, __LINE__, true_if_passed.message()).stream()

#endif  // !(OFFICIAL_BUILD && NDEBUG)
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switch (0) case 0: default：当宏被用在一个if子句中时，'switch’用于防止’else’不明确

以CHECK_GE为例分析源码

#define CHECK_GE(val1, val2) CHECK_OP(GE, >=, val1, val2)
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CHECK_OP(GE, >=, val1, val2)展开后

if (::logging::CheckOpResult true_if_passed = ::logging::CheckGEImpl((val1), (val2), "val1 >= val2")
  ;
else
  ::logging::LogMessage(__FILE__, __LINE__, true_if_passed.message()).stream()
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实际上跳到另一个宏Check##name##Impl去执行了，看返回结果true_if_passed是不是为真，若为假则打印出信息，这里的LogMessage是一个类，根据文件名、行号、日志级别构建对象，之后执行LogMessage::stream()输出信息
看看宏Check##name##Impl

  inline std::string* Check##name##Impl(int v1, int v2, const char* names) { \
    if (ANALYZER_ASSUME_TRUE(v1 op v2))                                      \
      return NULL;                                                           \
    else                                                                     \
      return ::logging::MakeCheckOpString(v1, v2, names);                    \
  }
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CheckGEImpl展开后，在这里#op是>=

    if (ANALYZER_ASSUME_TRUE(val1 >= val2))
      return NULL;
    else
      return ::logging::MakeCheckOpString(v1, v2, names);
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看看宏ANALYZER_ASSUME_TRUE，在clang_analyzer中加静态分析，返回参数val1 >= val2是否为真，若为真返回NULL，Check_xxx通过。ANALYZER_ASSUME_TRUE常与断言和类断言函数一起使用。

inline constexpr bool AnalyzerNoReturn() __attribute__((analyzer_noreturn)) {
  return false;
}

inline constexpr bool AnalyzerAssumeTrue(bool arg) {
  // AnalyzerNoReturn() is invoked and analysis is terminated if |arg| is
  // false.
  return arg || AnalyzerNoReturn();
}

#define ANALYZER_ASSUME_TRUE(arg) logging::AnalyzerAssumeTrue(!!(arg))
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若ANALYZER_ASSUME_TRUE返回假，则进到else中的MakeCheckOpString，负责构建错误消息字符串，就不再展开了

// Build the error message string.  This is separate from the "Impl"
// function template because it is not performance critical and so can
// be out of line, while the "Impl" code should be inline.  Caller
// takes ownership of the returned string.
template<class t1, class t2>
std::string* MakeCheckOpString(const t1& v1, const t2& v2, const char* names) {
  std::ostringstream ss;
  ss << names << " (";
  MakeCheckOpValueString(&ss, v1);
  ss << " vs. ";
  MakeCheckOpValueString(&ss, v2);
  ss << ")";
  std::string* msg = new std::string(ss.str());
  return msg;
}
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