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V1056. The predefined identifier '__fun…
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Contents

V1056. The predefined identifier '__func__' always contains the string 'operator()' inside function body of the overloaded 'operator()'.

Apr 16 2020

The analyzer has detected the '__func__' identifier in the body of the overloaded '()' operator.

Consider the following example:

class C
{
  void operator()(void)
  {
    std::cout << __func__ << std::endl;
  }
};

void foo()
{
  C c;
  c();
}

This code will output the string 'operator()'. This behavior may seem reasonable in code like this, so let's take a look at a less trivial example:

void foo()
{
  auto lambda = [] () { return __func__; };
  std::cout << lambda() << std::endl;
}

It is important to remember that '__func__' is not a typical variable, so the following versions will not work as intended and the program will be still outputting the string 'operator()':

void fooRef()
{
  auto lambda = [&] () { return __func__; };
  std::cout << lambda() << std::endl;
}
void fooCopy()
{
  auto lambda = [=] () { return __func__; };
  std::cout << lambda() << std::endl;
}

In the case of lambdas, this can be fixed by passing '__func__' explicitly using a capture list:

void foo()
{
  auto lambda = [func = __func__] () { return func; };
  std::cout << lambda() << std::endl;
}

To get full-fledged output of the function name even inside the overloaded 'operator()' or lambdas, you can use the platform/compiler-specific macros. The MSVC compiler provides three such macros:

  • '__FUNCTION__' – outputs the function name including its namespace. For example, this is what we will get for a lambda inside the main function: 'main::<lambda_....>::operator ()';
  • '__FUNCSIG__' – outputs the full function signature. Similarly, it can be helpful when combined with a lambda: 'auto __cdecl main::<lambda_....>::operator ()(void) const';
  • '__FUNCDNAME__' – outputs the decorated name of the function. This information is quite specific, so it cannot fully replace '__func__'.

Clang and GCC provide the following macros:

  • '__FUNCTION__' – outputs the same name that the standard '__func__' does;
  • '__PRETTY_FUNCTION__' – outputs the full function signature. For example, you will get the following output for a lambda: 'auto main()::(anonymous class)::operator()() const'.
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